Tuesday 23 February 2021

Cousteauvia kustovia: A Diving Duck from the Late Eocene of Kazakhstan.

Waterfowl (order Anseriformes) are characteristic elements of modern Avian faunas and are among the most common Birds in the late Cainozoic fossil record. The group comprises three commonly recognized living families, which may be classified within two suborders Anhimae (with one family Anhimidae, Screamers) and Anseres (with families Anseranatidae, Magpie Geese, and Anatidae, all other Ducks, Geese, and Swans). The early (pre-Late Oligocene) fossil record of the Anseriformes is rather scanty, and thus the early diversity and disparity of the group remains poorly known. Most early-diverged Palaeocene to early Eocene Anseriforms (Presbyornis isoni, Anatalavis oxfordi, Conflicto antarcticus, Naranbulagornis khun) were relatively large-bodied Birds, and the exceptionally well-studied smaller Presbyornithidae are characterized by highly mosaic and apparently somewhat aberrant postcranial morphology, in which they rather resemble Waders and Flamingoes.

Derived Anseriforms (family Anatidae) constitute the overwhelming majority of the modern taxonomic Anseriforms diversity and display an array of feeding and locomotor specialisations, including repeatedly occurring diving adaptations. Anatomically modern Anatidae sensu lato first appear in the fossil record in the early Oligocene of Belgium, but already in the late Oligocene they have a broad geographical distribution and are represented by several phylogenetical lineages. Many diving Anatids, including flightless forms, have been described from the late Oligocene to Pleistocene, clearly showing that such adaptations represent a common evolutionary pathway of the group. However, no specialised diving forms have been previously known among early-diverging Anseriforms, as the relationships between the specialised divers Vegaviidae and Anseri formes (and even Galloanseres) remain unclear. The oldest known Duck-like Anseriforms (the late Eocene to Oligocene Romainvilliidae) still resemble the Presbyornithidae morphologically, thus showing that Anatidae sensu lato may be evolutionary linked with these aberrant long-legged Palaeogene Anseriforms. The derived elaborate ability to filter-feed on small floating objects could have been the driving force that led to the origin of short-legged Anatidlike morphological type on the basis of plesiomorphic wading morphological type of Presbyornithidae. This scenario implies a possibility that the evolutionary occurrence of diving adaptations may only be implemented on the basis of the derived morphological type of modern Anatidae sensu lato, which is supported by the current fossil record of diving Anseriforms. However, the Eocene diversity of Anseriformes, and especially that of small- bodied forms, remains very poorly known.

In a paper published in the journal Acta Palaeontologica Polonica on 8 October 2020, Nikita Zelenkov of the Borissiak Paleontological Institute of the Russian Academy of Sciences, describes a tarsometatarsus from the late Eocene of Eastern Kazakhstan, which belongs to a new primitive diving Anseriform bird, representing the first evidence of the evolutionary occurrence of diving adaptations in early-diverged Anseriformes. The new form displays a previously undocumented mosaic of plesiomorphic and autapomorphic features, showing partial similarities with modern Anseranatidae and fossil Paranyrocidae, thus adding significantly to the known diversity of fossil Anseriformes. This is further only the second Eocene Anseriform bird from Asia, which supports previously expressed views that shallow epicontinental seas of Central Asia might have played an important role in the evolution of Anseriformes. The derived morphology of the intertarsal joint in Anatids is further explained by Zelenkov in terms of the characteristic aquatic locomotion of these Birds.

The specimen described by Zelenkov comes from the Kusto-Kyzylkain ('Plesh') locality in the Eastern Kazakhstan and was collected by Nina Shevyreva and Vyacheslav Chkhikvadze of the Paleontological Institute of Soviet Academy of Sciences in 1977. The locality is situated in the Zaysan Basin between the rivers Kusto and Kyzylkain some 20 km to the South of Zaysan lake and 60 km to the West of the Zaysan city. A short list of Amphibia and Reptilia has been reported previously from this locality. Fossil remains comes from the Kusto Svita ('Formation'), which is broadly exposed in the Zaysan Basin and is dated to the Ergilian (late Priabonian; latest Eocene). No Bird remains have been previously described from the Kusto-Kyzylkain locality, but Ergilornis sp. and Romainvillia kazakhstanensis are known from other localities of the Kusto Svita.

General outline map of Asia and Kazakhstan (A) showing the geographical position of Kusto-Kyzylkain locality at Zaysan Basin (B). Zelenkov (2020).

Comparative material on fossil and modern Anseriformes was examined by Zelenkov primarily at the collections of the Paleontological Institute of Russian Academy of Sciences and Smithsonian National Museum of Natural History.

The Anatidae of earlier authors often included the morphologically distinct Anseranatidae and hence distinct fossil forms (e.g. Romainvilliinae, Paranyrocinae) could also be easily included within broad-sensed Anatidae as subfamilies. However, recent authors classified Anseranatidae as a separate family of Anseriformes. The recently proposed exclusion of the phylogenetically basal and morphologically easily distinguishable Whistling Ducks, Dendrocygninae, from Anatidae and their treatment as a separate family allows recognition of fossil Romainvilliinae and Paranyrocinae as separate families as well. This taxonomic treatment, adopted by Zelenkov, corresponds with the distinctive osteology of these fossil groups as compared to modern Anatidae sensu stricte and Dendrocygnidae.

The new species is named Cousteauvia kustovia, where 'Cousteauvia' honours ocean explorer and prominent diver Jacques-Yves Cousteau (1910–1997), appealing to the fact that the new taxon is the oldest known (and presumably the first evolved) diving crown Anseriform Bird, and 'kustovia' is in reference to the geological origin of the specimen (Kusto Svita), which is itself named after the Kusto river.

The specimen is referred to Anseriformes based on the following combination of characters: the medial angle of the proximal end is narrow and prominent, the shaft is mediolaterally compressed, the hypotarsus with one closed canal (for the tendon of m. flexor digitorum longus), the crista medialis hypotarsi is dorsoplantarly high and proximodistally elongate, the cotyla lateralis forms a characteristic distally extending 'lip' on the dorsal surface of the bone, the plantar opening of the medial proximal vascular foramen is shifted proximally towards the proximal part of the hypotarsus. In particular, these features (except the structure of the hypotarsus) distinguish the new fossil from diving Alcidae. The structure of the hypotarsus is also more complicated in Alcidae. Yet another character of the new fossil that is shared with Anseriformes is the presence of a tubercle on the lateral surface of the bone just distal to the level of the hypotarsus. The hypotarsus of the new form is somewhat similar to that of some Galliformes, but aquatic forms with thick bone walls are not known for these ground and usually forest-dwelling Birds. Galliformes further have well-developed fossae parahypotarsales.

Tarsometatarsi of Anseriform Bird Cousteauvia kustovia and selected modern Anseriformes. (A) Cousteauvia kustovia holotype PIN 2612/4, latest Eocene of Kusto-Kyzylkain, Eastern Kazakhstan, in dorsal (A₁), medial (A₂), lateral (A₃), plantar (A₄), angled disto- dorsal (A₅), proximal (A₆), and distal (A₇) views. (B) The Surf Scoter, Melanitta perspicillata (Anatidae), PIN 41-9-1, Recent, in dorsal (B₁), medial (B₂), plantar (B₃), and proximal (B₄) views. (C) The Yellow-billed Duck, Anas undulata (Anatidae), PIN 40-32-2, Recent, in dorsal (C₁) and lateral (C₂) views. (D) The Magpie Goose, Anseranas semipalmata (Anseranatidae), USNM 621019, Recent, in dorsal (D₁) and proximal (D₂) views. (E) The Horned Screamer, Anhima cornuta (Anhimidae), USNM 345208, Recent, in dorsal (E₁) and proximal (E₂) views. (F) The Mallard Duck, Anas platyrhynchos (Anatidae), PIN 40-30-3, Recent, in distal view. (G) The Long-tailed Duck, Clangula hyemalis (Anatidae), PIN 41-7-8, Recent, in distal view. Abbreviations: cdl, dorsolateral crest of the shaft; cdm, dorsomedial crest of the shaft; cl, cotyla lateralis; cm, cotyla medialis; cmh, crista medialis hypotarsi; cpm, crista plantaris medialis; ei, eminentia intercotylaris; fdl, canal for tendon of m. flexor digitorum longus; fic, fossa infracotylaris; fpm, fossa parahypotarsalis medialis; fvp, foramina vascularia proximalia; itc, impressio m. tibialis cranialis; se, sulcus extensorius; sfdl, sulcus for tendon of m. flexor digitorum longus; l, lip-like distal extension of the cotyla lateralis. Scale bars: (A₁)–(A), (B₁)–(B₃), (C), (D₁) 10 mm; (A₆), (B₄), (D₂), (E₂) 5 mm; (A₇), (F), (G) 2 mm. Zelenkov (2020).

The new taxon represents stem-Anatidae sensu lato (Anatidae+Dendrocygnidae) because it does not possess an apomorphic dorsoplantarly low hypotarsus with four crest and three well-defined sulci, which invariantly characterises crown Anatids. Instead, the tarsometatarsus of the new form is high and has only two pronounced crests bordering one medial canal (for the tendon of m. flexor digitorum longus) together with a plantarly located sulcus. This structure of the hypotarsus is definitively more derived than in Anhimidae and fossil Presbyornithidae, which have only one medial sulcus. The presence of closed medial hypotarsal canal distinguishes the new taxon from fossil Paranyroca (Paranyrocidae), which otherwise have similar outlines of the hypotarsus, though a similarly- built hypotarsus with a canal might have been present in Saintandrea, a large-sized representative of Romainvilliidae, but the referred specimen is not sufficiently preserved to confirm it. Although affinities of the new taxon with Romainvilliidae seem probable, the type genus Romainvillia has a morphologically rather different tarsometatarsus, more similar to that of Anatidae. Anseranatidae also have one closed medial hypotarsal canal, but otherwise are characterised by a more complex and an autapomorphically strongly laterally shifted hypotarsus, which might have evolved independently from Anatidae. Thus, the familiar assignment of the new taxon is unclear; it may represent a separate undescribed family of belong to either stem Anseranatidae, specialised Romainvilliidae or Paranyrocidae. The latter assignment appears to be most probable. The presence of a distinct distally extending lip of the cotyla lateralis indicates that the new genus occupies a more derived position than Anseranatidae.

Cousteauvia differs from all known Anseriforms by having a highly prominent dorsomedial crest, running distally from the tip of the cotyla medialis, and further by the presence of a crista plantaris medialis, representing a continuation of the medial hypotarsal crest. Cousteauvia further differs from crown-group Ducks (Anatidae and Dendocygnidae) by the hypotarsus having only two (instead of four) crests and a common broad lateral sulcus.

Cousteauvia kustovia is a medium sized Anseriform, roughly similar in size to modern wild specimens of the Mallard Duck, Anas platyrhynchos. Proximal tarsometatarsus with bulbous and proximally prominent eminentia intercotylaris; cotyla medialis ovalshaped, narrow and dorsally protruding; articular surface of the cotyla medialis does not extend on the medial surface of eminentia intercotylaris; cotyla lateralis wider and with cut dorsolateral angle; dorsomedial margin of the shaft forming prominent ridge; dorsolateral ridge vestigial; tuberositas tibialis cranialis muscle short; hypotarsus moderately low, with closed medial canal for m. flexor digitorum longus and two plantar grooves, sulcus for the fibularis longus muscle absent; crista medialis hypotarsi is the best developed and extends moderately distally as a low crista medialis plantaris; caudal surface of the bone distal to hypotarsus slightly inclined medially (not flat); bone walls very thick.

The holotype represents a well-preserved proximal half of the left tarsometatarsus with complete proximal articular articular area and hypotarsus. The shaft is notably narrowed relative to the proximal end, being roughly similar to the condition in Presbyornithidae and Anseranatidae but unlike Anhimidae, Romainvillia and Anatidae sensu lato. A similarly widened proximal end might have also been present in large Romainvilliid Saintandrea. The cross-sectional area of the mid-shaft is sub-trapezoid. The bone walls are very thick as in modern diving Anatidae and in contrast to thin bony walls of Dabbling Ducks.

The eminentia intercotylaris is roughly symmetrical and bulbous, and it protrudes further proximally than the medial border of the cotyla medialis, as in Saintandrea and some non-Anseriform taxa (e.g. Palaelodidae), but unlike all other fossil and modern Anseriformes. In Anatidae and Anseranatidae, the eminentia is asymmetrical and its more pointed tip is inclined medially. Anhimidae have an even bigger, but also symmetrical eminentia intercotylaris. In proximal view, the eminentia intercotylar is also prominent and extends plantarly approximately up to the mid-point of the cotylae, whereas it is restricted to the dorsal part of the articular area in Anatidae. The articular surface of the cotyla medialis does not expand on the medial wall of the eminentia, in contrast to the condition in Anatidae.

The cotyla medialis is oval-shaped and rather narrow; it protrudes more dorsally than the eminentia intercotylaris unlike the condition in all other Anseriformes except the supposed late Presbyornithid Wilaru tedfordi, in which this cotyla is, however, unusually mediolaterally wide. The dorsal margin of the cotyla medialis is distinctly convex, which is different from nearly straight margin in all other Anseriforms except Anhimidae. The cotyla lateralis is wider than the cotyla medialis and is sub-circular in shape as in Paranyroca but unlike all other Anseriformes. The dorsomedial angle of the cotyla lateralis is prominent (as in Paranyroca) because of the well-developed 'lip', extending to the dorsal surface of the bone. This lip is positioned entirely on the dorsolateral edge of the bone in Anatidae, whereas in Cousteauvia it is strongly shifted medially. The condition in Romainvillia is unclear due to poor preservation. Such a 'lip' (distal extension of the cotyla lateralis) is not present in Anhimidae, Presbyornithidae and Anseranatidae and hence may represent an apomorphy of Anatidae sensu lato + Cousteauvia. The exact state of this character in Romainvilliidae is unclear, but Paranyroca may have a similar morphology. Plantar parts of both cotylae medialis et lateralis are bordered by not high but distinct beads, which are present in Presbyornithidae, but are absent or at least indistinct in Anatidae and other Anseriformes. The condition in Romainvilliidae is unclear.

Dorsally, there are two distinct concavities in the proximal margin of the bone, formed by the cotylae. The lateral concavity protrudes distinctly more distally than the medial one, as in Presbyornithidae and many Anatidae, but unlike Paranyroca. The condition of this feature in Romainvilliidae is unclear, but Saintandrea appears to be similar to Paranyroca in this respect. The fossa infracotylaris is deep proximally as in most Anseriformes except Anhimidae (Anseranatidae have a moderately excavated fossa). The foramina vascularia proximalia are small and relatively widely separated, as in Anatidae. The tuberositas muscle tibialis cranialis are short and proximally located, as in Anhimidae, Presbyornithidae, and Anseranatidae. In the more derived Anseriforms, the tuberositas are notably elongate and shifted distally.

The most unusual feature of Cousteauvia kustovia is the relative development of the dorsal lateral and medial crests, which border the fossa infracotylaris. In all known Anseriformes except Anseranatidae, the lateral crest is more prominent (both in robustness and the degree of dorsal protrusion) than the medial one, and this asymmetry is especially well-pronounced in diving taxa (e.g. Oxyurini, Mergini). Cousteauvia kustovia displays an alternative condition, with the medial crest being higher and more dorsally prominent than the lateral one. As a result, in medial view only the dorsomedial crest is visible in Cousteauvia kustovia and Anseranatidae, whereas in other Anseriforms the dorsolateral crest is also visible as it protrudes dorsally well above the medial one. This medial crest continues distally, forming a low medial wall of the dorsal sulcus (sulcus extensorius), as in Presbyornithidae and Anseranatidae, but not other Anseriformes. An apparently more poorly defined sulcus extensorius is visible in Cygnopterus alphonsi and Paranyroca magna. Modern Anserini also have a well-developed (although less prominent) dorsomedial crest.

The medial surface of the bone is slightly concave, forming a rather shallow but still distinct fossa parahypotarsalis medialis, which is totally absent in Anatidae, but a relatively- well defined fossa is present in Anseranatidae and Presbyornithidae. A large vascular foramen is present in the middle area of the fossa. The fossa parahypotarsalis lateralis does not form a concavity, being nearly vestigial. However, the lateral side of the shaft is not flat proximally, but have a slightly sloping plantar part. Two muscular/ligamental scars are present in this area, one is located dorsoproximally, and the other one is shifted distally and plantarly. Distally the lateral surface of the tarsometatarsus is flat as in modern Anatidae.

The hypotarsus is low and in general proportions is most similar to that of Paranyroca, from which it still differs in having a closed canal for the tendon of m. digitorum longus, as in Anseranatidae, Dendrocygnidae and most Anatidae. The medial hypotarsal crest, bordering this canal and a plantarly located sulcus, is prominent and continues distally as a gradually lowering crista medialis plantaris, a feature which is not present in any other Anseriformes (the medial hypotasal crest is still elongate in Anserini). This crest distally borders a distinct sulcus for the tendon of m. digitorum longus, which is also lacking in all other Anseriformes. Lateral of the abovementioned canal, the hypotarsus forms one broad sulcus, as in Paranyroca. Unlike Paranyroca and Anatidae, the hypotarsus extends as much laterally as the lateral margin of the cotyla lateralis. In medial view, the straight proximal wall of the hypotarsus is oriented sub-perpendicular to the long axis of the bone, unlike Anatidae, where this wall is angled and sometimes concave. Distally to the hypotarsus, the plantar surface of the shaft slopes laterally more notably than in Anatidae due to the plantar prominence of the medial part of the shaft (as in modern Cereopsini).

The Kusto Svita has yielded a stem-anatid taxon Romainvillia kazakhstanesis, which must be compared with Cousteauvia kustovia. Romainvillia kazakhstanesis is known from a coracoid that is very similar to the coracoid of Romainvillia stehlini from the contemporary latest Eocene of France. Given such a close similarity of the coracoids in two species, it would be reasonable to assume the same for their tarsometatarsi. However, the tarsometatarsus of Romainvillia stehlini significantly differs from that of Cousteauvia kustovia, preventing any possible association of the new Kazakh specimen with Romainvillia. In Romainvillia stehlini, the tarsometatarsus is of typical Anatid proportions and the shaft is not restricted relative to the proximal end, the dorsolateral margin of the proximal part of the shaft is thick and dorsally protruding, at least one visible tuberculum m. tibialis cranialis is elongate, the sulcus extensorius is poorly pronounced due to the absence of the dorsomedial ridge of the shaft, and the medial plantar ridge and a distally extending sulcus for the tendon of m. flexor digitorum longus are absent. These features thus clearly distinguish Romainvillia stehlini from Cousteauvia kustovia and strongly indicate closer affinities of Romainvillia with Anatidae. Additionally, in overall size, the tarsometatarsus of Romainvillia stehlini is comparable with modern small-sized Teals, whereas Cousteauvia kustovia was a larger Bird approximating wild specimens of Mallard Duck, Anas platyrhynchos, judging from the size of the articular surface.

The notably increased thickness of the bony wall of the tarsometatarsus indicates that Cousteauvia kustovia was a specialised diving Bird, thus being the first known Eocene and the oldest diving representative of the order Anseriformes. It displays an unusual combination of characters, but is definitively more derived than Anhimidae and is more primitive than Anatidae and fossil Romainvilliidae. Some similarities with modern Anseranatidae are notable and may indicate their closer relationships, but assignment of Cousteauvia kustovia to Anseranatidae is not supported by the structure of the hypotarsus. The new genus represents the first documented case of diving specialisations in the early-diverged Anseriformes and shows that pronounced ecological diversification of the group already took place in the Eocene. The occurrence of Cousteauvia kustovia in the late Eocene of Kazakhstan supports a previously expressed view that a retreat of large epicontinental Central Asian seas, which took place in the Priabonian epoch and resulted in dramatic changes in marine ecosystems in this geographical area, might have played a significant role in the evolution of derived Anseriforms. Currently, three taxa of primitive Anseriforms are known from the late Eocene of this regions: the Teal-sized Romainvillia kazakhstanensis, the medium-sized Cousteauvia kustovia, and the larger, Swan-sized 'Cygnavus' formosus.

Although the tarsometatarsus of Cousteauvia kustovia is similar to that of some modern marine Ducks (Mergini) in the general outlines (including the lateral flatness, which reduces profile drag), it nevertheless displays a series of profound structural differences, clearly indicating a different morpho-functional pattern. In particular, the proximally located tuberositas muscle tibialis cranialis of Cousteauvia kustovia show that speed was more important than power in the 'dorsiflexion' (anatomical hyperextension) of the foot. The position of the tuberositas muscle tibialis cranialis relative to the intertarsal joint vary depending on different ecological specialisations and hence different leg usage in modern Birds. In 1979 Ulla Norberg calculated that even a  slight (1–2 mm) increase in lever arm (due to distal shift of the insertion point of the muscle) results in a significant increase of the resultant force. In 2003 Anna Zeffer and Ulla Norberg revealed a difference in the mechanical properties of the leg geometry in fast-swimming versus slow-smimming Birds. In the former group, the speed is favored over force, because fast-swimming Birds catch their prey underwater and hence a high stroke frequency is important for them. This explains a shorter moment arms of the tibialis cranialis relative in this group relative to the condition in slow-swimmers. A similar mechanical requirement could explain the proximally positioned scars for muscle tibialis cranialis in Cousteauvia kustovia as well.

The most specialized modern Diving Ducks, Oxyura and Biziura, have a dorsoplantarly extended hypotarsus (especially, the medial hypotarsal ridge), which provides a mechanical advantage for the gastrocnemius muscle, the main plantar flexor of the feet, playing the key role in paddling. A similar posteriorly expanded hypotarsus is also characteristic of other specialised diving Birds. Cousteauvia kustovia has a moderately protruding medial ridge of the hypotarsus, more comparable with that of Anas Ducks, and thus apparently was specialised in a different way than Oxyura. However, the presence of the plantar medial ridge, which represents an ossified continuation of the Achilles tendon of the gastrocnemius muscle, may still imply an increased mechanical role of this muscle in Cousteauvia kustovia. The well-visible groove for the tendon of the flexor digitorum longus muscle, which characterises Cousteauvia kustovia , likely indicates a relative importance of the corresponding muscle. The proximal toe flexors are enlarged in diving Birds and during propulsive phase these muscle can effectively control unnecessarily hyperextension of toes, which would reduce effectiveness of the foot as a paddle.

The dorsal facies of the tarsometatarsus (sulcus extensorius) is the origin site of the extensor brevis digiti IV muscle, a short extensor of the fourth pedal digit, which is a relatively well-developed muscle with a long belly in Ducks. The deepened sulcus extensorius in Cousteauvia kustovia probably accounts for a better development of this muscle, which in turn implies a relatively greater functional role of the fourth toe in locomotion. The biological meaning of such adaptation, which might also be present in Anseranatidae, requires an explanation.

A suite of peculiar modifications of the tarsometatarsus in Cousteauvia kustovia is likely associated with diving specialisation of this taxon. Modern diving Anatids convergently evolved a set of shared morphological traits, and so the unusual nature of adaptations in Cousteauvia kustovia may be explained by the fact that they are realised on a different structural background, i.e. a more plesiomorphic osteology and function of the hindlimb. Indeed, Cousteauvia kustovia share many structural similarities with modern Anseranatidae and fossil Presbyornithidae, which are likely a sister taxon of Anseres. Presbyornithidae were long-legged waders and Anseranatidae also have a relatively long tarsometatarsus as compared with modern Anatidae sensu lato. The tarsometatarsus of Cousteauvia kustovia  thus might have also been longer than in modern Diving Ducks, and this proportional difference could be one of the factors explaining the overall structural modifications in the fossil taxon.

Yet another important factor could be the different kinematics of the intertarsal joint. The bulbous eminentia intercotylaris of Cousteauvia kustovia highly contrasts with the smaller, asymmetrical and pointed eminentia of modern Anatids and implies a different structure of the intertarsal joint. In anatids, the medial wall of the eminentia represents a continuation of the cotyla medialis and articulates with the lateral surface of the condylus medialis of the tibiotarsus, which is evenly narrow in these Birds. The eminentia does not act as a stop in Anatids and allows a considerable dorsiflexion of the tarsometatarsus, staying in full contact with the tibiotarsus at all possible angles of the dorsiflexion. The lack of the articular surface on the medial wall of the eminentia in Cousteauvia kustovia indicates that in this taxon it did not significantly articulate with the condyle, but the great proximal protrusal and wide apex of the eminentia intercotylaris in Cousteauvia kustovia show that it might have acted as a stop limiting dorsiflexion. However, the main function of the enlarged eminentia is likely to prevent significant rotation of the tarsometatarsus, as exemplified by the complete reduction of this structure in Parrots. Rotation of the tarsometatarsus relative to the long axis of the tibiotarsus is also present in modern Ducks and is correlated with the presence of small and asymmetrical eminentia intercotylaris. Such a rotation hence was obviously limited in Cousteauvia kustovia.

The intertarsal joint of Recent Anatidae (exemplified by the King Eider, Somateria spectabilis, PIN 41-2-2; (A) and fossil wading Bird (PIN 3104-65; early Eocene of Tsagaan-Khushu locality; southern Mongolia; (B) as related to the swimming locomotion. Two types of the general structure of the distal tibiotarsus (A₁), (B), illustrating the difference in the shape of the condylus medialis (cm1, cm2). Maximal anatomically possible dorsiflexion of the tarsometatarsus in Anatidae, in craniomedial view (A₂), showing full contact between the condylus medialis type 2 and the articular surface of the tarsometatarsus, in medial view (A₃). Position of the tarsometatarsus relative to the tibiotarsus at the beginning of the propulsive phase of the stride in the swimming Duck (C) and walking Wader (D); note the strongly dorsiflexed tarsometatarsus in the former. Abbreviations: cm1, cm2; condylus medialis in type 1 and type 2 intertarsal joints. Scale bars 10 mm. Zelenkov (2020).

Among Anseriformes, Anhimidae display a bulbous and symmetrical eminentia intercotylaris (somewhat similar to that of Cousteauvia kustovia), which in this taxon is associated with proximally restricted (subtriangular in cranial view) condylus medialis of the tibiotarsus, as in wading Birds. This proximal restriction of the articular surface results in that the area of contact between the condylus medialis of the tibiotarsus and the cotyla medialis of the tarsometatarsus gradually becomes smaller as the tarsometatarsus progressively dorsiflexes. Thus, a firm contact between the two bones is not possible at greatest angles of dorsiflexion, which itself implies that the main working diapason of such kind of intertarsal joint (as in Anhimidae; hereafter Type 1 intertarsal joint) covers a lower range of angles than in birds with an evenly narrow condylus medialis (as in Anatidae; Type 2 intertarsal joint). The condylus medialis of Cousteauvia kustovia thus might have been somewhat proximally restricted, as in the coeval fossil taxon 'Cygnavus' formosus.

It is hypothesised here that the Type 1 intertarsal joint, which is present in Anhimidae and Presbyornithidae among Anseriformes, but further occurs in Waders (Chara driiformes) and Phoenicopteriformes, is likely related with the wading (predominately walking) type of locomotion. A similar structure is also present in some Ground birds, e.g., Otidiformes, Gruiformes, and others. The structure of Type 1 joint prevents significant rotation and thus provides a greater stability at standing or during the stance phase, when the foot experiences the greatest load. Morphology of the eminentia intercotylaris and condylus medialis of the tibiotarsus adds to this stability, but nevertheless prevents effective loading at the joint during considerable degree of dorsiflexion. The type 2 joint, as in advanced Anseriformes (Anatidae sensu lato) and other swimming birds, represents an adaptation for transmitting the force at increased dorsiflexion angles, as it allows a full contact between the tibiotarsus and tarsometatarsus.

These proposed functional difference is in agreement with overall leg kinematics in Waders and Ducks. During walking, the long-legged Tringa Waders (Scolopacidae) dorsiflex their intertarsal joint only to 90°. A similar 70–90° flexion was recorded for walking/running in Lapwings, Vanellus, and Avocets, Recurvirostra, whereas this value may reach about 50° in Oystercatchers, Haematopus. However, in waders the maximal dorsiflexion of the intertarsal joint occurs during the middle of the swing phase, i.e. long before the contact between the leg and the substrate. The mechanical stress transmitted between the tibiotarsus and tarsometatarsus is higher at the beginning of the stance phase, when the intertarsal joint is extended. In swimming Ducks, the dorsiflexion at the intertarsal joint brings the foot to an appropriate protracted position required for the propulsion, which is being initiated when the dorsiflexion is maximal. Thus, the maximal dorsiflexion at the intertarsal joint in swimming Birds occurs during the moment of active interaction between the leg and the substrate, and hence the transmitted stress must be obviously much greater than during the mid-swing phase in Waders.

The presence of large eminentia intercotylaris without articular surface in Cousteauvia kustovia indicates the presence of generally Type 1 intertarsal joint in this diving Bird, and thus implies imperfect swimming adaptations as compared to modern Anatids.

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