Saturday 23 October 2021

Vishnuonyx neptuni: A new species of Otter from the late Miocene Hammerschmiede Fauna of Bavaria.

The Hammerschmiede locality in Bavaria, Germany, has been producing Late Miocene Vertebrate and Invertebrate fossils for almost five decades. The fossils are predominantly found within a series of river channels, and date to between 11.62 and 11.44 million years ago. Over 130 species of Vertebrate have been recorded from this location, several of which are unknown from any other location, most notably the Ape, Danuvius guggenmosi. Despite this high diversity, to date only two species of Carnivorans have been described from Hammerschmiede, the Palm Civets Semigenetta sansaniensis and Semigenetta grandis

In a paper published in the Journal of Vertebrate Palaeontology on 16 September 2021, Nikolaos Kargopoulos of the Department of Geosciences at Eberhard Karls University of Tübingen, Alberto Valenciano of the Departamento de Ciencias de la Tierra, and Instituto Universitario de Investigación en Ciencias Ambientales de Aragón at the Universidad de Zaragoza, Panagiotis Kampouridis, also of the Department of Geosciences at Eberhard Karls University of Tübingen, and Thomas Lechner and Madalaine Böhme, also of the Department of Geosciences at Eberhard Karls University of Tübingen, and of the Senckenberg Centre for Human Evolution and Paleoenvironment, describe a new species of 'Bunodont' Otter from the Hammerschmiede locality.

The phylogeny and classification of Otters, Lutrinae, is generally poorly resolved, with the relationship of fossil groups such as the Potamotheriinae and Bunodonts to modern Otters remaining unclear. Bunodont Otters are a (probably paraphyletic) group of large and very large fossil Otters found across Eurasia, Africa, and North America. The classification of this group is itself poorly resolved, with uncertainty about which fossils should be included within it, with one genus, Enhydrictis, having been recently reclassified and no longer thought of as an Otter at all. 

The new species is placed in the genus Vishnuonyx, which contains previously described species from the Miocene of South and Southeast Asia and the Miocene and Pliocene of Africa, and given the specific name neptuni, in reference to Neptune, the Roman god of the seas. The species is described from a right hemimandible, with a first premolar alveolus (canine tooth socket) and the second, third and fourth premolars and first molar intact. A left hemimandible and a number of isolated teeth are also referred to the species. All were recovered from river channel deposits at Hammerschmiede between 2011 and 2020, during excavations carried out by palaeontologists from Eberhard Karls University of Tübingen.

The lower dentition of Vishnuonyx neptuni. (A) SNSB-BSPG 2020 XCIV-0301, right hemimandible (holotype; original specimen and screenshots of its 3D model) in labial (A1), (A4), lingual (A2) and occlusal (A3) views; (B) GPIT/MA/16733, left hemimandible in labial (B1), lingual (B2), (B4) and occlusal (B3(, (B5) views; (C) SNSB-BSPG 2020 XCIV-1301 right p4 in labial (C1), lingual (C2) and occlusal (C3) views. Screenshots of the 3D models not in scale. Kargopoulos et al. (2021).

The third premolar of Vishnuonyx neptuni is distinctly asymmetrical, with the bak of the tooth being visibly lager than the front. The main cusp has three crista (ridges) deriving from it, one at the front, one at the back, and one on the inside edge. The inside crysta expands to form part of the wall of the tooth, which hosts one of the three roots of the tooth.

The upper carnassial tooth (front molar, modified to have a cutting edge which occludes the cutting edge on the lower tooth, a diagnostic trait of Carnivorans) has moderate wear on its cutting edge. The tooth has a strong cingulum (convex protuberance below the crown). The tooth has a high paracone (front outer) cusp, which is attached to a ridge a metastyle (ridge on the outer surface). There is no carnassial notch (notch used to hold bone for a crushing bite, a common Carnivoran trait). The protocone, parastyle, and paracone cusps form a line, with the hypocone outside this, behind the paracone. 

The upper dentition of Vishnuonyx neptuni. (A) SNSB-BSPG 2020 XCIV-1022, left P3 in occlusal (A1), labial (A2) and lingual (A3) views; (B) GPIT/MA/17347, right P4 (original specimen and screenshots of its 3D model) in occlusal (B1), (B4), labial (B2), (B5) and lingual (B3), (B6) views; (C) SNSB-BSPG 2020 XCIV-1552, left M1 (original specimen and screenshots of its 3D model) in occlusal (C1), (C4), labial (C2), lingual (C3), mesial-occlusal (C5) and mesial (C6) views. Abbreviations: meta, metacone; metal, metaconule; lp, lingual platform; para, paracone; paral, paraconule; prot, protocone; protl, protoconule. Kargopoulos et al. (2021).

The upper molar is complete with some wear to the inner sides of the paracone and metacone. A distinct cingulum is present, although this is less developed towards the forepart of the toorh. The tooth is nearly rectangular in outline, and somewhat slim, although the outer suface is slightly higher at the fore than at the rear. The inner cusps are roughly the same height, and connected by a low crest with a notch towards its middle. Behind these two further cusps, the protoconule and metaconule rear cusps are connected by another ridge.

Neither of the two hemimandibles is complete. The right specimen preserves part of the socket of the canine, part of the angular process (bottom part of the back of the jaw) and part of the masseteric fossa (muscle attachment above the angular process), and everything in between. The muscle attachment is deepest at the top, the hind part of the hemimandible bent inwards, and the angular process small and hook-like. The left mandible is less well preserved, with only part of the canine socket, the cheek teeth and part of the masseteric fossa muscle attachment, however, on this specimen the attachments for the masseter pars superficialis and pars profunda muscles can be seen.

The canine is absent in both hemimandibles. The second premolar is two-rooted and has a distinct cingulum (convex protuberance beneath the crown), particularly on the inner side and towards the rear. The long axis of this tooth is not in line with the long axis of the tooth row. The third and fourth premolars are high and pointed, with their largest cusps towards the middle and inclined backwards. In the third premolar only a single cusp is present, and the tooth has a wrinkled surface. The fourth premolar is taller than the third (and the first molar), and has a second, smaller cusp behind the first. The first molar is broad, with a talonid (crushing region) covering about a third of its surface. The first cusp is wider than this crushing area. This tooth also has wide cingulum. 

The dentition of Vishnuonyx neptuni differs considerably from that of modern Otters, but fits well with the genus Vishnuonyx, which has been confidently assigned to the group based upon more complete material. This genus has previously been described from Asia and Africa, with the Asian specimens being older and less robust than the African specimens; Vishnuonyx neptuni, the first described European member of the genus, appears to be intermediate between these groups in both age and size.

Temporospatial distribution of the known species of the genus Vishnuonyx. Kargopoulos et al. (2021).

The oldest known member of the genus, Vishnuonyx maemohensis, comes from Mae Moh in Thailand, and is dated to the Middle Miocene, between 14.2 and 13.2 million years old. Slightly younger is the late Middle Miocene Vishnuonyx chinjiensis from the Siwalik Hills of northwest India, which is thought to be between 13.8 and 12.7 million years old. A second specimen assigned to Vishnuonyx chinjiensis has been described from Ngorora in western Kenya, this specimen being about 12 million years old. Also known from Africa is the Latest Miocene Vishnuonyx? angololensis from Lower Nawata in Lothagam, near the southwestern shores of Lake Turkana, Kenya, which is dated to between 7.3 and 6.6 million years ago. Finally, an Early Pliocene specimen Vishnuonyx sp. has been recorded from Ethiopia and is thought to be between 5.2 and 4.85 million years old. The HAM 4 fossiliferous layer at Hammerschmiede, which produced Vishnuonyx neptuni, has been dated to 11.44 million years ago, making Vishnuonyx neptuni slightly younger than Vishnuonyx chinjiensis.

This suggests that the genus Vishnuonyx originated in Southeast Asia, and migrated westwards through the Indian subcontinent, reaching Europe and East Africa by the end of the Middle Miocene, with the genus persisting in East Africa into the Pliocene. Vishnuonyx neptuni is sufficiently different from other members of the genus to imply a reasonably long period of reproductive isolation, leading Kargopoulos et al. to reason that its ancestors reached Europe before 11.5 million years ago.

Vishnuonyx is thought to have had a semi-aquatic lifestyle similar to that of modern Otters, suggesting that it is most likely to have dispersed from Southeast Asia into South Asia, Europe, and Africa, by water. During the Middle Miocene the Central Paratethys Ocean separated East Africa and Arabia from southern Asia, with the Eastern Paratethys, separated from the Cenral Paratethys by the Araks Straight, forming a connection with Central Europe. This would have provided a plausible route for the westward expansion of the genus Vishnuonyx.

The proposed dispersal path of Vishnuonyx from South Asia towards Central Europe and East Africa during the Konkian around 13 million years ago (late Badenian, early Serravalian). Kargopoulos et al. (2021).

The teeth of Otters are adapted for two slightly different diets; catching slippery Fish, and breaking open hard shells. All living Otters pursue both these feeding methods to some extent, although they vary in the extent to which they depend on each method, which is reflected in their dental morphology. In species which primarily feed on hard-shelled prey such as Clams or Crustaceans, the molars tend to be broad and flat with well-developed crushing areas, whereas those that specialise in catching Fish tend to have high pointed cusps useful for catching slippery prey. The teeth of Vishnuonyx neptuni are apparently more adapted towards a piscivorous diet, with high, pointed premolars and a strong cutting blade on the molar. This is at odds with other 'Bunodont' Otters, which appear to have been primarily durophages specialising in hard prey, and suggests an ecology similar to that of the modern Amazonian Giant Otter, Pteronura brasiliensis.

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