Sunday, 29 March 2020

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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