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Enhydriodon, known as the bear otter, is an extinct genus of typically large otters that lived in what is now Kenya, Ethiopia, Uganda, Chad, South Africa, and India from the late Mioceneup to early Pleistocene. The otter is thought to be a relative of modern-day sea otters. E. omoensis and E. dikikae are described as the largest mustelids to have ever existed, though only fragments of the genus have been found such as the skull, femur, and dental remains in Ethiopia. Multiple estimates put them at about 200 kilograms (440 lb) while E. omoensis was described to be lion-sized, making them the largest mustelids described so far. Most species of the Enhydriodongenus are presumed to be semi-aquatic given most of the fossil isotope values being similar to fossilized semi-aquatic animals like hippopotamuses. The largest species, Enhydriodon omoensis, however, was determined to be a terrestrial predator, capable of hunting herbivorous terrestrial prey. Enhydriodon is part of the bunodont otters group, referring to otter genera with non-bladelike carnassials including the extant Enhydra genus and its extinct relatives that lived from the late Miocene to the early Pleistocene.

Enhydriodon


Temporal range: Late Miocene to Early Pleistocene, 5.8–2 Ma

Enhydriodon omoensis right femur faced at different sides.
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Carnivora
Family: Mustelidae
Tribe: Enhydriodontini
Genus: †Enhydriodon

Falconer, 1868

Type species
Enhydriodon sivalensis

Falconer, 1868

Other species
  • E. africanus Stromer, 1931
  • E. latipes? Pilgrim, 1931
  • E. falconeri Pilgrim, 1931
  • E. ekecaman Werdelin, 2003
  • E. hendeyi Morales, Pickford & Soria, 2005
  • E. kamuhangirei Morales & Pickford, 2005
  • E. soriae? Morales & Pickford, 2005
  • E. dikikae Geraads, Alemseged, Bobe & Reed, 2011
  • E. afman Werdelin & Lewis, 2013
  • E. omoensis Grohé, Uno, & Boisserie, 2022
Synonyms
  • Amyxodon Falconer & Cautley, 1835

Taxonomy[]

1868 Illustrations of the 2 craniums of E. sivalensis (Figure 3-4 are different views of the same specimen). The drawings were based on specimens at the British Museum.

Enhydriodon was first described in 1868 in a collected memoir by Dr. Hugh Falconer when he erected the genus based on several craniums attributed to E. sivalensis in Siwalik Hills, India and described it as a lutrine animal the size of a panther. He explained that the scientific name, meaning "otter tooth," is derived from the Ancient Greek terms ἐνυδρίς (otter) and ὀδούς (tooth) and isn't a reference to the genus Enhydra, which he said has a similar derivation. According to the paleontologist, the Siwalik Hill fossils belonging to E. sivalensis were previously sorted under the name Amyxodon in 1835 which eventually changed the year he fully described the genus. Falconer calculated the dental formulas of Lutra and Enhydra as 3.1.53.1.5 and 3.1.42.1.5, respectively (the molar and premolar teeth were presumably calculated together). Using this information and the available cranium specimens, he calculated the upper dental formula of E. sivalensis as 3:1:4, matching up more with the Enhydra genus. 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 otter genera.

As more debated Enhydriodon species such as E. campanii (later sorted into different genera) were introduced and as more prehistoric bunodont otter groups such as Sivaonyx and Vishnuonyx were discovered in the early 20th century, the 19th and 20th centuries created a particularly complicated history for the earliest-discovered prehistoric otter genus. In 1931, Pilgrim described more fossils discovered in the Siwalik Hills, including a newer species named E. falconeri. The year after, he diagnosed the species as being smaller than E. sivalensis. He implied that despite the similarity between Enhydriodonand Sivaonyx, the two genera were differentiated by the structure of the maxillary 4th premolar and apparent lack of the anterior upper premolar (P1) that's presumed to be reflected at the bottom jaw as well (both of which are debated up to today). In the same year that E. falconeri was described, Ernst Stromer described E. africanus of the late Pliocene, its teeth fossils being located in South Africa and the first species described in the continent of Africa.

The taxonomies of individual otter species and genera continued to evolve into the 21st century as more prehistoric otter species had been discovered while paleontologists continually debated the genera levels. For instance, in 1999, Paludolutra (Hürzeler & Engesser, 1976) was originally sorted as a sub-genus of Enhydriodon by Willemsen, but in 2005, Jorge Morales and Martin Pickford noted that the dental morphology of the Paludolutragenus was distinct enough to be its own genus (meaning that P. campanii and P. maremmanawould no longer be sorted under Enhydriodonby technicality). Eventually, Paludolutra was considered a separate genus again. E. ekecaman, E. hendeyi, and E. kamuhangireiwere all initially sorted into the Sivaonyx genus upon their respective times of discovery but were eventually assigned to their current genus placement (E. soriae remains a disputed species). In 2005, Enhydriodon was sorted into the newly created Enhydriodonti tribe, which Morales and Pickford described as hosting genera of extinct bunodont otters from the Siwalik Hills and Africa including Vishnuonyx, Sivaonyx, and Paludolutra(Enhydra was explicitly excluded from the category, and Paludolutra was later reclassified as a sister taxon).

In 2011, a team of researchers described E. dikikae based on its partial skull and femur remains in the Lower Awash of Dikika, Ethiopia. It was described as having a notably heavier skull (albeit broken) than other Enhydriodonspecies or the modern sea otter, attributing it to be bear-sized and weighing 100 kg minimum and 200 kg on likely estimates. It was deemed as the largest species of Enhydriodonuntil another species whose remains were also found in Ethiopia, E. omoensis, was described in the Lower Omo Valley in Ethiopia in 2022. It is generally observed that the Enhydriodon was a result of a Miocene-Pleistocene trend that gave prehistoric otters bunodont teeth and large-scale sizes compared to their extant relatives.

EvolutionEdit[]

Enhydriodon's closest extant relative, the sea otter, which is the only extant bunodont otter.

Enhydriodon belongs to the tribe Enhydriodontini in the subfamily Lutrinae, which first appeared in Eurasia and Africa during the late Miocene epoch. It is perhaps the most well-known prehistoric otter given its old taxonomic history and primary source of comparisons to other bunodont otter genera. It is classified as a member of the bunodont otters group, which also includes Sivaonyx, Paludolutra, Vishnuonyx, Torolutra, Enhydritherium, Djourabus, Paralutra, Tyrrhenolutra, Siamogale and Enhydra.Bunodont otters are defined as large to very large otters of North America, Eurasia, and Africa that had robust dentition compared to most of the extant otters, generally allowing them to prey upon hard-armored creatures.

The following cladogram defines some of the following extant and extinct otter species and genera within the subfamily Lutrinae based on a 50% majority consensus (the bunodont otter genera are bolded):

Lutrinae
Pteronura brasiliensis
Lontra canadensis
Lontra felina
Lutra lutra
Aonyx capensis
Paralutra jaegeri
Siamogale melilutra
Siamogale thailandica
Enhydra lutris
Enhydriodon
Vishnuonyx
Sivaonyx
Tyrrhenolutra helbingi
Paralutra garganensis
Enhydritherium terranovae
Paludolutra
Lutra aonychoides

As shown in the above phylogeny, Enhydriodonshared a closer morphology with its other extinct relatives and Enhydra than the other extant otters that lack bunodont carnassial teeth (Lutra aonychoides was described as not being related to Lutra). Although the majority consensus tree displays a close morphological relation between Enhydriodon and Enhydra, the authors of the consensus tree also created a Bayesian inference tree proposing that Enhydrais a separate clade. Regardless, they argued that Enhydra is closer to the Enhydriodontinitribe (Enhydriodon, Sivaonyx, and Vishnuonyx) than any other bunodont otter genus. Nonbunodont otters likely branched out separate from bunodont otters during or before the Pliocene epoch, but their poor fossil records and restriction to Plio-Pleistocene deposits in comparison leave little understanding in their evolutionary phylogenies.

Enhydriodon
Scientific classification
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