Four new Emperor Penguin colonies discovered in satellite images of Antarctica.

All current predictions for the future of Emperor Penguns, Aptenodytes forsteri, suggest that the species is in serious trouble, due to the effects of anthropogenic climate change, with predictions suggesting that all known colonies will be extinct or quasi-extinct (the point at which a population has shrunk so much that it is unlikely to be able to recover) by the end of the century if carbon dioxide emissions continue to rise at the current rate. If conservation measures are to be developed which will give the species any hope of survival, then monitoring Emperor Penguin colonies is essential. However, the extremely remote location of these colonies makes observing them from the ground impractical. This has caused scientists studying Emperor Penguins to turn to satellite imagery as a way to detect and monitor colonies of these Birds. This has proven to be a useful method not just for monitoring the health of Penguin colonies, but also of discovering new ones, with eight new colonies located in 2019, using data from the European Space Agency's Sentinel-2 satellite.

In a paper published in the journal Antarctic Science on 20 January 2024, Peter Fretwell of the British Antarctic Survey reports the discovery of four new Emperor Penguin colonies, using imagery from Sentinel-2 and Maxar WorldView.

The first new colony described by Fretwell lies on the Dronning Maud Land coast on the northern side of the Lazarev Ice Shelf, and is tentatively named the 'Lazarev North Colony'. A colony has previously been recorded on the Lazarev Ice Shelf. This colony known as the 'Lazarev Colony' was first observed by the Soviet Antarctic Survey in 1959, and was subsequently found in a number of satellite images. However, it has not been seen since 2014, and was recorded as being extinct in 2019. 

Map of Lazarev Ice shelf sites, showing the old location and the new location. Note that the new breeding site is around 65 km around the coast from the old site and although not yet counted, looks much smaller than the 4,500 pairs recorded in 2013. Fretwell (2024).

Fretwell considers it highly likely that the Lazarev North Colony is a relocated remnant of the old Lazarev Colony. The oldest image in which it is visible dates to 2018, and it can be seen in images from 2019, 2020, 2021, and 2022, although it is not present in all images over this period. No very high-resolution images of this colony, which would enable an estimate of the population size, but it appears to be significantly smaller than the Lazarev Colony in 2014, which contained about 4500 pairs of breeding Penguins. Fretwell suggests the colony may have relocated due to the extension of the ice tongue or a change in sea-ice conditions.

Sentinel-2 image of the Lazarev North Colony. Arrow shows the location of the Penguins. Fretwell (2024).

The second new colony is located at Verleger Point on the coast of Marie Byrd Land in West Antarctica. This colony is visible in all Sentinal-2 images of the region from 2018 to 2022, and is also visible in the higher resolution Maxar WorldView images, allowing the size of this colony to be estimated at about 500 pairs of Birds. \The colony is about 53 km to the east of the abandoned Soviet Russkaya research station, which was abandoned in 1990, although if the scientists at this base ever made observations of Penguins in the area, they did not publish them.

(Top) Map of the location of Verleger Point colony. This small colony is located equidistant from Ledda Bay and Curzon Island. It is approximately 53 km east of the site of the abandoned Russkaya research station. (Bottom) Sentinel-2 image of the Verleger Point Colony. Arrow shows the location of the Penguins. Fretwell (2024).

The third new colony is located on the eastern side of the West Ice Shelf. It is some way offshore, on stable fast ice which has formed around icebergs which have grounded on the shallow seas of the area. This colony is located about 65 km to the east of the Karelin Bay colony and about 180 km to the west of the Burton Glacier Ice Shelf colony, with all three colonies appearing to exist at the same time. Another colony was reported at Gaussberg, about 150 km from the new colony, in 1958. This colony appears to have subsequently vanished, making it possible that one of the current populations is in fact this colony relocated. However, since the closest population to the old Gaussberg colony site id the Burton Glacier Ice Shelf colony, Fretwell feels it is unlikely that the new colony is that colony, and suggests that this is instead a well-established colony, which was missed in earlier surveys because it is so far from shore. Because there is already a colony named for the West Ice Shelf, Fretwell names this colony the 'Vanhoeffen Colony' in honour of Ernst Vanhoeffen, the biologist on board the First German South Polar Expedition of 1901-1903.

Map showing the location of the Vanhoeffen Colony. There are a number of other breeding sites in the general area of this large new site, satellite imagery shows that there all exists at the same time and are not a movement of one group. Fretwell (2024).

The Vanghoeffen Colony is visible in all Sentinal-2 images from 2018-2022, as well as in very high-resolution Maxar WorldView images, allowing an estimate of the size of the colony to be made, at about 5000 breeding pairs of Penguins.

Sentinel-2 image of the Vanghoeffen Colony. Arrow shows the location of the Penguins. Fretwell (2024).

The final new colony described is located on the northten side of the Gipps Ice Rise, which itself forms the southern margin of the Larsen C Ice Shelf. This is a noteworthy discovery, as no colony has been discovered between the Jason Peninsula and Dolleman Island until now, despite a number of searches. 

Location of Gipps Colony. Fretwell (2024).

This colony is extremely small, estimated at about 200 breeding pairs of Penguins, and located in an area which is often obscured by cloud cover, making it extremely hard to detect in satellite images, although once identified it could be found in Maxar WorldView images dating back as far as 2016. The colony was located against ice cliffs or in a small ice creek north of the ice rise, until 2021, when the calving of a large ice berg changed the topology of the region, forcing the Penguins onto open fast ice, where the colony became easier to detect.

Sentinel-2 image of the Gipps Ice Rise Colony. Arrow shows the location of the Penguins. Fretwell (2024).

The discovery of the four new colonies, together with the re-discovery of a colony at Umbeashi in Amundsen Bay, which was thought to be extinct in 2019, but reformed in 2021 and 2022, brings the number od known Emperor Penguin colonies to 66, as well as filling several gaps in the distribution of these Birds. Despite this, it only raises the known global population of Emperor Penguins by about 5700 pairs. Given that the former Lazarev Colony was thought to comprise about 4500 pairs, and the new Lazarev North Colony is thought to be much smaller, this probably does not represent a major increase in the total Emperor Penguin population.

Newly reported emperor penguin colonies, shown in red boxes. Light blue boxes denote other known extant colony sites. Fretwell (2024).

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All Emperor Penguin colonies on the Bellingshausen Sea believed to have failed in 2022.

Emperor Penguins, Aptenodytes forsteri, are the largest species of Penguin, and have become iconic symbols of Antarctic wildlife, being the only Vertebrates that overwinter on the Antarctic continent. The species is dependent on the presence of anchored sea ice (sea ice attached to land rather than drifting) during the breeding season, with breeding and moulting taking place on the ice, while foraging for food is largely accomplished in waters around the ice shelf. Emperor Penguins typically arrive at their breeding grounds in March and April and lay eggs in May and June, with chicks hatching in July and August and fledging in December and January. This means that stable, land-fast ice needs to remain in place between April and January for breeding to succeed.

This means the Emperor Penguins are directly threatened by climate change, with loss of sea ice, or changes in its distribution, known to have caused failure at individual breeding sites in the past. Attempts at predicting the future of the species have painted a bleak picture, with current climate predictions suggesting that 90% of Emperor Penguin colonies will no longer be viable by the end of the twenty first century due to global warming and the accompanying loss of sea ice. This is the first anthropogenic threat the species has faced; Emperor Penguins have never been hunted, never suffered direct habitat loss due to Human expansion, and their fishing grounds have not been overfished by Human competitors, making them the only known Vertebrate species for which climate change is the sole threat to their long-term survival. 

A group of Emperor Penguins, Aptenodytes forsteri, on the Antarctic Ice Shelf. Ty Hurly/ICUN Red List of Threatened Species.

There are five known Emperor Penguin colonies on the Bellingshausen Sea, at (from east to west) Rothschild Island, Verdi Inlet, Smyley Island, Bryant Peninsula and Pfrogner Point. All of these colonies have been discovered within the past 14 years, using medium resolution satellite imagery, and subsequently had their populations assessed using very high-resolution imagery. Only the Rothschild Island colony has ever been visited, although the Smyley Island colony has been sighted from the air. The sites are assumed to be breeding colonies because they are occupied between October and December, when large aggregations of non-breeding Emperor Penguins have never been recorded. The largest of the Bellingshausen Sea colonies is on Smyley Island, where an average of 3500 pairs have been recorded, while the smallest is on Rothschild Island, with only 630 pairs recorded.

In a paper published in the journal Communications Earth and Environment on 24 August 2023, Peter Fretwell of the British Antarctic Survey, Aude Boutet, an independent researcher from Paris, and Norman Ratcliffe, also from the British Antarctic Survey, discuss the impact that loss of sea ice across much of the Bellingshausen Sea in November and December 2022 had upon Emperor Penguin breeding colonies of the area.

Antarctic sea ice extent in 2022–2023. The red line shows sea ice extent (more that 15% concentration) for 2022–2023, blue line shows 2021–2022 and the orange line is the 1981–2010 mean. The yellow ribbon is the satellite record (1979–2022). The grey shading refers to breeding stages of Emperor Penguin chicks. Critically sea ice must be stable for Emperor Penguins until the end of the fledging stage for all chicks to survive. Between October and January 2022–2023 sea ice around the continent has been at or below to the lowest ever recorded in the 45 year satellite record. Only briefly in mid-November did the concentration fleetingly rise to the second lowest extent. This low period intersected with the end of créching and fledging period in the emperors breeding cycle. Data courtesy of National Snow and Ice Data Center, Boulder, Colorado. Fretwell et al. (2023).

Four of the five Penguin colonies were affected by early sea-ice loss in November 2022; at this time much of the Southern Ocean around Antarctica was affected by sea-ice loss, but the Bellingshausen Sea suffered the most significant loss. Three of these colonies had been visible in satellite images in late October and early November, but were abandoned by the start of December, when the chicks should have begun fledging. The Pfrogner Point colony, which is the most westerly on the Bellingshausen Sea, and which was outside the area of the sea-ice anomaly, was also abandoned at some point between 29 October and 9 November 2022, probably due to earlier loss of sea ice.

Antarctic sea ice anomaly for November 2022. Blue areas in the map show positive sea ice anomaly, red shows negative. Although most of the continent has witnessed negative sea ice extent, the Bellingshausen Sea area has been particularly badly affected with up to 100% loss of ice in the region. Fretwell et al. (2023).

Exact timings of chick hatching and fledging on the Bellingshausen Sea have never been made, so assumptions about when these events occur have been based upon observations on Cape Washington and Pointe Géologie in the east Antarctic, where fledging begins in early- to mid-December, and finishes in late December or early January. This makes it likely that the Bellingshausen Sea colonies suffered total failure in 2022, due to losing their sea-ice before these dates. It is possible that some chicks were able to survive on grounded icebergs, but this is unlikely to have been a significant proportion of the whole, and the three colonies which disappeared before the onset of December were simply abandoned by the adult Penguins.

Emperor penguin colonies in the central and eastern Bellingshausen Sea. The locations of the five emperor penguin colonies in this region superimposed over the regional sea ice concentration anomaly for November 2022 shown in red. Fretwell et al. (2023).

Although satellite data is only available for the entire of the Bellingshausen Sea from 2018 onwards, only one of the colonies is known to have previously suffered a complete loss of sea-ice (Bryant Peninsula in 2010), and statistical models which predicted the colony failures in 2022 from satellite data suggest that it is unlikely that other such losses have gone undetected. 

The timing of satellite imagery showing sea ice break up and colony disappearance at the five colonies, in comparison with the timing of the breeding stage of the species. The dark blue circles denote Sentinel2 images where the colony can be seen. Light blue hexagons denote where ice was still present but there was no sign of the colony (no brown pixels) and orange squares denote images where sea ice had broken up or dispersed. Note that four of the five colonies were abandoned by the start of the fledging season. Fretwell et al. (2023).

The Verdi Inlet colony was first detected in 2018, and has been found in satellite imagery each year since. in 2018-2021 sea-ice around the inlet did not break up until January, with an estimated 3000 pairs of Penguins using the site in November 2021. The colony was observed in September 2022, but was much smaller than in previous years. The last of the land-bound sea-ice around Verdi Inlet broke up between 31 October and 4 November 2022, with all ice having vanished by early December. No signs of Penguin activity were detected after the initial break-up of the ice, suggesting the colony was abandoned at this point.

The Smyley Island colony was first detected in Landsat imagery in 2009, and the colony has been monitored in Very High-Resolution satellite imagery by the British Antarctic Survey since that time. The colony was home to between 1000 and 6500 pairs of breeding Penguins each year, with a ten-year average of 3000 pairs. Bound sea-ice was observed around the island until at least early December, until 2022, when the ice broke up in mid-November. Prior to this, the colony had split into two groups about 4 km apart (suggesting the Penguins were aware there was a problem, and had reacted to it), Some Penguins were detected on a grounded iceberg in December, although it is unclear if any chicks survived.

The Bryant Coast colony was first detected in Landsat data in 2014, and subsequently found in images dating back as far as 2000. The colony was absent in 2010, and reached a maximum size of 2000 breeding pairs in 2014. Multi-year bound-ice was present at the colony site from 2010 until 2021, providing the Penguins with a stable year-round platform. In 2022 the colony was detected in mid-November, but again seemed smaller than usual. By 25 November, the sea-ice could be seen retreating close to the colony, and by 29 November the bound sea-ice around the colony had gone, although floating pack-ice was still present, with some brown staining (indicative of the presence of Penguins) observed on this ice. However, by 2 December all signs of Penguin activity had vanished, and the colony is presumed to have failed. 

The Pfrogner Point colony was first detected in satellite imagery in 2019, and has been found in satellite images from 2018-2022, with a single estimate putting the population at 1200 pairs of Penguins. The colony is situated on an ice shelf (part of a glacier flowing out over the sea) rather than directly on the sea-ice, and appears to have shifted between the main shelf and a tongue of ice associated with an outflowing creek. The colony was detected on 9, 22, and 29 October 2022, although it appeared smaller than usual, but was not seen in an image taken on 8 November, nor any subsequent image. by 12 December all sea ice in the area had vanished, and no Penguins could be observed. This colony appears to have been abandoned before the ice began to break up, although why this was the case is unclear. Very high resolution satellite images suggest that the ice cliff at the edge of the shelf was about 4.5 m high, with a snow ramp between the shelf and the sea ice below at the foot of the ice creek. Satellite images from October suggest that the sea ice beneath this ramp may have broken close to the ice cliff, which would have made it impossible for the adult Penguins to return from the sea to their chicks, forcing them to abandon the colony.

The Rothschild Island colony is the furthest north of the Bellingshausen Sea colonies, being found on sea ice between Alexander Island and Rothschild Island within the Wilkinson Sound embayment. It is a small colony, home to about 700 pairs of breeding Penguins. The colony was directly observed by helicopters deployed from the luxury cruise ship Commandant Charcot on 20 November 2022, which counted 228 adult Penguins, and 820 chicks. Satellite images revealed that there was still ice beneath the colony on 5 and 17 December, although several large patches of open water had appeared within the ice shelf, with the ice around the colony starting to break up on 20 December, making likely that at least some of the chicks fledged successfully. Rothschild Island was close to the heart of the 2022 sea-ice anomaly, and yet the sea-ice appears to have remained sufficiently intact for the Penguins chicks to fledge. It is possible that the sheltered position within the shallow Wilkinson Sound and the many icebergs in the surrounding sea helped to stabilize the sea for longer than at other locations.

Sentinel2 imagery from the five colonies in 2022 showing the progressive sea ice extent though the créching and fledging season. Each of the five columns shows multiple images from a single colony, with the earlier images at the top and the later images below. Images where the brown pixels of guano staining, indicative of emperor penguin colonies can be seen are highlighted with yellow circles. Fretwell et al. (2023).

Scientists have been monitoring Emperor Penguins by satellite since 2009, and other instances of breeding colonies being lost to rapid ice break-up. Some colony sites, such as the Leda Bay colony in Marie Byrd Land appear to be particularly prone to this, and fail regularly. However, the 2022 event is the first recorded instance of a widespread sea-ice failure affecting multiple colonies before the chicks fledge. Moreover, only one of the Bellingshausen Sea breeding colonies had previously undergone such a failure, suggesting that the 2022 sea-ice collapse event was genuinely significant.

Emperor Penguin colonies have been known to relocate in response to repeated sea ice failure. A colony of Penguins which formerly bred at Halley Bay in the Weddell Sea, but the ice here began to fail regularly from 2016 onwards, prompting the Penguins to relocate their breeding site to a more stable location on Dawson Lambton Glacier, 85 km to the south. However, more widespread failures of the ices shelf due to global warming would make such relocations impossible, although some respite might be offered by refugia such as the Rothschild Island location.

It is difficult to predict exactly how climate change will affect the future of the ice shelf, but all current models suggest that a long-term decline in the ice cover is to be expected. Satellite records of the extent of the ice shelf go back 45 years, with four of the lowest sea-ice coverages recorded since 2016, and the lowest two coverages being in the 2021-22 and 2022-23 seasons. It is possible that this loss is part of an episodic cycle rather than a genuine long-term trend, and answering this question is now a priority for scientists studying the Antarctic climate and sea-ice. The extreme sea-ice loss seen in 2021-22 and 2022-23 is likely to have been influenced by the three years of La Niña conditions in the southern Pacific, which tends to lead to warmer conditions and lower sea ice in the waters off western Antarctica, and that the switch to El Niño conditions in the Pacific in 2023 will lead to a return of cooler conditions and more stable sea ice, but the failures seen in 2022 still represent a warning about the future of Emperor Penguin  in a warming global climate.

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Vertebrate fossils from the Late Eocene of Seymour Island, Antarctica.

The Southern Hemisphere biota has been profoundly influenced by Mesozoic-Cainozoic continental breakup and climatic change. Before its fragmentation, the supercontinent Gondwana facilitated dispersal of terrestrial organisms between now-separated southern landmasses. Early discoveries suggest that Antarctica was central to this pattern of terrestrial movement, acting as a bridge between what is now South America and Australia. This widespread dispersal ended with the final breakup of Gondwana. Through the last part of this breakup, the Antarctic climate shifted from being warm and seasonally wet to increased periods of ice cover by the early to middle Eocene; by the earliest Oligocene (about 33.9 million years ago) Antarctica experienced complete glaciation. Insights into how the Antarctic biota was shaped by tectonic and climatic shifts have come from the upper Eocene La Meseta and Submeseta formations on Seymour Island, the best-studied fossil Vertebrate fauna from Antarctica. This assemblage has been proposed to most closely resemble contemporaneous faunas from Patagonia which were separated from what is now the Antarctic Peninsula by the flooding of the Weddellian Isthmus at the end of the Palaeocene. The fossil record of the La Meseta and Submeseta formations is famously dominated by stem Penguins, including some of the tallest Penguins that ever lived. The non-Penguin Vertebrate fossil record mostly comprises isolated teeth and bones representing an array of Marsupial, Gondwanathere, Ungulate, Cetacean, and other Eutherian Mammals as well as a possible non-Therian Dryolestoid as well as non-Penguin Birds. 

In a paper published in the journal PeerJ on 9 January 2020, Sarah Davis of the Department of Geological Sciences at the University of Texas at Austin, Christopher Torres of the Department of Integrated Biology, also at the University of Texas at Austin, Grace Musser, also of the Department of Geological Sciences at the University of Texas at Austin, James Proffitt of the School of Medicine at the University of Missouri, Nicholas Crouch and Ernest Lundelius, again of the Department of Geological Sciences at the University of Texas at Austin, Matthew Lamanna of the Section of Vertebrate Paleontology at the Carnegie Museum of Natural History, and Julia Clarke, once again of the Department of Geological Sciences at the University of Texas at Austin, report additional Mammalian and Avian specimens recovered from Antarctica by the Antarctic Peninsula Paleontology Project, including a gruoid and a xenarthran fossil, and discuss their biogeographic implications.

The fossils described by Davis et al. were collected from Eocene deposits on Seymour Island. The island is located approximately 100 km east of the Antarctic Peninsula in the James Ross Basin and contains fossiliferous marine sedimentary units ranging from Late Cretaceous to late Eocene/earliest Oligocene in age. These deposits can be divided into the Marambio Group (Santonian-Danian) and the unconformably overlying Seymour Island Group (Palaeogene). The Marambio Group is made up of the Santa Marta (Santonian to Campanian), Snow Hill Island (Campanian to Maastrichtian), López de Bertodano (Maastrichtian to Danian), and Sobral (Danian) formations, while the Seymour Island Group includes the Cross Valley (upper Paleocene), La Meseta (Eocene), and Submeseta (upper Eocene to lowermost Oligocene) formations. 

The La Meseta Formation is predominantly made up of mudstones and sandstones and is interbedded with conglomerates. The formation was divided into numbered informal units called Tertiary Eocene La Meseta 1-7. These units were later divided into allomembers (from bottom to top: Valle de Las Focas, Acantilados I, Acantilados II, Campamento, Cucullaea I, Cucullaea II, and Submeseta), with the Submeseta Allomember (comprising the upper portion of Tertiary Eocene La Meseta 6 and all of Tertiary Eocene La Meseta 7) later reassigned as the Submeseta Formation. The Submeseta Formation represents the late Eocene (Priabonian) to earliest Oligocene, between 43.4 and 33 million years old. This unit is composed predominantly of fine sandstones and mudstones from a shallow marine environment, but may reflect a sea level rise towards the top of the section. The Submeseta Formation itself has been divided into three units defined by their boundary discontinuities: the lower Submeseta I, middle Submeseta II, and upper Submeseta III allomembers.

Fossils were surface-collected at four localities on the northeastern end of Seymour Island from cross-bedded fine sandstone units interpreted as shallow marine or estuarine environments. A Mammalian metacarpal was collected from locality S124, located in the Cucullaea I Allomember of the La Meseta Formation (52.8-49 million years old). A Penguin mandible was collected from locality S074, within the Cucullaea II Allomember of the La Meseta Formation (49-45.9 million years old), and all tarsometatarsi were collected from the Submeseta I Allomember of the Submeseta Formation (43.4-41 million years old) at localities S123 and S117/122.

  

Map and geology of Seymour Island, James Ross Basin, Antarctic Peninsula, showing the localities where new material was recovered. Fossil localities are marked by dots in the La Meseta and Submeseta formations. The Mammalian metacarpal was recovered from S124, the Penguin mandible from S074, and the three tarsometatarsi from S123 and S117/122.  Davis et al. (2020).

The first specimen described is TMM 44190-1, the left metacarpal II of an unknown Xenarthran Mammal, from the Late Eocene Cucullaea I Allomember (Tertiary Eocene La Meseta 4) of the La Meseta Formation, of location S124 on Seymour Island, off the Antarctic Peninsula.

TMM 44190-1 is weathered and missing the distal epiphysis. The distal surface is extensively pitted and shows no sign of breakage, indicating that TMM 44190-1 likely belonged to a juvenile individual. It has a maximum proximodistal length of 33 mm as preserved and a maximum mediolateral width of 21 mm at both the proximal and distal ends.

 

Left metacarpal II (TMM 44190-1) referred to Xenarthra. (A) lateral; (B) medial; (C) palmar; (D) dorsal; (E) proximal; and (F) distal views. Features are tentatively indicated, as they are difficult to assign given the isolated and fragmentary nature of the element. Abbreviations: ?mcc, facet for either the metacarpal-carpal complex or for metacarpal I; ?mcIII, probable facet for metacarpal III; ?t, trapezoid facet. Scale bar is 10 mm. Davis et al. (2020).

In medial view, the articular facet for what could be the metacarpal-carpal complex or for metacarpal I is sharply- defined, forming the proximal part of the palmar margin and projecting well- palmarly of the rest of this margin. The remainder of the medial face is marked by two rugosities: one at the proximopalmar part and the other across the entire distal half. These rugosities are separated by a smooth sulcus, resulting in a notched medial margin in dorsal/palmar views. In lateral view, an articular facet, potentially for metacarpal III, is rugose and worn. Due to the bone being hourglass-shaped in medial and lateral views but sub rectangular in dorsal and palmar views, the latter faces are broadly concave and saddle-shaped. The articular face for the ?trapezoid carpal is triangular with sharply-defined medial and dorsal margins, and slopes slightly towards the medial edge.

The majority of Mammal fossils from the Eocene of Seymour Island comprise teeth and isolated postcranial material of Marsupials, Astrapotheria, Gondwanatheria, South American Native Ungulates (Litopterna), and potential Xenarthrans. Given the juvenile status and isolated nature of TMM 44190-1, as well as the relative paucity of described mammalian metacarpals from this time period, it is difficult to make comparisons with other contemporaneous fossils. Davic et al. therefore limit detailed comparison to clades known from Seymour Island, except for Gondwanatheres for which no metacarpal material has been described.

The metacarpals of Palaeogene Marsupials are more elongate and gracile than TMM 44190-1, with more rounded proximal ends. Metacarpal II specifically shows less prominent articular facets for metacarpal III compared to the new fossil. In Astrapotheres, the second metacarpal is subrectangular and apparently dorsopalmarly compressed while widening distally, but has a prominent ridge along the lateral side that is not seen in the new fossil. In ?Parastrapotherium the proximal facet appears somewhat flattened similar to the condition in TMM 44190-1, but the metacarpal contains a shallow pit along the medial surface not seen in the new fossil. The metacarpal also does not taper as dramatically towards the midpoint of the element. However, there are no detailed illustrations of individual metacarpals of ?Parastrapotherium, and so these apparent similarities are difficult to further assess. South American Native Ungulate metacarpals are more elongated than the new fossil, have a saddle-shaped articulation for the trapezoid, and have more points of articulation with the carpals than are seen on TMM 44190-1.

Overall the robustness and proportions of TMM 44190-1 are most consistent with the metacarpals of Xenarthrans, though the specimen does not have conspicuous affinities with a particular subclade. Within Cingulata (Glyptodonts, Armadillos), the metacarpals and metatarsals are most often figured in articulated posture which complicates comparisons, especially of their articular facets. The second metacarpals of the North American Glyptotherium texanum and South American Glyptodon reticulatus are of similar proportion and share the hourglass configuration in lateral and medial views as TMM 44190-1, though they have less dorso palmar tapering towards the center of the diaphysis than is seen in the new fossil. The articulation for the trapezoid in both taxa is dramatically concave, in contrast to the flat, triangular surface in TMM 44190-1, and there are two points of contact along the proximal end between metacarpals II and III rather than one. The articular surface for metacarpal III in the Miocene Armadillo relative Proeutatus is similarly shaped to that of TMM 44190-1, though it is only figured as articulated and this cannot be confirmed. The Oligocene genus Peltephilus is most similar amongst representatives of the Cingulates to the new fossil, though it appears longer. The dorsal edge of the articular surface for the trapezoid appears similarly flat and sloped as in TMM 44190-1, and the facets for metacarpal III and the metacarpal-carpal complex appear proportional to the new fossil as well. However towards the distal end the metacarpal tapers in a way that TMM 44190-1 would not, even if the missing epiphysis were present.

The new fossil also shares some features seen in Pilosa (Anteaters, Sloths). Although no second metacarpals have been recovered for Palaeogene Folivorans (Sloths), there are similarities with the metacarpals of younger Sloth taxa. The overall shape of the metacarpal resembles that of adult specimens of Megalonyx spp., Hapalops spp., and Eucholeops spp. TMM 44190-1 is more robust than the metacarpal II of some other Xenarthrans such as Thalassocnus, and Mionothropus cartellei, but more closely matches the proportions of Pleistocene ground-dwelling taxa such as Megatherium urbinai. At the distal end of the metacarpal, TMM 44190-1 is more mediolaterally wide than it is dorsopalmarly tall, a proportion that does not match other measured Sloth metacarpals. The ratio of proximodistal length to dorsopalmar depth of TMM 44190-1 is 1.57, which falls far below most values reported for other Sloth taxa and near Glossotherium, well within the range considered stout. The trapezoid facet is sub-planar, similar to that of Pseudolestodon hexaspondylus and Simomylodon uccasamamensis. However, this is different than the concave facets seen in other Sloths such as Eremotherium eomigrans and Thalassocnus. In TMM 44190-1, the articular facet for the metacarpal-carpal complex does not extend distally to the midpoint of the shaft as in younger forms such as Hapalops (Miocene) and Nothrotherium (Pleistocene). The new fossil is also lacking facets seen in some Sloth taxa, such as one for the magnum as in Eremotherium eomigrans, Megatherium americanum, and Thalassocnus or the unciform as seen in Scelidotherium. It is unclear if the rugose surface texture along the medial face is consistent with its juvenile status or could be due to a closely-appressed digit I (as in for example Mionothropus). There are no figured second metacarpals for extinct Vermilingua (Anteaters), but in extant Anteaters the second metacarpals are long and thin and quite different from the new fossil.

Being Eocene in age, and therefore close to the estimated early radiation of Xenarthran groups, the likelihood of a single metacarpal having a diagnostic characteristic of one particular Xenarthran subclade may be expected to be low. The fossil shows several features seen in extinct members of both Pilosa and Cingulata, and particularly with early diverging examples such as Peltephilus. While these similarities suggest TMM44190-1 is a Xenarthran, further subclade attribution is not possible.

The second specimen described is TMM 44189-2, the distal end of a left tarsometatarsus of an unknown Gruiform Bird, from the Late Eocene Submeseta I Allomember (Tertiary Eocene La Meseta 7), Submeseta Formation of location S123, on Seymour Island off the Antarctic Peninsula.

TMM 44189-2 preserves the bases of trochleae IIIV as well as the dorsal and plantar openings of the distal vascular foramen. The maximum mediolateral width as preserved is 25 mm. The distal vascular foramen is proximodistally elongate in dorsal and plantar views, and the dorsal opening of the foramen is set in a deep sulcus. The plantar opening of the foramen is slightly lateral to and near the midline and is positioned distal to the juncture of trochleae II and III. The fossa the supratrochlearis plantaris muscle is shallow. Most of trochlea II is missing but appears to lack a well-defined plantar crest extending proximally from the ala of the trochlea. In distal view, trochlea II is plantarly deflected, and trochleae III and IV are widely spaced.

 

Distal end of a left tarsometatarsus (TMM 44189-2) compared to those of extant Gruiforms. The fossil (A)-(E) and comparative materials (F)-(Y) shown in dorsal, plantar, medial, lateral, and distal views. Abbreviations: II, trochlea II; III, trochlea III; IV, trochlea IV; dvf, distal vascular foramen. Scale bar is 10 mm. Davis et al. (2020).

Aportion of a tarsometatarsus (MLP 90-I-20-9) recovered from Seymour Island was previously figured as Gruiform but not described. This specimen comprises a distal diaphysis that is broken proximal to the trochleae, of which only the proximal most part of trochlea IV is preserved. Despite the partial preservation, this fossil does not clearly show the splayed trochlear arrangement present in extant Gruoidea (Cranes, Trumpeters, and Limpkins) and in TMM 44189-2. The lack of published measurements or description of MLP 90-I-20-9 make comparisons with the new fossil difficult, but as figured it appears that this fossil is narrower than TMM 44189-2. Further evaluation is needed to determine the exact relationship between the two fossils, but TMM 44189-2 exhibits a suite of character states that allows for a more detailed assessment.

Phylogenetic studies that have recovered a monophyletic Gruiformes have not recovered synapomorphies from the distal tarsometatarsus that can be assessed in this specimen. However, TMM 44189-2 presents a combination of character states most consistent with Gruiformes: (Cranes, Rails, and allies): (1) trochleae III and IV projecting well distal of II; (2) plantar deflection of trochlea II (as inferred from the base and preserved ala); (3) trochlea III positioned dorsal to trochlea IV in distal view; (4) dorsoplantar flattening and mediolateral broadening of the supratrochlear region; (5) position of the distal vascular foramen near the midline and away from the lateral margin in plantar view; and (6) wide spacing of trochleae, III and IV. Within Gruiformes, TMM 44189-2 is more similar to Gruoidea than Ralloidea (Rails, Finfoots, and Flufftails) based on the following characteristics: (1) trochlea II is not as plantarly deflected in the new fossil as in Ralloids; (2) trochlea II projects farther distally relative to III and IV in the new fossil than in Ralloids; (3) the distal vascular foramen is located midway between the midline and the lateral margin in plantar view, unlike in Ralloids where it is located on the midline; (4) the supratrochlear region of the new fossil is mediolaterally broader and trochleae II and IV are more widely spaced than in Ralloids; (5) the distal margin of the distal vascular foramen is in line with the proximal extent of trochlea III in dorsal view, unlike in Ralloids; (6) the distal vascular foramen is located closer to the intertrochlear incisure in plantar view than in Ralloids; and (7) in dorsal view, the proximal extents of trochleae III and IV are subequal, whereas IV is proximal to III in Ralloids.

The morphology of TMM 44189-2 is not unambiguously consistent with any particular Gruoid subclade. Trochlea II is not as plantarly deflected as in Gruidae (Cranes) or Aramus guarauna (Limpkin), and is more like the condition observed in Psophiidae (Trumpeters). As in Aramus guarauna and Gruidae, trochlea III is the most dorsally positioned trochlea. There is a shallow depression at the plantar base of trochlea IV along the beginning of an ala that is most like that of Balearica pavonina and Grus canadensis among compared Gruiformes. However, the trochlear bases of TMM 44189-2 are not as dorsoventrally thick as those of Aramus guarauna and Gruidae and are more like those of Psophia viridis. The distal vascular foramen of TMM 44189-2 is ovoid in plantar view, with the long axis at an oblique angle to the long axis of the shaft, as in Gruidae but unlike A. guarauna and Psophia. In dorsal view, the distal vascular foramen is set in a broad, shallow sulcus as in Balearica pavonina and Psophiidae; by contrast, this sulcus is deep and sharply defined in Gruoidea. The fossil lacks the sharp plantar crest extending proximally from the ala of trochlea II observed in Gruoidea. A marked, circular depression is located between trochleae II and III, and is most like the condition in Psophia, Aramus guarauna, and Balearica pavonina, although the observed depth may be an artifact of preservation.

The next specimen, TMM 44189-1,  is a left tarsometatarsus assigned to the Penguin Delphinornis sp., from the late Eocene Submeseta I Allomember (Tertiary Eocene La Meseta 7) of the Submeseta Formation of location S123 on Seymour Island, off the Antarctic Peninsula.

TMM 44189-1 is missing its proximal end and trochlea IV. It is the more complete example of the two tarsometatarsi (including TMM 44188-2) recovered by the 2016 Antarctic Peninsula Paleontology Project expedition that represent a small-bodied Penguin. The specimen is similar in size to the tarsometatarsus of the extant Spheniscus humboldti (Humboldt Penguin). The hypotarsal crests are not preserved; however, an abraded surface appears to mark the former distal most extent of the medial hypotarsal crest. The medial proximal vascular foramen is positioned directly medial to the abraded surface that potentially corresponds to the medial hypotarsal crest. The distal vascular foramen is partially preserved, including a distinct plantar foramen just lateral to the trochlea of metatarsal II.

 

New Sphenisciform fossil material. A small left tarsometatarsus (TMM 44189-1) in (A) dorsal, (B) plantar, (C) proximal, and (D) distal views; a small left tarsometatarsus (TMM 44188-2) in (E) dorsal, (F) plantar, (G) proximal, and (H) distal views; a large left tarsometatarsus (TMM 44188-1) in (I) dorsal, (J) plantar, (K) proximal, and (L) distal views; and a partial mandible (TMM 44187-1) in (M) dorsal, (N) ventral, (O) right (with associated material), and (P) left (with associated material) views. Abbreviations: II, trochlea II; III, trochlea III; dvf, distal vascular foramen; es, extensor sulcus; fs, flexor sulcus; h, hypotarsus; ia, intercotylar area; ie, intercotylar eminence; lc, lateral cotyla; mc, medial cotyla; ms, mandibular symphysis; pvf, proximal vascular foramen. Scale bar is 10 mm. Davis et al. (2020).

TMM 44189-1 is referable to Sphenisciformes (Penguins) based on its overall proportions, morphology, and extreme osteosclerosis. The specimen possesses both intertarsal grooves, unlike the much larger Penguin tarsometatarsus (TMM 44188-1). Although apparent, the medial intertarsal groove is shallower than that of all comparable extant species. The lateral intertarsal groove is present and deep, similar to the condition in extant species as well as the extinct taxa Delphinornis, Marambiornis, and Mesetaornis. The groove does not taper strongly distally, as in Marambiornis and Mesetaornis. The trochlea of metatarsal II is positioned more medially than those of all comparable extant Penguin species, resulting in a wide medial intertrochlear incisure that appears most similar to that of Delphinornis. A distal vascular foramen is present as in taxa from the Palaeocene of New Zealand, including Muriwaimanu, as well as the small Antarctic taxa Delphinornis, Marambiornis, and Mesetaornis. The distally-opening passage of the extensor brevis digiti IV muscle is confluent with the distal vascular foramen, as in Delphinornis, Marambiornis, and Mesetaornis. The plantar opening of the distal vascular foramen is more distally positioned than in Marambiornis and Mesetaornis and is similar in morphology to that of Delphinornis. Based on these traits, we assign this fossil to Delphinornis. Tarsometatarsal traits that distinguish among the three species of Delphinornis, namely morphology of the intercotylar eminence, medial hypotarsal crest, proximal vascular foramina, and relative sizes of all three trochleae, are not preserved in this specimen.

The next specimen described is TMM 44188-2, a left tarsometatarsus, also assigned to Delphinornis, from the Late Eocene Submeseta I Allomember (Tertiary Eocene La Meseta 7) of the Submeseta Formation at location S122, on Seymour Island, off the Antarctic Peninsula.

TMM 44188-2 is missing the proximal end and all three trochleae. It is the less complete of the two 2016 specimens that represent a Small Penguin morphotype. It is comparable in size to TMM 44189-1 and identical to that specimen in all preserved morphologies. Therefore, it likely also represents a fragmentary specimen of the clade Delphinornis.

The next specimen described is TMM 44188-1, another left tarsometatarsus, assigned to the Penguin Palaeeudyptes sp., from the Late Eocene Submeseta I Allomember (Tertiary Eocene La Meseta 7), of the Submeseta Formation at location S117, on Seymour Island, off the Antarctic Peninsula.

TMM 44188-1 is a mostly complete tarsometatarsus that is missing trochlea IV. It has a proximodistal length of 45mmand proximal mediolateral width of 39 mm. The medial and lateral proximal cotyla are separated dorsally by a pronounced intercotylar eminence and plantarly by a planar intercotylar area. The medial proximal vascular foramen is positioned just distal to the distal terminus of the medial hypotarsal crest, and is more developed than the lateral proximal vascular foramen. A scar for the tibialis cranialis muscle is present on the dorsal face as a short ridge that extends distally from the proximal margin. The tarsometatarsus lacks an appreciable medial dorsal intertarsal sulcus but exhibits a lateral sulcus.

TMM 44188-1 most closely resembles the tarsometatarsus of the contemporaneous Seymour Island Penguin Palaeeudyptes gunnari based on the following features: (1) a concave medial margin, (2) a medial proximal vascular foramen that is larger than the lateral vascular foramen, (3) absence of an osseous ridge from the intermediate hypotarsal crest to the medial margin as in Palaeeudyptes antarcticus, and (4) a proximally-positioned scar for the tibialis cranialis muscle. The new specimen can be differentiated from the contemporaneous and similarly sized Archaeospheniscus, known from Seymour Island and New Zealand, based on the lack of a medial dorsal intertarsal sulcus and unequally sized proximal vascular foramina. The new specimen is also distinct from the contemporaneous Anthropornis, known from Seymour Island and New Zealand, in which the scar for the tibialis cranialis muscle is positioned more distally and the medial proximal vascular foramen is larger than the lateral. Lastly, TMM 44188-1 is significantly smaller than Palaeeudyptes gunnari, Palaeeudyptes klekowskii, and the two species of Palaeeudyptes known from New Zealand, Palaeeudyptes antarcticus and Palaeeudyptes marplesi, and may therefore represent a new species within Palaeeudyptes.

The final specimen, TMM 44187-1, comprises therostral portion of the mandible of an unknown Penguin, with associated fragments of caudal rami, from the Late Eocene Cucullaea II Allomember (Tertiary Eocene La Meseta 5), of the La Meseta Formation at location S074, on Seymour Island, off the Antarctic Peninsula.

TMM 44187-1 comprises the rostral end of a mandible that includes most of the symphyseal region, with the left mandibular ramus being more complete than the right. The rostralmost tip is missing. The preserved portion of the left ramus measures 171 mm in length and 7 mm in maximum width. An additional fragment of this ramus measures 81 mm in length, demonstrating that, when complete, the left mandibular ramus was at least 252 mm in length. However, the articular regions of the mandible are missing, indicating that the original length of the bone was even greater. The preserved portion of the symphysis measures 37 mm in length and 10 mm wide at its rostrocaudal midpoint. Davis et al. estimate the length of the complete symphysis at 40 mm. The mandible is slender and pointed but sturdily constructed, and is excavated by vascular canals throughout much of its length. The tip of the mandible is straight, and the rami meet the symphysis along a straight line rather than at an angle. Mandibular fossae are not preserved.

Few penguin mandibles have been reported from Seymour Island, and to date only one has been referred to a known species (MLP 14-XI-27-84, assigned to Anthropornis grandis). TMM 44187-1 has a shorter symphysial region than that reported for MLP 14-XI-27-84 (about 45 mm) and lacks the ventrally convex, dorsally concave condition seen in Anthropornis grandis. TMM 44187-1 differs from other partial mandibles described from Seymour Island (MLP 91-II-4-221, MLP 92-II-2-195, IB/P/B-0653), which are more tapered towards the distal end and have thinner rami. However, the overall morphology of the symphysis is comparable to other Seymour Island fossils described despite differing in overall dimensions (MLP 96-I-6-48, MLP 78-X-26-144, IB/P/B0617e, MLP 14-XI-27-27). The fossil MLP 96-I-6-48 has vascular pitting and a flattened dorsal surface similar to those seen on TMM 44187-1, and the pitting is consistent with the morphology of extant adult Aptenodytes forsteri (Emperor Penguin). The shape of the rami of MLP 96-I-6-48, MLP 78-X-26-144, IB/P/B-0617e, and MLP 14-XI-27-27 are all similar to that of TMM 44187-1. An unassigned, mostly complete mandible with associated maxilla most closely resembles the new fossil; MLP 14-XI-27-27 appears to have similarly straight mandibular rami and appears to have a flattened dorsal surface. However, only a dorsal view is figured and the specimen lacks formal description, making it difficult to determine if the two fossils are from the same taxon. These fossils pertain to Paleogene Penguins with spear- or dagger-like bills characteristic of stem species. However due to the new fossil's differences to assigned mandibular material and its partial preservation, TMM 44187-1 is not considered referable to any known Eocene taxon at this time.

Though Davis et al.'s recently collected Eocene material is fragmentary, it provides additional support for records of the presence of Mammalian and Avian taxa previously proposed from even more fragmentary and controversial single elements. These new records are also consistent with those expected for the Eocene of Antarctica given longstanding hypotheses of a biotic connection between Antarctica and South America during the Palaeogene as well as penecontemporaneous fossil discoveries from Patagonia. The Eocene Mammalian record otherwise comprises Gondwanatheres, Marsupials, Cetaceans, `South American Native Ungulates' (e.g. a Litoptern, Astrapotheres), and additional, enigmatic Eutherians. Indeed, in addition to the described metacarpal, the 2016 Antarctic Peninsula Paleontology Project expedition recovered a vertebra (from locality S123) consistent with referral to a Basilosaurid Archaeocete. Although new collections improve our understanding of biodiversity on Antarctica during the Eocene, they also highlight the need to recover and describe more material to elucidate a nuanced understanding of biotic exchange during this key time period.

Previous reports of Xenarthrans from the Eocene of Seymour Island, based on a distal ungual phalanx and an incomplete toothwere initially assigned to Tardigrada (Folivora), but were later questioned. The phalanx was recovered from the Cucullaea I Allomember, approximately 0.7 km away from where TMM 44290-I was collected, but lacks formal description and reportedly has been lost, precluding reevaluation. The phalanx was noted to be indistinguishable from the earliest known Vermilingua (Anteater) fossil from Patagonia, and based on histological study the tooth was reassigned to Mammalia indet. Therefore, the newly described metacarpal is either further evidence, or new evidence, that Xenarthra was indeed present on Antarctica during the Eocene depending upon one's stance with regard to prior controversies. This record is consistent with the estimated timing of origin for Folivora by the early Eocene, and of Xenarthra in the Palaeocene. 

Xenarthra is proposed to have originated in South America, and thus is plausibly anticipated in the Palaeogene of Antarctica given inferred land connections between these continents during the early Cainozoic. The new metacarpal extends the known Palaeogene geographic range of Xenarthra into Antarctica. Xenarthran limb bones and osteoderms have been reported from the Early Eocene (55-50 million years ago) of Brazil, but the earliest reported members of Pilosa date to 31.5 million years ago in Chile and Argentina while described early Cingulates from Patagonia are potentially Early Eocene. If Pilosan, the new material would indicate that this clade was present in Antarctica by at least 35 million years afo, four million years before it is known in South America. A Cingulate affinity would support the presence of the group in both Antarctica and southern South America around the same time. However, the paucity of other described Palaeogene Xenarthran postcranial material limits definitive analysis of the phylogenetic affinities and ecology of this individual.

Antarctic Bird fossils from non-Penguins are rare, and only a few have been named as species. They account for less than half of known extinct Avian species diversity on the continent, but comprise an even smaller fraction of unnamed material in collections. Therefore, the distal tarsometatarsus, although fragmentary, expands our understanding of Antarctic avian diversity during the late Eocene. A proposed Gruiform from Seymour Island was previously figured but its relation to the new fossil is difficult to assess. The new fossil exhibits preserved characters that allow for a more confident referral to core-Gruiformes, providing new evidence for the presence of the clade in Antarctica.

Our understanding of the palaeobiogeography of Gruoidea remains incomplete due to a near lack of known remains of Gruidae from the Palaeogene of the Southern Hemisphere and of reported parts of stem Aramidae and Psophiidae from the Palaeogene. Of these three clades, Gruidae has the most extensive fossil record, with Eocene fossils primarily restricted to the Northern Hemisphere. The new tarsometatarsus cannot confidently be referred to a subgroup within Gruoidea, and as such has different biogeographic implications depending on its affinities. If more closely related to Psophiidae or Aramidae, the new record suggests that these largely South American Gruoid families were more broadly distributed at least as far back as the late Eocene and supports hypotheses of a distribution across Antarctic landmasses. If placed within Gruidae, the new tarsometatarsus could suggest that the gruid radiation may have been multi-directional; one radiation of cranes could have dispersed from North America to Eurasia via the Bering Land Bridge during the early Eocene and then dispersed towards west Eurasia over time, and another radiation could have arrived in Antarctica by the late Eocene via South America. However, more fossils are needed in order to gain a better understanding of the biogeography of this group and core-Gruiformes as a whole within the Southern Hemisphere.

The Penguin mandible described by Davis et al. adds to the record of spear-billed Penguins reported from the Eocene of Antarctica. Although fossil Penguin cranial material is rare from Seymour Island, two beak morphotypes are known: long and narrow, spear-like shapes (proposed to indicate a primarily piscivorous diet) and shorter, broad morphs (proposed to indicate feeding on small Crustaceans). The shape of the mandible is consistent with a spear-billed morphology seen in other Antarctic remains and similar to those of penecontemporaneous species from Peru (Perudyptes devriesi, mid-Eocene; Icadyptes salasi, mid-to late Eocene) as well as Palaeocene Penguins from New Zealand (Muriwaimanu tuatahi; Sequiwaimanu rosieae). The morphology of the mandible is consistent with the spear-billed morphology typical of stem species. Measurements of the symphysis and estimates of mandible length indicate that the individual represented by the new mandible would have been larger than the older New Zealand species Muriwaimanu tuatahi and between size estimates reported for other Eocene Antarctic spear-bills recovered from Seymour Island. The mandible does not reach the maximum mandibular length recorded for the South American Icadyptes salasi, further supporting that a potential intermediate size class of these spear-billed taxa was present on Antarctica.

Penguins were diverse across the globe during the Eocene, with 14+ species described from Seymour Island alone. The material described by Davis et al. add to our understanding of this diversity with new material from a range of size classes: one large spear-billed taxon, one medium-sized taxon represented by a tarsometatarsus, and one small taxon represented by two tarsometatarsi. It has been proposed that Penguins were diverse in the mid- to late Eocene in part because of the increasing productivity of the southern oceans. The range of body sizes and bill morphotypes observed have also been hypothesised to be the result of increased interspecific competition and size-based resource partitioning. The morphological diversity reported here may lend further support to these hypotheses.

New records from Antarctica expand our understanding of Tetrapod biodiversity on the continent during the Eocene and support previously controversial reports of Gruiformes and Xenarthra. A metacarpal is proposed to possibly represent a new early record of Cingulata or Folivora, and lends support to previously reported Xenarthran materials that have been subsequently questioned or lost. A new tarsometatarsus supports the presence of Gruiformes in Antarctica during the Eocene, adding to our understanding of the Avian fossil record of Seymour Island. Newly reported Penguin remains, including a spear-shaped mandible and three tarsometatarsi, add to the diversity of Penguins known from this time. The nature of the Antarctic fossil record is characterised by isolated elements and is dominated by Penguins, making new discoveries vital to furthering our understanding of diversity during a period of climate change and tectonic shifts. While historically fragmentary, new material from Antarctica is needed to elucidate trends in biodiversity and biotic exchange during a key episode of Earth history.

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Penguins from the Palaeocene of Chatham Island, New Zealand.

There now exists a wealth of literature dedicated to the origin and diversification of crown group Birds, or Neornithes, which has progressively elucidated the evolutionary history and framework of major modern Bird clades. Most modern Birds are included within Neoaves, a clade which most recent molecular-based phylogenetic studies estimate emerged during the Late Cretaceous. Subsequently, Neoavian lineages are shown to have rapidly diversified into the abundance of ecological niches that immediately became available following the Cretaceous/Palaeogene mass extinction. Consistent with a lack of molecular support for their extensive diversification before the conclusion of the Cretaceous. Late Cretaceous fossil Neornithines, especially those proposed to have Neoavian affinities, are particularly scarce and fragmentary. Comparatively, there are many definitive, well-preserved Neoavian Birds known from the early Palaeogene onwards. The fossil record evidences that stem group representatives of almost all modern Neoavian orders were present in the early Eocene, corroborated by molecular estimates for divergence of most distinct lineages by 50 million years ago. Fossils Penguins, Sphenisciforms, are relatively abundant in southern high-latitude Cainozoic sites, possibly due to their greater fossilisation potential, considering their shallow marine habitat and robust limb bones. Until recently Sphenisciform fossils from the Palaeocene were scarce, however, the origin of basal stem-Penguin evolution remains poorly resolved. The oldest described Sphenisciforms are from the Waipara Greensand in the Waipara River area of Canterbury, New Zealand. These fossils include the larger and more basal Waimanu manneringi, constrained to between 60.5 and 61.6 million years old, and the slightly younger (58-60 million years old) and smaller Muriwaimanu tuatahi. As aquatic wing-propelled divers, these fossils exhibit many derived characteristics of extant Penguins, yet also display the most plesiomorphic morphology of Sphenisciformes to date; superficially similar to diving Alcids (Alcidae; Auks, Guillemots, Puffins etc.), and the extinct Penguin-like Plotopterids (Plotopteridae). Slightly more derived forms recovered from the same locality as remains attributed to Waimanu and Muriwaimanu include Sequiwaimanu rosieae, described from a partial skeleton, and an unnamed giant form that is represented by distal leg bones, of Middle Palaeocene (approximately 61 million-year-old) age. Most recently, ?Crossvallia waiparensis, was described from leg bones, representing an additional very large form, which was also recovered from the Paleocene Waipara Greensand. The late Palaeocene (59.5-55.5 million-year-old) giant penguin Kumimanu biceae, from the Moeraki Formation on Hampden Beach, Otago, New Zealand, further expands the known diversity of the oldest Sphenisciformes. Outside of New Zealand, the only representative of these earliest Penguins is the giant Crossvallia unienwillia, from the late Paleocene (59.2-56 million-year-old) Cross Valley Formation of Seymour Island, Antarctica. While Crossvallia unienwillia has been recovered in a basal position in a phylogenetic analysis, the fragmentary and incomplete nature of the fossils prohibits comparison with most stem group Penguins.

In a paper published in the journal Palaeontologica Electronica in December 2020, Jacob Blokland of Biological Sciences at Flinders University, Catherine Reid of the School of Earth and Environment at the University of Canterbury, Trevor Worthy, also of Biological Sciences at Flinders University, Alan Tennyson of the Museum of New Zealand Te Papa Tongarewa, Julia Clarke of the Department of Geological Sciences at the University of Texas at Austin, and Paul Scofield of the Canterbury Museum, describe two novel basal Penguins, from numerous fossils recovered from the Takatika Grit of Chatham Island, New Zealand.

Specimens were recovered in situ from the same wave platform and relatively narrow 'Bird Horizon', overlying the ‘Nodular Phosphorite-bone Package’, between 2006 and 2011 by Jeffrey Stilwell and parties, and likely represent numerous individuals. Blockland et al. taxonomically describe and examine the phylogenetic affinity of the medium-sized taxon and, due to its incompleteness, only comment on the second, larger form. Dated to the late Early to Middle Paleocene (New Zealand Teurian stage, 62.5-60 million years old), these specimens are among the oldest known fossils of Sphenisciformes and are significant to the understanding of basal members of this clade, as well as early Neoavian Waterbird evolution. As some of the oldest Avifauna recovered from the continental block associated with New Zealand, examination of these specimens is additionally important in understanding the ecology of early Zealandian seas.

The reported fossil specimens were recovered from main Chatham Island (Rēkohu), part of the Chatham Islands located 860 km off the east coast of New Zealand’s mainland on the largely submerged Chatham Rise. Collectively referred to as the Chatham Islands, Chatham and Pitt Island (Rangiaotea), and several smaller islands, are the only exposed land areas on the largely submerged Chatham Rise. The Chatham Islands, together with New Zealand and New Caledonia, and the interconnecting submerged Chatham Rise, Campbell Plateau, Lord Howe Rise, and Norfolk Ridge form the continental geological block that is referred to as Zealandia.

Locality information. (1) Relative position of New Zealand and the Chatham archipelago; (2) The locality on the Chatham Islands where the fossils were found; (3) Stratigraphic column showing the Takatika Grit, from which the fossils were recovered. Blockland et al. (2020).

On northern Chatham Island, the Takatika Grit outcrops as steep, low-lying coastal cliffs and a 2 km span of wave-cut platforms and isolated blocks eroded from the cliff-line along the length of Maunganui Beach. The Takatika Grit additionally occurs inland along Tutuiri Creek in a series of creek cuttings. At a maximum thickness of 10 m the Takatika Grit unconformably overlies the regional basement Chatham Schist and is conformably succeeded by the Tutuiri Greensand. From an exclusively inland basal breccia, the Takatika Grit outcrops along Maunganui Beach as a fossiliferous, dark green-grey, well-bedded, poorly-sorted, glauconitic lithic wackestone with predominately fine glauconitic grains, quartz and metamorphic lithic inclusions, imbedded within a clay matrix, and often supported by siliceous cement. A minor volcanic constituent is also observed. Three horizons containing macrofossils are known from the Takatika Grit, which show increasing fossil abundance up-section, and are laterally consistent across the areas the Takatika Grit outcrops. In the lower to mid-section of the glauconitic lithic wackestone, an abundance of differentially preserved fossils and authigenic phosphorite nodules of pebble to boulder size exist as a succinct package of several beds, together known as the Nodular Phosphorite-bone Package. Preserving the majority of fossils, phosphorite nodules and skeletal elements in this package are almost conglomeratic in some areas. The lower section of the Nodular Phosphorite-bone Package is characterised by poorly sorted, phosphatised grit among phosphate nodules and macrofossils, while the upper part is characterised by nodular-bedded sandstone and grit. The Takatika Grit culminates in a bioclastic-quartz arenite package, sucsucceeding the Nodular Phosphorite-bone Package, which lacks nodules, but is also fossiliferous.

Associated Bird fossils, including the Penguin material described by Blockland et al, were recovered from a relatively narrow greensand horizon, overlying the Nodular Phosphorite-bone Package, and distinguished from the Nodular Phosphorite-bone Package by a lack of phosphate nodules. Specifically, these Penguin fossils were found within crevasses and depressions created by the upper topography of the Nodular Phosphorite-bone Package in the uppermost P1 layer, and in a narrow concretionary interval in the lowermost P2 layer. Fossils also recovered from this section include an abundant Hexactinellid Sponge fauna, teeth from the Frilled Shark, Chlamydoselachus tatere, and isolated Theropod Dinosaur bones. Due to the presence of semi-articulated Avian remains in these beds overlying the Nodular Phosphorite-bone Package, they are considered unlikely to have been reworked.

The Takatika Grit formed as a product of extensional activity and progressive rifting from eastern Gondwana, as Zealandia separated from West Antarctica about 83-79 million years ago (though possibly at or before 84 million years ago), and continued rifting from eastern Australia until the Eocene. Through related post-rift thermal relaxation and subsidence, Zealandia experienced widespread marine transgression throughout this interval. interval. In association with the oceanic inundation of the region, and the formation of a basin and basement range style landscape, the Takatika Grit formed as an accumulation of thin sandstones, greensands, and marine fossiliferous assemblages, deposited within half-grabens on the Chatham Rise simultaneously with intraplate volcanics. Based on recent palynomorphic research (studies based upon pollen, which is a very efficient dating tool), the Takatika Grit has been found to effectively preserve an initial marine transgression in the early Campanian (82-80 million years ago), followed by an interval of non-deposition in the latest Cretaceous and earliest Palaeocene, and renewed transgression and marine sedimentation in the late early to middle Palaeocene (62.5-60 million years ago,)

The new species described is named Kupoupou stilwelli, where 'Kupoupou' means 'Diving Bird' in Te Re Moriori, the native language of Chatham Island, in recognition of where the fossils were recovered, and 'stilwelli' honours palaeontologist Jeffrey Stilwell, who led and organised the parties to recover the holotype and the only known referred specimens.

The species is described from five specimens; NMNZ S.47312, an associated left tarsometatarsus, left radius, and caudal vertebra, NMNZ S.44729, a left coracoid, NMNZ S.47303, an associated partial skeleton comprising of a distal right carpometacarpus, the left radius, the proximal right radius, the right proximal phalanx of the second digit, the right phalanx of the third digit, an almost complete axis, four cervical vertebrae, a caudal vertebra, a left rib, and a partial worn ilium, NMNZ S.47308,  a right femur, a left humerus, a sternal section of a left coracoid, and a left ulna. NMNZ S.47339, the  omal part of a scapula, a distally eroded left humerus, a right ulna, a right radius, a distal left femur, a distal left tibiotarsus, two cervical vertebrae and five other vertebrae in differing degrees of preservation and exposure at the rock surface, and two partial ribs.

Axis vertebra (1)-(6), cervical vertebrae (7)-(30), pelvis (31)-(34), caudal vertebrae (35)-(38), and rib elements (39), (40) referred to Kupoupou stilwelli. Axis, NMNZ S.47303 in (1), dorsal; (2) ventral, (3) right lateral; (4) left lateral; (5) cranial; (6) and caudal views. Cervical vertebra (i) possibly third in vertebral column, NMNZ S.47303 in (7),dorsal; (8) ventral; (9) right lateral; (10) left lateral; (11) cranial; (12) and caudal views. Cervical vertebra (ii) NMNZ S.47303 in (13) dorsal; (14) ventral; (15) right lateral; (16) left lateral; (17) cranial; (18) and caudal views. Cervical vertebra (iii) NMNZ S.47303 in (19) dorsal; (20) ventral; (21) right lateral; (22) left lateral; (23) cranial; (24) and caudal views. Cervical vertebra (iv) NMNZ S.47303 in (25) dorsal; (26) ventral; (27) right lateral; (28) left lateral; (29) cranial; (30) and caudal views. A partial ischium, from the right side of the pelvis, NMNZ S.47303, in (31) dorsal; (32) ventral; (33) medial; (34) and right lateral views. Caudal vertebra, NMNZ S.47303, in (35) cranial and (36) and caudal views. Caudal vertebra, NMNZ S.47312, interpreted to have been located further caudally in the vertebral column compared to the caudal vertebra in NMNZ S.47303, in (37) cranial and (38) caudal views. A left rib, NMNZ S.47303 in (39) lateral and (40) caudal views. Abbreviations: ac, ansa costotransversaria; fac, facies articularis caudalis; facr, facies articularis cranialis; fov, fovea at base of processus spinosus; ft, foramen transversarium; fv, foramen vertebrale; iav, incipient projections of the arcus vertebrae; li, lacuna interzygapophysialis; pc, processus costalis; pca, processus caroticus; ps, processus spinosus; pt, processus transversus; pvc, processus ventralis corporis; tc, tuberculum costae; td, torus dorsalis; zca, zygapophysis caudalis; zcr, zygapophysis cranialis. Scale bars equal to 20 mm. Blockland et al. (2019).

Kupoupou stilwelli is referred to Sphenisciformes because it shares the synapomorphy (characteristic present in an ancestral species and shared by its evolutionary descendants) of having flattened long bones of the forewing/flipper. It is characterised by the combination of the following osteological apomorphies (derived traits distinct to a certain species or group): a bifurcated processus transversus of the axis with a dorsally protruding torus dorsalis; the processus acrocoracoideus has a rounded and protruding omal crista acrocoracoidea of the coracoid, the insertion for ligamenti acrocoraco-procoracoidale on the facies articularis clavicularis is weakly hooked with a rounded facies apicalis, a weakly defined tuberculum for the insertion of plica synovialis coracoidea, joined by a low ridge to the impressio ligamenti acrocoraco-acromiale, the latter of which is separated by the impressio ligamenti acrocoraco-procoracoidale by a groove; a welldefined labrum internum of the coracoid that is compressed in the sternal-omal direction; the distal margin of the crista bicipitalis on the humerus is nearly perpendicular to the long axis of the shaft; the distal caudal border of the olecranon of the ulna is distinctly angled, with a marked bony caudal protuberance; a dorsocaudally situated sub-triangular insertion scar for the musculus supinator on the proximal radius; a distinct caudally projecting tuberculum aponeurosis ventralis from the ventral caudal margin of the distal radius and an associated prominent ulnar depression; a proximally directed process on the phalanx III-1; a marked laterally protruding epicondylus lateralis on the femur; the sulcus for the tendon to the muscle flexor hallucis longus is bounded by medial and lateral hypotarsal crests of distinct subequal plantar projection on the tarsometatarsus; a strongly plantar projecting flange on the lateral rim of trochlea metatarsi IV.

Coracoids and scapula referred to Kupoupou stilwelli. (1)-(2), (6)-(7), (8)-(9) compared to other Palaeocene taxa (3)-(5). Two left coracoids assigned to Kupoupou stilwelli, NMNZ S.44729, in (1) ventral and (2) dorsal views; and NMNZ S.47308 in (6) ventral and (7) dorsal views. Dorsal perspectives of left coracoids of Muriwaimanu tuatahi, CM zfa 34 (3) and Sequiwaimanu rosieae, CM 2016.6.1 (4); right omal part coracoid of NMNZ S.47302 (larger Chatham Island form) (5). Left cranial part scapula referred to Kupoupou stilwelli NMNZ S.47339, (8) in medial and (9) lateral views. Abbreviations: acr, acromion; al, angulus lateralis; am, angulus medialis; ce, crista epimarginalis; coa, collum acrocoracoidei; cos, collum scapulae; cs, cotyla scapularis; csb, crista subcapitalis; fa, facies apicalis; fac, facies articularis clavicularis; fas, facies articularis sternalis; fg, facies glenoidalis (facies articularis humeralis); ic, impressio coracobrachialis; ilaa, insertion for ligamenti acrocoraco-acromiale; ilah, impressio ligamenti acrocoracohumeralis; ilap, insertion for ligamenti acrocoraco-procoracoidale; is, impressio sternocoracoidea; ipsc, tuberculum for the insertion of plica synovialis coracoidea; lacs, insertion for ligamenti acrocoraco-claviculare superficiale; lg, labrum glenoidale (facies articularis humeralis); li, labrum internum; not, notch adjacent to the facies articularis clavicularis; oca, protruding omal extremity of crista acrocoracoidea; pac, processus acrocoracoideus; pcc, processus procoracoideus; pl, processus lateralis; sms, sulcus musculi supracoracoideus; tc, tuberculum coracoideum. Scale bars equal to 20 mm. Note that 5 is a tomographic rendering image. Blockland et al. (2019).

Kupoupou stilwelli is a medium sized Sphenisciform (relative to all known fossil and extant Penguins), likely slightly smaller than a modern adult King Penguin, Aptenodytes patagonicus. The referred specimens are assigned to Kupoupou stilwelli based on similarity of overlapping skeletal elements, size, and their origin in the same horizon of the same bed in the Takatika Grit. The dimensions of the forewing elements reveal that Kupoupou stilwelli was likely smaller than both Muriwaimanu tuatahi and Sequiwaimanu rosieae from the Palaeocene Waipara Greensand of Canterbury, New Zealand. Its humeri and coracoids show that it was smaller than the larger Chatham Island form described by Blockland et al.

The humeri of Kupoupou stilwelli. Left humerus of NMNZ S.47308 in (1) dorsal; (2) caudal; (3) ventral; (4) cranial; (5) distal; and (6) proximal views. Left humerus of NMNZ S.47339 in (7) dorsal; (8) caudal; (9) ventral; (10) cranial; and (11) proximal views. Abbreviations: cb, crista bicipitalis (bicipital crest); cd, condylus dorsalis (radial condyle); cdf, crus dorsale fossa; ch, caput humeri (humerus head); cv, condylus ventralis (ulnar condyle); dc, crista deltopectoralis (deltopectoral crest) and insertion of the musculus deltoideus major; dtr, dorsal trochlear ridge; el, insertion for entepicondylar ligament; fpd, fossa pneumotricipitalis dorsalis (secondary tricipital fossa); fpv, fossa pneumotricipitalis ventralis (tricipital fossa); ic, incisura capitis (capital incisura); iic, incisura intercondylaris; imp, impressio musculus pectoralis; itr, intermediate trochlear ridge; mb, fossa musculus brachialis; mcc, attachment scar of musculus coracobrachialis caudalis; mcd, margo caudalis; ms, trochlea for tendon musculi scapulotricipitalis; mcl, margo cranialis; mcr, insertion for musculus coracobrachialis cranialis (impressio coracobrachialis); mh, trochlea for tendon musculus humerotricipitalis; msc, crista musculi supracoracoidei as an accessory insertion site for the tendon of the musculus supracoracoideus, extending distally from the tuberculum dorsale; nf, nutrient foramen; psd, processus supracondylaris dorsalis (dorsal supracondylar tubercle); td, tuberculum dorsale (dorsal tubercle) the attachment site of the musculus deltoideus minor and the principal part of the musculus supracoracoideus; ts, sulcus transversus (transverse sulcus); tv, tuberculum ventrale (ventral tubercle/internal tuberosity); vtr, ventral trochlear ridge. Scale bars equal to 20 mm. Blockland et al. (2019).

Besides Kupoupou stilwelli, Blockland et al. (2019) recognise another markedly larger form of Penguin from the Takatika Grit, of the same late early to middle Palseocene age. This form is represented by specimens NMNZ S.47302 and NMNZ S.47304, recovered from the same wave platform and horizon of the Takatika Grit as the specimens of Kupoupou stilwelli. NMNZ S.47302 is one of four blocks preserving parts of one skeleton, the other three of which (whereabouts unknown) were unavailable to study and was collected February 2008. NMNZ S.47302 is an associated partial skeleton comprising of a caudal portion of the left mandible, a partial furcula, a fourth cervical vertebra, an omal part of the right coracoid, a portion of the sternum, and a vertebra fragment. The second specimen, NMNZ S.47304, is a single humerus. While markedly larger than Kupoupou stilwelli, the lack of overlap in skeletal elements between the two specimens, means their association as one taxon is only tentative. Furthermore, their relative incompleteness precludes a formal taxonomic description. Nevertheless, some comparative observations are made by Blockland et al. assuming they are of one taxon. Based on the humerus length, this larger Chatham Island form was between the size of an adult Emperor Penguin, Aptenodytes forsteri, and an adult King Penguin, Aptenodytes patagonicus. 

Views of the caudal end left mandible of NMNZ S.47302. (1) Dorsal; (2) ventral; (3) rostral; (4) caudal; (5) left lateral; (6) left medial aspects. Reconstruction assuming proportions similar to Paleocene Penguins in (7) in left lateral view. Abbreviations: cc, cotyla caudalis; cl, cotyla lateralis; cm, cotyla medialis; facm, fossa aditus canalis mandibulae; fmc, fenestra mandibulae caudalis; mp, insertion of musculus pterygoideus; pc, processus coronoideus; plm, processes lateralis mandibulae; pmm, processus mandibulae medialis; pr, processus retroarticularis; si, sulcus intercotylaris. Scale bars equal to 20 mm.

These bones minimally represent one larger taxon than Kupoupou stilwelli. It differs from Kupoupou stilwelli in coracoid morphology including: a proportionally smaller diameter of cotyla scapularis; a collum acrocoracoidei (acrocoracoid neck) that is proportionally mediolaterally thinner and slender sternal to the processus acrocoracoideus; a more gracile shape of the corpus coracoideum sternal to the processus procoracoideus; a pronounced depression for the impressio coracobrachialis and a fossa sternal to it, and the lack of a rounded and omally directed apex of the crista acrocoracoidea; a better defined labrum glenoidale. The humerus of the larger Chatham Island form (NMNZ S.47304) differs from Kupoupou stilwelli in its more robust form; the proximal apex of the caput humeri located nearer to the midline of the humerus shaft; and a crista deltopectoralis that is proximally incurvate and extends more proximally. The extremitas sternalis claviculae is narrower and more curved in dorsal and ventral views.

Cervical vertebra IV (1)-(6), coracoid (7)-(10), and furcula (11)-(16) of NMNZ S.47302, as part of the larger Chatham Island form. Cervical vertebra IV in (1) dorsal; (2) ventral; (3) right lateral; (4) left lateral; (5) cranial; (6) caudal. Omal right coracoid in (7) dorsal; (8) ventral; (9) medial; and (10) lateral views. Partial furcula in (11) caudal; (12) cranial; (13) right lateral; (14) left lateral; (15) dorsal; and (16) ventral views. Abbreviations: cs, cotyla scapularis; esc, extremitas sternalis claviculae; fac, facies articularis clavicularis; faca, facies articularis caudalis; facr, facies articularis cranialis; fg, facies glenoidalis (facies articularis humeralis); fo, fossa; ft, foramen transversarium; ic, impressio coracobrachialis; ilaa, impressio ligamenti acrocoraco-acromiale; ilah, impressio ligamenti acrocoracohumeralis; ipsc, tuberculum for insertion of plica synovialis coracoidea; lg, labrum glenoidale (facies articularis humeralis); pc, processus costalis; pca, processus caroticus; pcc, processus procoracoideus; ps, processus spinosus; pvc, processus ventralis corporis; td, torus dorsalis; zca, zygapophysis caudalis; zcr, zygapophysis cranialis. Scale bars equal to 20 mm. Blockland et al. (2019).

Kupoupou stilwelli and the specimens belonging to a larger Chatham Island form are among the oldest described representatives of the Penguin clade, from deposits that are dated to late Early to Middle Paleocene (62.5-60 million-years-old). Fittingly, they are recovered in phylogenetic analyses, alongside similarly-aged New Zealand Palaeocene counterparts Waimanu manneringi, Muriwaimanu tuatahi, Sequiwaimanu rosieae, Kumimanu biceae, the unnamed Waipara Greensand Giant Penguin, ?Crossvallia waiparenis, Crossvallia unienwillia from Seymour Island, and Kaiika maxwelli reportedly from the early Eocene. Importantly, however, support values for placement of fossil taxa, including Palaeocene forms, are notably low. This is perhaps expected given many fossil taxa have much missing data, including a complete lack of molecular data. Kaiika and the unnamed Waipara Greensand Giant Penguin are represented by a nearly complete humerus and a partial tarsometatarsus with pedal phalanges, respectively. Effectively, relatively incomplete fossils can impede on topologic resolution, including potential obfuscation of relationships among relatively more complete taxa, leading to reduced branch support across the tree. 

Parsimony majority-rule (50%) consensus tree of 16 300 most parsimonious trees (length = 5,234). Percentage of most parsimonious trees recovering each node is indicated at each internode in the consensus tree, and bootstrap support values (over 40% only) are numbered below them italicised in red. Legend and branch colouration correspond to percentage of most parsimonious trees that recovered each node. Darkened area indicates the topological region occupied by Paleocene taxa. Ages associated with taxa are shown in thickened black lines. Nodes illustrated are not calibrated in association with age. Blockland et al. (2019).

Nonetheless, although the consensus trees depicted by Blockland et al. may show particular topologies, specific placement of fossil taxa in these trees with low support values should be treated tentatively, while those recovered in a supermajority of trees may be hypotheses more confidently interpreted as approaching reality. This is especially relevant for the majority-rule consensus tree under parsimony criterion, where even though a topological relationship may be recovered in more than 50% of most parsimonious trees, in the case of Palaeocene taxa, no justification exists for the preference of one topology over an alternate equally parsimonious topology.

Phylogenetic tree based on Bayesian inference (majority-rule consensus, undated). Colour of branches indicate the gradient of posterior probability values, the numbers of which are specified next to their respective branches. Darkened area at base of Sphenisciformes indicates the topological region occupied by Paleocene taxa. Scale bar corresponds to the given degree of change across branch lengths. Blockland et al. (2019).

The implication is that using both parsimony and Bayesian inference methods a clade including both Waimanu and Muriwaimanu branching from the most basal Sphenisciformes node, and sister to all other ingroup taxa, may be treated with a degree of confidence. However, the specific arrangement of nodes and branches pertaining to other Palaeocene forms (and the majority of other fossil Penguins) should be viewed more tentatively. This topological uncertainty is also illustrated by the posterior distribution favouring the recovery of most Palaeocene taxa within a monophyletic clade sister to a clade leading to the crown group in the Bayesian analysis (albeit with very low support), compared to their various taxonomic groupings or stepwise relationships commonly found under parsimony. Their phylogenetic placement does, however, support the interpretation that Kupoupou stilwelli, the larger Chatham Island form and other Palaeocene taxa possess more derived morphologies compared to Waimanu manneringi and Muriwaimanu tuatahi. The fossil humerus of Kaiika maxwelli from South Canterbury, New Zealand, commonly reported as early Eocene in age was found nested among Palaeocene taxa in all analyses, supporting recognition that it may have been derived from older sediments. Except for Kaiika maxwelli, most parsimonious trees consistently recovered Delphinornis larseni as the most basal of Eocene taxa, with low bootstrap support. While Delphinornis is still recovered in a relatively basal position among Eocene Penguins in the Bayesian majority-rule consensus tree, the poor-moderately supported position of middle Eocene Perudyptes devriesi one node crownwards of the Palaeocene Waipara Greensand Giant Penguin, implies a contrasting evolutionary scenario to that depicted in most parsimonious trees where it is one node more basal to a node that supports a clade including Delphinornis, Marambiornis exilis, and Mesetaornis polaris, and another clade that includes all other geologically younger Sphenisciforms. The consistency of close relationships between Palaeocene taxa, and their absence from Eocene clades across both Bayesian and parsimony trees, however, does support their phylogenetic restriction to the base of Sphenisciformes.

Long bones of the forewing of Kupoupou stilwelli. Left ulna of NMNZ S.47308 in (1) dorsal; and (2) ventral views. Left ulna of NMNZ S.47339 in (3) dorsal; and (4) ventral views. Dorsal view of radii, (5) left NMNZ S.47312; (6) left NMNZ S.47303; (7) right NMNZ S.47303; (8) right NMNZ S.47339. Right radius of NMNZ S.47303 (without suggested eroded extent) in caudal (9) and ventral (10) views. Caudodistal view of left radius of NMNZ S.47303, (11) and Muriwaimanu tuatahi, right radius (mirrored) CM 2009.99.1 (12). Abbreviations: bl, bony lobe; cd, condylus dorsalis; ch, cotyla humeralis; cv, cotyla ventralis; drp, incisura radialis (depression radialis proximalis); fr, fracture; fur, furrow; jut, edge-like jut on dorsal ulna face; mb, scar for insertion of musculus brachialis; mela, groove for musculus extensor longus alulae; memr, groove for the musculus extensor metacarpi radialis; ms, insertion scar for musculus supinator; nf, nutrient foramen; ol, olecranon; pcd, processus cotylaris dorsalis; tav, tuberculum aponeurosis ventralis; tc, tuberculum carpale; ud, depressio ligamentosa (ulnar depression). Dotted lines represent suggested erosion to respective elements. Scale bars equal to 20 mm. 11 and 12 are not to scale. Blockland et al. (2019).

The close association of Kupoupou stilwelli and the larger Chatham Island Penguin with other Palaeocene forms in phylogenetic simulations reflects the numerous anatomical similarities drawn between these similarly aged species. Kupoupou stilwelli is further phylogenetically distinguished from other Palaeocene taxa by morphological characters of the humerus, ulna, proximal manual phalanx of digit three, and the tarsometatarsus, while the larger Chatham form is distinguished with regard to mandibular (NMNZ S.47302) and humeral (NMNZ S.47304) characters. With the material available, clear plesiomorphic features are observed in these early Penguins, which do not persist in geologically younger taxa, and which Blockland et al. consider ancestral in Sphenisciformes. These include: the lack of a coracoidal fenestra on the medial margin of the coracoid, a relatively slender and less dorsoventrally flattened humerus that is longer than the coracoid, dorsoventral flattening of forelimb elements, but not as broad and as heavily flattened as in more crownward spheniscids, and the presence of a processus cotylaris dorsalis on the proximal ulna.

Images of the distal right-wing elements in NMNZ S.47303 (1)-(2), (4)-(17). Distal carpometacarpus in (1) ventral, (2) dorsal, and (4) distal aspects. Left carpometacarpi of Muriwaimanu tuatahi, (3) CM zfa 34, mirrored, and Sequiwaimanu rosieae, (5) CM 2016.6.1, mirrored, are presented for comparison, in ventral aspect. The right proximal manus phalanx of the second digit is shown in (6) dorsal, (7) ventral, (8) caudal, (9) cranial, (10) distal, and (11) proximal views. The right manus phalanx of the third digit is presented in (12) dorsal, (13) ventral, (14) caudal, (15) cranial, (16) distal, and (17) proximal aspects. A left-wing reconstruction of Kupoupou stilwelli is shown in (18), using mirrored carpometacarpus and phalanges. Scale bars are equal to 20 mm. Abbreviations: ee, eroded end; fad, facies articularis digitalis major; fam, facies articularis metacarpalis; fma, facies articularis digitalis major; fmi, facies articularis digitalis minor; mII, os metacarpale majus (metacarpal II); mIII, os metacarpale minus (metacarpal III); pc, pila cranialis phalangis; pp, proximally directed process; si, sulcus interosseous; sim, spatium intermetacarpale; smd, symphysis metacarpalis distalis. Blockland et al. (2019).

Modern Penguins are well known for the assortment of specialised adaptations they possess in association with a subaquatic lifestyle. However, this morphological transition towards the modern form has been a gradual one, where the aforementioned differences observed in basal counterparts reflect an earlier stage in this evolution. Given the various structures preserved it is possible to make broad functional inferences with regards to the Palaeocene Chatham Island Penguins, and their adaptive significance.

Hindlimb elements. Right femur of Kupoupou stilwelli NMNZ S.47308 in (1) cranial and (2) caudal views. Left distal femur of NMNZ S.47339 in (3) cranial and (4) caudal aspects. Right femur of Sequiwaimanu rosieae in caudal view (CM 2016.6.1), (5) for comparison. Fragmentary right distal tibiotarsus of NMNZ S.47339 in (6) caudal and (7) cranial aspects, compared to cranial view of right distal tibiotarsus of Waimanu manneringi (CM zfa 34), (8). Abbreviations: ce, distal opening of canalis extensorius; cf, caput femoris; cl, condylus lateralis; cm, condylus medialis; cof, collum femoris; csm, crista supracondylaris medialis; ct, crista trochanteris; ctf, crista tibiofibularis; stf, semicondylus tibiofibularis; sf, semicondylus fibularis; epl, epicondylus lateralis; fac, facies articularis antitrochanterica; faf, facies articularis fibularis; fat, facies articularis tibialis; flc, fovea ligamenti capitis; fpo, fossa poplitea; ii, incisura intercondylaris; lcr, linea intermuscularis cranialis; lic, linea intermuscularis caudalis; sic, sulcus intercondylaris; sf, semicondylus fibularis; slf, sulcus fibularis; sp; sulcus patellaris; stf, semicondylus tibiofibularis; tct, trochlea cartilaginis tibialis; tlg, tuberculum musculus gastrocnemialis lateralis. Scale bars equal to 20 mm. Blockland et al. (2019).

Both Kupoupou stilwelli and the larger Chatham Island form possess a caudally directed and blade-like processus spinosus on cervical vertebrae, that is directly linked to mechanical ability of the cervical system in bringing the head back to the body. Mechanical folding of the cervical series in the neck is observed in many Birds, and in extant Penguins is especially important in the formation of a more hydrodynamic shape for pelagic aqueous flight, as well as maintaining erect posture on land. This shared characteristic may infer that neck length reduction, associated reduction of drag, and acquisition of hydrodynamic form may have been present in these Chatham Island Paleocene Penguins.

Tarsometatarsus of Kupoupou stilwelli compared to other fossil taxa. Tarsometatarsi in dorsal aspect, (1) Paleocene Waimanu manneringi, right (mirrored), CM zfa 35; (2) Palaeocene Kupoupou stilwelli, left, NMNZ S.47312, (3) Eocene Delphinornis larseni, left, IB/P/B-0062. Left tarsometatarsus of NMNZ S.47312 in (4) distal, (5) proximal, (6) lateral, (7) plantar, and (8) medial views. Abbreviations: ait, area intercotylaris; cl, cotyla lateralis; cl(fdl), crista lateralis flexoris digitorum longus; cl(fhl), crista lateralis flexoris hallucis longus; clh, crista lateralis hypotarsi; cm, cotyla medialis; cm(fdl), crista medialis flexoris digitorum longus; eit, eminentia intercotylaris; fbl, sulcus for muscularis fibularis longus; fcdq, fovea ligamentae collateralis digitorum quarti; fdl, sulcus for tendon of musculus flexor digitorum longus; fhl, sulcus for tendon of musculus flexor hallucis longus; fid, fossa intercotylaris dorsalis; fidm, fossa intercotylaris dorsalis medialis; fphl, fossa parahypotarsalis lateralis; fphm, fossa parahypotarsalis medialis; fsp, fossa supratrochlearis plantaris; fvd, foramen vasculare distale; fvpl, foramen vasculare proximale laterale; fvpm, foramen vasculare proximale laterale; iim, incisura intertrochlearis medialis; ilcl, impressio ligamentosae collaterale laterale intertarsi; ilcm, impressio ligamentosae collaterale mediale intertarsi; ilcma, impressio ligamentosae collaterale mediale intertarsi accessorium; irel, impressiones retinaculi extensorii lateralis; irem, impressiones retinaculi extensorii medialis; madII, insertion site of musculus adductor digiti II; madIV, insertion site of musculus adductor digiti IV; sf, sulcus flexorius; sldl, sulcus longitudinalis dorsalis lateralis; sldm, sulcus longitudinalis dorsalis medialis; slg, sulcus ligamentosus; tII, trochlea metatarsi II; tIII, trochlea metatarsi III; tIV, trochlea metatarsi IV; tfb, tuberculum muscularis fibularis brevis; tmtc, tuberositas muscularis tibialis cranialis. Scale bars equal to 20 mm. Blockland et al. (2019).

Modern Penguins are known for having a very specialised flight apparatus. The coracoid is a key element in underwater Penguin locomotion, where the acrocoracoid process, furcula, and scapula create the canalis triosseum, which acts as a pulley for the musculus supracoracoideus to raise the wing in the upstroke. The coracoids of Kupoupou stilwelli and the larger Chatham Island form display a medioventrally directed processus acrocoracoideus that is more elongate compared to aerially flighted Birds, but not as long as some phylogenetically more derived Penguins (e.g., the Late Eocene Colossus Penguin of Seymour Island, Antarctica, Palaeeudyptes klekowskii, or the extant Cape Penguin of Southern Africa, Spheniscus demersus).The musculus supracoracoideus is relatively enlarged in extant Penguins, allowing them to raise their wing and produce greater forward thrust against the resistance of water, which has 800 times the density of air. Similarly, greater acrocoracoid process elongation in Kupoupou stilwelli and the larger Chatham Island form compared to volant counterparts may relate to an increased space for this muscle and confer aquatic locomotory advantages.

Partial sternum NMNZ S.47302, of the larger Chatham Island form. In dorsal (1); ventral (2); right lateral (3); left lateral (4); cranial (5); and caudal (6) views. Abbreviations: cs, carina sterni; fp, foramen pneumaticum; se, spina externa. Scale bar equals 20 mm. Blockland et al. (2019).

A shorter coracoid relative to the length of the humerus is a plesiomorphic character Kupoupou stilwelli shares with other Palaeocene Penguins Muriwaimanu tuatahi and Sequiwaimanu rosieae and aerially flighted birds. By contrast, the opposite is true for the hyper-elongate coracoid of modern Penguins, which acts to displace the canalis triosseum relative to the sternum, increasing space for the pectoralis muscles, and leverage for the supracoracoideus muscle for the upbeat of the wing. The length of the coracoids associated with Kupoupou stilwelli, Muriwaimanu tuatahi and Sequiwaimanu rosieae implies an intermediate adaptation towards diving proficiency, compared to the more specialised hyper-elongated coracoids of extant Penguins. While the full length of the coracoid associated with the larger Chatham Island form (NMNZ S.47302) is not preserved, approximate length extrapolation and comparison to the humerus of the larger form reveals that the coracoid may have been equal in size or longer than the humerus of NMNZ S.47304. Should they represent the same taxon, this would be the earliest occurrence of more elongate coracoid proportions within Sphenisciformes and may have indicated increased diving efficiency.

Humeri of Sphenisciforms from the Chatham Island, compared to those of various early Penguins. Right humerus NMNZ S.47304 of unnamed large form in (1) dorsal, and (2) ventral views. Humeri in ventral aspect, left Sequiwaimanu rosieae, CM 2016.6.1 (3); left Kupoupou stilwelli, NMNZ S.47308 (4); left Kupoupou stilwelli, NMNZ S.47339 (5); right Muriwaimanu tuatahi, CM zfa 34 (6); right Muriwaimanu tuatahi, 2008.145.4 (7); right Muriwaimanu tuatahi, 2008.145.3 (8); left Muriwaimanu tuatahi, 2008.145.4 (9); right Muriwaimanu tuatahi, CM 2010.108.3 (10); left Kaiika maxwelli, OU 22402 (11). Abbreviations: cb, crista bicipitalis (bicipital crest); cd, condylus dorsalis (radial condyle); ch, caput humeri (humerus head); cv, condylus ventralis (ulnar condyle); dc, crista deltopectoralis (deltopectoral crest) and attachment site for musculus propatagialis (dorsally) and musculus pectoralis; fpd, fossa pneumotricipitalis dorsalis (secondary tricipital fossa); fpv, fossa pneumotricipitalis ventralis (tricipital fossa); ic, incisura capitis (capital incisura); imp, impressio musculus pectoralis, particularly for insertion of musculus pectoralis thoracica; itr, intermediate trochlear ridge; mcc, attachment scar of musculus coracobrachialis caudalis; ms, trochlea for tendon musculus scapulotricipitalis; mcr, insertion for musculus coracobrachialis cranialis; mh, trochlea for tendon musculus humerotricipitalis; msc, crista musculi supracoracoidei as an accessory insertion site for the tendon of the musculus supracoracoideus, extending distally from the tuberculum dorsale; psd, processus supracondylaris dorsalis (dorsal supracondylar tubercle); td, tuberculum dorsale (dorsal tubercle) and attachment site of musculus deltoideus minor and the principal part of the musculus supracoracoideus; ts, sulcus transversus (transverse sulcus); tv, tuberculum ventrale (ventral tubercle/internal tuberosity); vtr, ventral trochlear ridge. Scale bar equal to 20 mm. Blockland et al. (2019).

The pronounced dorsoventral flattening and shortening of the forewing is another notable example of the morphological transition to aquatic life in Penguins, related to more efficient aquaflight with increasing body mass. Indeed, the reduced marrow cavity observed in radii of Kupoupou stilwelli, provides evidence of a more robust and dense bone structure than volant Birds, approaching that of modern forms. This adaptation acts to counteract buoyancy and allows greater ability for diving and underwater foraging. Basal Penguins had more elongate and less flattened humeri than extant forms and would have been less resistant to torsion imposed by the stresses of swimming in the dense water medium. In this way, the Paleocene Chatham Island Penguins bear closer resemblance to the other earliest Penguins, however, the humerus of the larger Chatham Island form (NMNZ S.47302) is markedly more robust than Kupoupou stilwelli, which may be reflected in aquatic flight potential. Compared to Muriwaimanu tuatahi, it is observed that Kupoupou stilwelli had proportionally shorter, wider, and more flattened ulnae and radii converging on the morphologies of Eocene Penguins such as species of Anthropornis. These structural modifications likely increased bone mass and strength, potentially enhanced flight stroke rate, and submarine propulsion ability during the up and downstroke, and lowered energetic costs, yet are still far removed from the broader, more specialised, forewing elements of modern Penguins.

NMNZ S.47302, (1) the extent of the specimen that has been physically prepared, (2) the three-dimensionally rendered elements within the block. Fossils numbered in (2) are as follows: (1) coracoid; (2) unidentified, possibly a radiale; (3) sternum; (4) furcula; (5) mandible; (6) cervical vertebra; (7) unidentified; (8) unidentified; (9) cervical vertebra IV; (10) unidentified. Scale bar is equal to 50 mm. Blockland et al. (2019).

In addition, the humeral condylus ventralis in Kupoupou stilwelli and the larger Chatham Island form are rounded with a shelf-like articulatory surface adjacent to it, reminiscent of other basal Sphenisciforms. This joint morphology would have increased relative rigidity of the wing in the downstroke, but would have been less effective at counteracting the ventrodistal flexion against water during the upstroke. The humerus-ulna joint of modern Penguins is a comparatively flat surface, contributing to a relatively narrow range of wing motion, and allowing it to act as an efficient hydrofoil. Effectively, Paleocene Penguins such as Kupoupou stilwelli and the larger Chatham Island form may have had a greater wing flexibility and movement range at the elbow than in modern counterparts.

NMNZ S.47303, (1) the extent of the specimen that has been physically prepared, (2) the three-dimensionally rendered elements within the block. Fossils numbered in (2) are as follows: (1) ischium; (2) manus phalanx III-1; (3) radius; (4) radius; (5) cervical vertebra; (6) cervical vertebra; (7) carpometacarpus; (8) cervical vertebra; (9) possibly cervical vertebra III; (10) manus phalanx II-1; (11) rib; (12) caudal vertebra; (13) axis. Scale bar is equal to 50 mm. Blockland et al. (2019).

In a rare circumstance amongst fossil Penguins, a manus phalanx II-1 and manus phalanx III-1 were recovered with the Kupoupou stilwelli material. The manus phalanx III-1 has a proximally directed tubercle similar to that in extant Penguins, in contrast to its absence from all known fossil taxa. While incomplete, preserved distal tapering of manus phalanx III-1 indicates that it may not have exceeded the length of manus phalanx II-1, which would be indicative of a more tapered wing tip like Icadyptes salasi (a Giant Penguin from the Late Eocene tropics of South America) and volant Birds, than in modern Penguins. Such morphology is correlated with an increased wing loading and a higher aspect ratio compared to extant Penguins, reflective of primitive proportions, though contribution to aquatic flight efficiency was likely almost negligible.

NMNZ S.47303 in Materialise Mimics, cross-sectional view of right radiusin top two images (1) and left radius in two lower images (2). Cross-sectional perspective reveals a reduced marrow cavity compared to modern aerially flighted Birds, but not as dense as extant Penguins. Blockland et al. (2019).

Another distinctive morphological change in Penguins through their evolution is the progressive shortening and widening of the tarsometatarsus. Among Penguins, Waimanu manneringi and Muriwaimanu tuatahi have the most primitive and elongate proportions in this element. The complete tarsometatarsus of Kupoupou stilwelli however, as well as that of the less complete and less well-preserved unnamed Giant Penguin and ?Crossvallia waiparensis from the Waipara Greensand present the earliest occurrence of a relatively shortened, stout, and robust morphology. In particular, the specific morphology of the tarsometatarsus in Kupoupou stilwelli seems to approach that of Eocene Penguins of Seymour Island, especially those of similar inferred body size such as species of Delphinornis, Mesetaornis, and Marambiornis, in a transition to the characteristic modern Penguin hind-limb.

Undescribed vertebrae and ribs referred to Kupoupou stilwelli, (1)-(7) vertebrae, NMNZ S.47339; and (9) and (10) ribs, NMNZ S.47339. (8) an incomplete vertebra, is part of NMNZ S.47302, associated with the larger Chatham Island form. Scale bar is equal to 10 mm. Blockland et al. (2019).

While ancient Penguins (including some of the largest, e.g., species of Anthropornis) are recognised to have had relatively more elongate tarsometatarsi compared to recent forms. A more robust structure is also mechanically required to support a greater mass and would imply that shortened tarsometatarsi of Penguins may be an adaption related to supporting their increased weight relative to volant ancestors. Contrarily, the elongate tarsometatarsi that some of the heaviest birds (e.g., Ratites) bear suggest that shortened tarsometatarsi in Penguins may have an additional functional significance. Modern Penguins use their cornified feet and tarsometatarsi for much more than walking or resting, including propelling themselves in prone positions on land or ice, gripping and holding onto icy surfaces, and are also very important in underwater flight, assisting with steering the bird as it swims. It has also been observed that the presence, shape, and position of feet in extant Penguins during underwater flight reduced drag and completed a more hydrodynamic shape, and may potentially facilitate heat retention when feet are placed in line with the body. Effectively, the evolution of the distinctive shortened tarsometatarsi in some of the earliest Penguins may have evolved as an adaptation that augmented swimming capabilities. While this likely led to greater reproductive success, a consequence of such shortened-hind limbs may have meant Kupoupou stilwelli, ?Crossvallia waiparensis and the Waipara Greensand Giant were prone to high metabolic costs while walking, but also large lateral displacement of their feet, characteristic of an energetically conservative waddling gait observed in modern forms, compared to the relatively less phylogenetically derived Waimanu manneringi and Muriwaimanu tuatahi.

Specifics of hypotarsal morphology. (1) Diomedea antipodensis left (mirrored); (2) Aphrodroma brevirostris, left (mirrored); (3) Hydrobates castro, left (mirrored); (4) Waimanu manneringi, right, CM zfa35; (5) Muriwaimanu tuatahi, right, 2009.99.1 (.STL file, tomographic rendering); (6) Marambiornis exilis, right, IB/P/B-0490; (7) Delphinornis gracilis, right, IB/P/B-0279a; (8) Anthropornis nordenskjoeldi, MLP 95-I-10-142 (mirrored); (9) Palaeeudyptes klekowskii, IB/P/B-0485 (mirrored); (10) Palaeeudyptes antarcticus, right, BM A.1048; (11) Palaeospheniscus bergi, NHMUK A694 (mirrored); (12) Spheniscus magellanicus, NHMUK 2001.45.1 (mirrored); (13) Eudyptes chrysocome, NHMUK 1898.7.1.15 (mirrored); (14) Aptenodytes forsteri, NHMUK 1905.12.30.419 (mirrored); (15) Pygoscelis adeliae, unassigned from IB/P/B (mirrored); abbreviations: cl(fdl), crista lateralis flexoris digitorum longus; cl(fhl), crista lateralis flexoris hallucis longus; cm(fdl), crista medialis flexoris digitorum longus; fbl, sulcus for musculus fibularis longus; fdl, sulcus/canal for tendon of musculus flexor digitorum longus; fhl, sulcus/canal for tendon of musculus flexor hallucis longus; tfb, tuberculum musculus fibularis brevis. Dotted line represents estimated extent of bone. Not to scale. Blockland et al. (2019).

Further distinguishing the tarsometatarsus of Waimanu manneringi and Muriwaimanu tuatahi from Kupoupou stilwelli is the comparatively reduced plantar deflection of trochlea metatarsi II observed in Kupoupou stilwelli and the giant Waipara Greensand Penguin. Definitive and reliable comparisons relating to this feature in ?Crossvallia waiparensis are limited, however, due to damage to the plantar surface of trochlea metatarsi II. The plantar deflection and medial ridge of trochlea metatarsal II is typical of foot-propelled diving Birds, facilitating the movement of the inner toe behind the other toes in the recovery stroke while swimming at the surface of and within the water. This morphology is exhibited to a small degree in Waimanu manneringi and Muriwaimanu tuatahi relative to Birds that use foot-propelled diving as a primary form of locomotion, allowing the postulation that these early Penguins may have utilised foot-propelled propulsion in underwater locomotion, in conjunction with their comparatively less specialised flippers. The contrastingly shortened tarsometatarsus, with more dorsally aligned toes may support that early Penguins such as Kupoupou stilwelli and the giant Waipara Greensand taxon used their feet in a more similar way to modern Penguins than Waimanu manneringi and Muriwaimanu tuatahi, perhaps in underwater steering. Indeed, differing locomotory function and behaviour may have promoted ecological separation and niche partitioning in these Palaeocene Penguins, considering their likely co-existence.

The abandonment of aerial flight in Penguin evolution can be viewed as the elimination of volancy-related constraints, to allow specialised adaptations for underwater propulsion efficiency. turn, numerous morphological adaptations have allowed Penguins to better exploit the marine realm, many of which were in place by the Middle Palaeocene.

Unrestricted from aerial body mass constraints, Penguins attained larger sizes early in their evolution. In addition to being associated with greater muscle mass required for more powerful aquatic wing-propulsion, larger size is hypothesised to be related to increased mating success, capacity to dive longer and to a wider range of depths, facilitate niche separation, and confers advantages in catching more prey. This evolution does not, however, seem to be correlated with migration into higher latitudes or cooler temperatures. While early Penguins like Muriwaimanu tuatahi were likely capable wing-propelled divers, their forewing structure suggest that they and other Palaeocene forms were neither as powerful nor efficient as their modern relatives. Although Kupoupou stilwelli  was not a Giant Penguin, a potential higher body mass, and a more hydrodynamic morphology may have given it a competitive advantage in diving capabilities, and may have permitted foraging at greater depths, or allowed a wider exploitation of marine environments and ecological niches compared to coexisting penguins such as M. tuatahi. Effectively, other, more massive Palaeocene forms such as ?Crossvallia waiparensis, and the unnamed Waipara Greensand Giant Penguin, that also had stout tarsometatarsi, may have explored and exploited the water column to an even greater extent.

Until recently, archaic penguins such Waimanu manneringi and Muriwaimanu tuatahi, from the Canterbury area in New Zealand, were thought to be the oldest Sphenisciforms. Recent discoveries reveal that similarly aged taxa including Sequiwaimanu rosieae, ?Crossvallia waiparensis, and the morphologically dissimilar giant Waipara Greensand Penguin shared the same environment. Less than 300 kilometres away from these Canterbury Penguins, an additional giant species, Kumimanu biceae, revealed further Palaeocene diversity. Only 800 km from Canterbury, on the Chatham Peninsula of Zealandia, Kupoupou stilwelli and the larger Chatham Island form would have inhabited marine or nearshore environments, in areas where we know mainland penguins regularly visit today, likely from ephemeral oceanic islands, surrounded by deep ocean to the north and south. Also from Canterbury, Kaiika maxwelli may have also coexisted alongside these Palaeocene Penguins. This unprecedented diversity of Paleocene Penguins living in a relatively close proximity implies that numerous ecological niches must have been present in the region now known as the eastern coast of New Zealand’s South Island during this time. Ecological segregation for an area such as this is not unparalleled, however, in consideration that some sub-Antarctic islands and the Antarctic Peninsula today are known to host breeding populations of several modern Penguin species sympatrically.

A south polar orthographic projection of the Earth around 60 million years ago. Approximate site locations of Palaeocene Penguin fossils are indicated. Locations are associated with following fossils: CANTERBURY, Waipara Greensand, Waimanu manneringi, Muriwaimanu tuatahi, Sequiwaimanu rosieae, giant Waipara Greensand Penguin, ?Crossvallia waiparensis; OTAGO, Moeraki Formation, Kumimanu biceae; CHATHAM ISLAND, Takatika Grit, Kupoupou stilwelli and larger Chatham Island form; SEYMOUR ISLAND, Cross Valley Formation, Crossvallia unienwillia. Blockland et al. (2019).

While still connected to Australia, early Cenozoic Zealandia had drifted north after it had completely separated from the eastern Gondwanan margin becoming increasingly isolated, and geographically and biologically distinct. As a unique and important sector of the south-west Pacific, and with the discoveries of an apparently diverse assemblage of archaic Penguins on this landmass, Zealandia, and by extension the exposed landmass of present day New Zealand, is currently recognised as the apparent cradle in which Sphenisciformes evolved. The lack of exposed outcrops available to study, however, may obscure the true nature of their origin. The only non-Zealandian Sphenisciform of this early interval is represented by the late Paleocene Crossvallia unienwillia, and while fossils attributed to it are relatively fragmentary and incomplete, visual observations and phylogenetic analyses find it closely associated with Zealandian Paleocene Penguins. Taxa such as these would have lived in a greenhouse interval with exceptionally warm poles, predating the formation of the Circum-Antarctic Current, in a world before the southern polar ice cap; when ocean circulation and climate was drastically different as a result. While sub-tropical to tropical surface water temperatures existed in the warmer early Cainozoic, the sub-surface water would have still been cooler than Penguin body temperature. Indeed, modern Penguins have geographic distributions that are largely correlated with specific aquatic temperature ranges, and also possess numerous thermoregulatory adaptations related to survival in cooler waters. It has been hypothesised that the evolution of the rete mirabile of the forelimb in early Penguins would have promoted greater foraging duration at cooler sub-surface water temperatures, and increased their ability to forage for longer durations and greater distances. Evidence of a humeral plexus of this fashion has not been confidently observed in any Palaeocene Penguin, and the observation that the majority of these early taxa have been recovered in a relatively close proximity, near the east coast of New Zealand’s South Island, may imply that a humeral plexus may not have yet evolved, effectively restricting them to a relatively near-shore foraging habitat. Adaptations such as this, in conjunction with giant size and greater hydrodynamic body shape, may have been significant in the dispersal of Palaeocene Penguins from inshore habitats and their radiation across the ocean to Antarctic shores. Unfortunately, while of massive proportions, the highly weathered bones attributed to Crossvallia unienwillia limit the evolutionary inferences that can be made surrounding the presence of this species in the late Palaeocene Antarctic.

The ecological release provided by the vacuum in the aftermath of the Cretaceous/Palaeogene mass extinction allowed near simultaneous divergence of Neoavian Birds into newly available niches, followed by rapid population isolations, specialisations, and speciation events. It has been hypothesised that the lineage of Birds leading to Penguins evolved flightlessness in the wake of the Cretaceous/Palaeogene mass extinction, whereby Penguins may have inherited a world devoid of many marine predators such as large Sharks and Marine Reptiles. While Reptilian predators were subsequently replaced in the earliest Cainozoic, this event potentially facilitated the transition to flightlessness in the ancestors of Penguins, especially in areas largely free of predation pressures. The large and growing diversity of early Penguins may coincide with the niche availability following the mass extinction, and a rapid radiation of early penguin forms into the early Cainozoic. The large osteological variation observed within some species may fall within the range of sexual dimorphism and other intraspecific variation, significant levels of which have been recognised for modern Sphenisciforms. This could explain morphological differentiation across elements assigned to Kupoupou stilwelli and in specimens attributed to Muriwaimanu tuatahi. Conversely, the disparities evident across Palaeocene forms may reflect the existence of hitherto unrecognised taxonomic diversity of Palaeocene species.

Recent genomic studies have implied a mid-Palaeocene divergence of the Sphenisciformes clade from its sister taxon, the Procellariiformes (Albatrosses, Petrels, Shearwaters, and Storm Petrels), in contrast, however, some earlier molecular estimates had suggested that this split occurred within the Late Cretaceous. Palaeogene Procellariiforms are scarce, and fossils from deposits of the latest Cretaceous or earliest Palaeocene have only been tentatively referred to the Procellariiformes. However, the earliest Penguin fossils from the Palaeocene are relatively well-preserved and are diverse in size and form. The existence of at least two Chatham Island Penguin taxa, in addition to an already diverse Sphenisciform fauna on what is now the eastern coast of New Zealand’s South Island, with morphologies significantly dissimilar to the earliest Procellariiforms during the early-middle Palaeocene suggests that origin of both Sphenisciforms and Procellariiforms occurred before the Palaeocene. 

Blockland et al. conclude that a deeper, Late Cretaceous divergence of the Penguin lineage from that leading to Procellariiformes better conforms with the fossil record, whereby the earliest Sphenisciformes intensely radiated in the South Pacific oceans following the Cretaceous/Palaeogene mass extinction. Freed from aerial flight constraints, these nonvolant archaic Sphenisciforms evolved numerous adaptations and morphologically disparate forms related to exploiting the aquatic realm and diving efficiency, culminating in the highly specialised Penguins of today.

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