Enhydriodon

Enhydriodon is an extinct genus of otters known from Africa and South Asia that lived from the late Miocene to the early Pleistocene. It contains nine confirmed species, two debated species, and at least a few other undescribed species from Africa. The genus name means "otter tooth" in Ancient Greek and is a reference to its dentition rather than to the Enhydra genus, which includes the modern sea otter and its two prehistoric relatives. Enhydriodon belongs to the tribe Enhydriodontini in the otter subfamily Lutrinae.
The exact sizes and lengths of Enhydriodon species are unknown given the lack of complete fossils of it and most related fossil lutrines. Indian subcontinental species are estimated to be of weights similar to that of the extant sea otter, but African species are estimated to be heavier than extant lutrines. In particular, several species such as E. kamuhangirei, E. dikikae, and E. omoensis were estimated to weigh over. Given these weight estimates, the three species likely reached sizes comparable to extant bears or lions, making them the largest known mustelids to exist, although a lack of complete specimens makes precise estimates impossible.
Its advanced dentition is well-known, its broad, bunodont carnassials allowing the lutrine to consume prey by crushing them rather than shearing them like the modern sea otter and unlike most other extant otters. As such, it is grouped among the bunodont otters, a categorical term referring to fossil lutrines with non-bladelike carnassials in the premolars or molars of the Miocene to Pleistocene and the sea otter of the sole extant Enhydra genus. Its I³ teeth are canine-like and much larger than its other incisors, a trait not seen in extant and extinct lutrine genera. It is hypothesized that Indian species of Enhydriodon were semiaquatic and consumed bivalves because their bunodont dentitions would have allowed them to consume hard-shelled invertebrates. It is unknown whether African species were generally aquatic, semiaquatic, or terrestrial, but their potential diets suitable for bunodont dentitions include bivalves, catfish, reptiles, eggs, and carrion. E. omoensis of Ethiopia in particular could have been a terrestrial locomotor that at least semiregularly hunted or scavenged terrestrial prey with C₄ plant diets which if true makes its behaviour unlike any extant otters. It is unknown whether the species is an outlier amongst African bunodont otter species, but it has been suggested that Enhydriodon dikikae and Sivaonyx beyi were both large terrestrial bunodont otters of Africa as well.
The taxonomic status of Enhydriodon species have been complicated by its affinities and similarities with other bunodont lutrine genera like Sivaonyx and Paludolutra up to the modern day, although Paludolutra is presently considered a distinct genus not closely related to Enhydriodon. Currently, the Enhydriodontini tribe is considered evolutionarily closer to the modern Enhydra genus than any other known bunodont otter genus that may have gained bunodont dentition as a result of parallel evolution, but the extent to which they are closely related remains unresolved.

Taxonomy

Early history

Enhydriodon was first erected in 1868 by Hugh Falconer based on several crania from Siwalik Hills, India that he attributed to E. sivalensis. He explained that the scientific name, meaning "otter tooth", is derived from the Ancient Greek terms ἐνυδρίς and ὀδούς and is not a reference to the genus Enhydra, which includes the modern sea otter. According to Falconer, the Siwalik Hill fossils belonging to E. sivalensis were previously classified by Falconer and Proby Cautley under the genus and species name Amyxodon sivalensis in an 1835 synopsis of the fossil genera in the Siwalik Hills that the two palaeontologists found, in which the fossil taxon was considered to be a carnivoran of an unknown family, although no holotype or diagnostic descriptions were attributed to it. As a result of the rename, Amyxodon had been considered a "dead name" or synonym of Enhydriodon despite being the older genus name. Using the available specimens of E. sivalensis, Falconer calculated that there were four premolars and molars in the species's upper jaw, one less than in the extant genus Lutra but matching the total count of Enhydra. He described the upper carnassial of E. sivalensis as the most unique feature of its upper jaw, being nearly square and its coronal lobes being developed from conical mamelons unlike the two extant lutrine genera.
During the 19th and 20th centuries, more species of Enhydriodon such as E. campanii were introduced and more lutrine genera with bunodont dentition such as Sivaonyx and Vishnuonyx were described, creating a particularly complicated history for the earliest-described prehistoric otter genus. In 1931, Guy Pilgrim described more fossils discovered in the Siwalik Hills, including a newer species named E. falconeri. He also implied that Enhydriodon and Sivaonyx, despite their similarities, were differentiated by the structure of the maxillary 4th premolar and apparent lack of the anterior upper premolar that is presumed to be reflected at the bottom jaw as well. In the same year that E. falconeri was described, Ernst Stromer described E. africanus of the late Pliocene, its fossil teeth being located in South Africa and the first described species from the continent of Africa.

Perceived relationships with ''Paludolutra'' and ''Enhydra''

In 1976, Charles Repenning brought about the idea that Enhydriodon was related to the extant Enhydra genus due to the supposed species of the former being an evolutionary "branch" of "crab-eating otters" in Italy, Spain, and California, eventually leading to the modern sea otter. He correctly introduced the idea that Enhydra was related to Enhydriodon given their bunodont dentitions, but the supposed European "branch" of the Enhydriodon genus was later reclassified by Johannes Hürzeler and Burkart Engesser into the newer genus Paludolutra in 1976, although it remained relatively obscure in the palaeontological record until later research revised its taxonomic state.
The taxonomies of individual lutrine species and genera continued to be revised into the 21st century as more prehistoric otter species were described while palaeontologists continually revised the fossil bunodont lutrine species to different genera. Paludolutra was originally reclassified as a subgenus of Enhydriodon by Gerard F. Willemsen in 1992. However, in January 2005, Martin Pickford and colleagues diagnosed Paludolutra as a synonym of Sivaonyx on the basis of Pilgrim's diagnosis of the latter, rejecting Willemsen's synonymy of Paludolutra to Enhydriodon. Additionally, they erected a species of Enhydriodon named E. hendeyi from the type locality of Langebaanweg, South Africa, which dates to the lower Pliocene and was named after the palaeontologist Quinton B. Hendey, who they said described the first known specimens that were since attributed to the species. In December of the same year, Jorge Morales and Pickford instead described Paludolutra as a distinct genus that might be related to Sivaonyx based on dentition convergences. In 2007, the two palaeontologists reaffirmed that the dental morphology of Paludolutra was distinct enough to be reclassified as a genus based on full generic differentiation, suggesting that the species P. campanii, P. lluecai, and P. maremmana would no longer be classified under Enhydriodon under the basis of Paludolutra being a subgenus.

Modern revisions of African species

In 2003, Lars Werdelin erected the species E. ekecaman from the Kanapoi palaeontological site of the Turkana Basin in Kenya, describing it as one of the earliest members of the African Enhydriodon lineage. The species was named after the Turkana language term "ekecaman", which means "fisherman" because he suggested that fish may have been a diet for the species. He also declared the species "E. pattersoni ", described by R. J. G. Savage in 1978, as a nomen nudum of E. ekecaman since no type specimen or valid diagnosis was designated to it, a view supported by Morales and Pickford in December 2005.
E. africanus, E. ekecaman, and E. hendeyi were reclassified into Sivaonyx by Pickford and Morales in December 2005, where they additionally described a new species named Sivaonyx kamuhangirei. The reclassification of African fossil bunodont otters into Sivaonyx had brought about continuous debate regarding the practicality of the differences between Enhydriodon and Sivaonyx, with some researchers claiming neutrality due to preferred focuses on researching the individual species instead of their genus placements. In 2022, the four species were eventually reclassified into Enhydriodon in a research paper by Camille Grohé et al. E. soriae was also initially sorted unto Sivaonyx but was eventually assigned to Enhydriodon, although its genus placement remains disputed. In 2005, Morales and Pickford sorted Enhydriodon into the newly created Enhydriodontini tribe, which they described as hosting genera of extinct bunodont otters from the Siwalik Hills and Africa including Vishnuonyx, Sivaonyx, and Paludolutra. In 2007, Pickford synonymized the species "E. aethiopicus ", previously described by Denis Geraads et al. in 2004, to Pseudocivetta ingens, an extinct member of the Viverridae family. In 2017, Enhydra was explicitly excluded from the Enhydriodontini tribe despite its similarities, and Paludolutra was reclassified as a sister taxon to the tribe.
In 2011, Denis Geraads and colleagues described E. dikikae based on its remains of a partial skull and femurs in the Lower Awash of Dikika, Ethiopia, the locality dating to the middle Pliocene. It was described as having a notably heavier skull than other Enhydriodon species or the modern sea otter. The species named was based directly on the site of Dikika. It was deemed as the largest species of Enhydriodon until another species also from Ethiopia, E. omoensis, was described from the Lower Omo Valley in 2022, dating from the late Pliocene up to the Plio-Pleistocene boundary. Similar to E. dikikae, the species name was derived directly from the site in which it was recovered. Similarly, the youngest known possible fossils of Enhydriodon from eastern Africa are also dated to the Early Pleistocene, circa 1.8-1.6 million years ago. In a September 2022 conference by Alberto Valenciano, Morales, and Pickford, however, they referred to certain lutrine species previously reclassified to Enhydriodon as Sivaonyx, namely S. hendeyi and S. africana.