Enhydriodon

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 (otter) and ὀδούς (tooth) and is not a reference to the genus Enhydra, which includes the modern sea otter (Enhydra lutris). 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 (the dental formula was calculated as ), 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 (P4) and apparent lack of the anterior upper premolar (P1) that is 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 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. 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. 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 (early Pliocene, ca. 5.2-4.0 Ma), 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. 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. 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 (3.44-2.53 Ma). Similar to E. dikikae, the species name was derived directly from the site in which it was recovered. In a September 2022 conference by Alberto Valenciano, Morales, and Pickford (the same month as the research paper on E. omoensis), however, they referred to certain lutrine species previously reclassified to Enhydriodon as Sivaonyx, namely S. hendeyi and S. africana. Classification . It 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. Bunodont otters are defined as large to very large mustelids 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. ==Description==

Skull . Its I3, while larger than its other incisors, is not hypertrophied in size unlike the Enhydriodon's I3. It is generally estimated that some species of Enhydriodon are similar in weight to modern large-sized otters while others are estimated as much larger than them (It should also be noted that weight estimates are more often made for bunodont otters like Enhydriodon than size estimates, although size comparisons to modern animals may be referenced). E. dikikae of Ethiopia was estimated to have weighed minimum and maximum (the latter mentioned to be more likely), its holotype suggesting a bearlike size. Compared with most other Enhydriodon or Enhydra species, it had an estimated skull length of about . E. omoensis was later estimated to weigh more than , making it heavier than E. dikikae and modern lions. According to Grohé et al., E. omoensis'' was potentially "lion-sized", making it the largest mustelid species to ever exist. ==Palaeobiology==

As fossil bunodont otter genera including Enhydriodon generally lack complete specimens and postcranial elements, their locomotion and ecological niches remain uncertain. A common theory of the Indian subcontinental species of Enhydriodon is that based on their robust, bunodont dentition similar to Enhydra, E. falconeri and E. sivalensis were both specialized for commonly eating shellfish. Several catfish genera were present in Africa starting from their first appearances during the late Miocene coinciding with the presence of Enhydriodon, including the extant genera Clarotes, Bagrus, Auchenoglanis, and Chrysichthys and the extinct genus Nkondobagrus. In contrast to the slow-moving, abundant catfish, crabs in Africa were excluded as potential prey for African species of Enhydriodon given the lack of fossilized crabs at Dikika, unlikeliness for biomasses of crabs to support populations of large otters, and apparent incompatibility for enamel dentition. Fast-swimming fish might have been unlikely to have been regular food sources due to the specialized dentition for crushing hard food in addition to large animals likely not having the ability to catch fast prey. Other armored prey, such as juvenile crocodiles, turtles, and ostrich eggs, were also suggested prey of E. dikikae. Because of the hypothesis that S. beyi was a terrestrial predator, E. dikikae is speculated to have been mostly terrestrial based on its shared fossil location with both aquatic and terrestrial fauna at Dikika. It is also pointed out that African species of bunodont otters like Enhydriodon and Sivaonyx were always found in sites in association with permanent bodies of water as opposed to the Upper Laetolil Beds in Laetoli, Tanzania which lacked such a feature, putting a question to the extent of the possibly terrestrial lifestyle of African Enhydriodon and Sivaonyx species. E. hendeyi was analysed based on femoral robustness index (FRI) and the femoral epicondylar index (FEI), in which its FRI value is comparable to the extinct S. beyi, Enhydritherium, and Satherium (the latter two which are analogous to the large sea otter and giant river otter (Pteronura brasiliensis) respectively and have larger values in femoral indexes than most other extant otters) while its FEI value is analogous to the extant African clawless otter and Asian small-clawed otter (Aonyx cinereus). Since both the African clawless otter and Asian small-clawed otter are typically less associated with water bodies compared to other extant otters, it is hypothesized that E. hendeyi and S. beyi were both semiaquatic locomotors that had lower associations with water than aquatic locomotors Enhydritherium and Satherium, although S. beyi was said to be more terrestrial than E. hendeyi. Meanwhile, the lowest values correspond with E. dikikae, which has similar values to terrestrial semifossorial (adapted to digging and living somewhat underground) musteloids such as the American badger and the striped skunk, thereby reinforcing the hypothesis that E. dikikae was a more generalized terrestrial mustelid similar to S. beyi. With the overall lack of consensus on the lifestyle of African Enhydriodon species considered, a 2022 study on E. omoensis measured the stable carbon and oxygen isotope ratios of Enhydriodon species in comparison to extant terrestrial mammals such as felids, hyaenids, and bovids along with semiaquatic mammals such as hippopotamids. The authors explained that using oxygen isotopic ratios, or δ18O, can be used to understand a taxon's dependency on water, in which extant aquatic and semiaquatic taxa, which includes river and sea otters, have significantly lower oxygen isotopic deviations compared to terrestrial carnivorans. The researchers who studied E. omoensis found that its tooth enamel δ18O values had a standard deviation of 2.7%, falling outside the δ18O standard deviations of the sea otter, and the North American river otter (Lontra canadensis), which were recorded to be 0.6% and 0.3%-0.9% respectively. The standard deviation of Omo Enhydriodon aligns itself more within the range of extant terrestrial carnivorans such as hyaenids, suggesting that E. omoensis was not as semiaquatic as initially thought. The results of the study contradict the 2008 assumption that the Omo Enhydriodon species was aquatic. Grohe et al. initially considered that the diet of Enhydriodon could have been the oyster Etheria elliptica, which was present in the continent at the same time range. Based on investigations using carbon stable isotopes, a diet of pure oysters would result in an enamel δ13C value of −11.3%. The diet of E. omoensis, however, was not based purely on Etheria as its minimum-maximum carbon values (-9.7% to -4.7%) are ~2-7% more positive than the expected pure oyster diet value. Its enamel δ13C values fall within the range of mixed C3-C4 feeders, only partly falling within the range of diets of aquatic feeders of C3 plants such as fish, turtles, or bivalves. The δ13C standard deviation of Omo Enhydriodon, however, falls outside the range of studied extant freshwater otter populations. It is instead considered that E. omoensis consumed terrestrial prey with a C4 diet at least semi-regularly via hunting and/or scavenging. The large bunodont dentition of the species suggests durophageous abilities that allowed it to feed on carrion, including bones, in potentially a similar manner to hyeanas or bone-crushing mustelids. ==Palaeoecology==

Pakistan and India gigantea, a species of percrocutine hyaenid, which lived in the Indian subcontinent and coexisted with E. falconeri'' and other hyaenids during the late Miocene E. falconeri and E. sivalensis, while both Enhydriodon species that were present in the Siwalik Hills in India and Pakistan during the Neogene period, did not coexist for the same epochs based on their formation deposit appearances. E. falconeri remains were present at the Nagri Formation (Dhok Milan and Sethi Nagri, Pakistan) and the Dhok Pathan Formation (Dhok Pathan and Hasnot, Pakistan), both formations dating back to the middle Siwaliks representing late Miocene. The species was also present at the Tatrot Formation (Tatrot, India) of the Upper Siwaliks from the early or middle Pliocene. In the Nagri and Dhok Pathan Formations, E. falconeri was shown to have existed with several archaic mammalian carnivorous families that went extinct before the Pliocene, such as hyainailourine hyaenodonts and amphicyonids. The early otter species also existed with various extinct carnivorous members of extant families during the late Miocene representing other mustelids, ursids, felids (felines and machairodontines), hyaenids (percrocutinae hyaenids ictitheres, and hyaenines), viverrids, and herpestids. It is suggested that the extinction of the amphicyonids and percrocutids left empty predatory niches that were quickly filled by other hyaenid genera, which became highly diversified and coexisted with felids in the subcontinent. Other extinct members of extant and extinct mammalian families were found in the Nagri Formation and thereby existed with E. falconeri including bovids, giraffids, anthracotheres, tragulids, suids, hipparionine equids, rhinocerotids, chalicotheres, gomphotheres, hominids, and spalacids. An extinct reptilian species of gharial, Gavialis lewisi (?), is reported from the Dhok Pathan Formation of Pakistan and is Pliocene in age. Mammal genera that were found in the Dhok Pathan Formation are generally consistent with the mammal genera found within the Nagri Formation but also include other bovids, giraffids, cervine cervids, anthracotheres, suids, hipparionines, rhinocerotids, "tetralophodont gomphotheres", cercopithecids, and hystricids. equids such as Hipparion to Eurasia are representative of major Eurasian faunal turnovers of the late Miocene The transition from the middle Miocene to the late Miocene reflected a period in which the evergreen to deciduous tropical forests once covering a large part of the Indian subcontinent shrank and were replaced by grasslands because of global cooling, drier conditions, and the intensification of Asian monsoons. A change from the Nagri floodplains to the Dhok Pathan floodplains suggests less draining in the fluvial system of the latter compared to the former with Dhok Pathan's smaller rivers having more seasonal flow than before. This reflects the general trend of late Miocene climate forcing resulting in more seasonality, bringing about large faunal turnovers. The drier and more seasonal climates along with fluvial changes gradually brought about larger, open woodlands predominantly consisting of C4 plants near the Potwar Miocene rivers while communities exclusively or predominantly consisting of C3 plants diminished greatly and eventually disappeared by 7.0 Ma along with the C3 feeders that depended more on closed vegetation. These changes occurred shortly after the arrivals of the hipparionines and marked decreases in mammal groups within the Indian subcontinent such as the extinctions of the hominid Sivapithecus and the deinothere Deinotherium as a result of the fragmentation of closed habitats in favour of open habitats that would eliminate food for C3 browsers and frugivores. was a typical grazer of C4 plants from the Pliocene-Pleistocene. It adapted its diets to mixed feeding of C3 plants by middle Pleistocene while Stegodon'' was a consistent C4 browser that failed to adapt and went extinct. The carnivoran fossil records of the Tatrot Formation in India are scarce, but amongst the extinct members that existed with E. falconeri in the Pliocene were other lutrines, machairodontines, and hyaenids. The crocodilians Crocodylus and Rhamphosuchus, the pelican Pelecanus, turtles (Batagur, Geoclemys, Hardella, and Pangshura), and the freshwater crab Acanthopotamon are reported from at least the Tatrot or Pinjor Formations of India as well, indicating an active freshwater habitat that E. falconeri and later E. sivalensis were present in. Amongst carnivoran taxa, Enhydriodon is the longest-lasting caniform genus to have ever existed within the Siwaliks of the Indian subcontinent, identified from the Nagri-Pinjor formations. However, the species identified within the Pinjor Formation of the Plio-Pleistocene epochs is E. sivalensis, which suggests that E. falconeri after a long time of relative success eventually might have gone through anagenesis by the Pliocene. Other carnivoran genera that were found in the Pinjor Formation included the newly arrived canids as well as mustelids, ursids, felids (machairodontines, pantherines, and felines), hyaenids, and viverrids. Ethiopia E. dikikae and E. omoensis were large lutrine species found in different locations within modern-day Ethiopia. E. dikikae fossils were found within the bottom two sequences of the Hadar Formation of the Lower Awash Valley, Ethiopia, indicating that its fossils range from 4 Ma to 3.2 Ma. Fossils of E. omoensis were located at the Usno Formation and Shungura Formation of the Lower Omo Valley in Ethiopia, the fossils ranging from 3.44 Ma to 2.53 Ma. E. dikikae was named after the Dikikae Basal Member of the Hadar Formation while E. omoensis had its name derived from the Lower Omo Valley. E. dikikae fossils are known from the formation's Basal and Sigi Hakoma members and are unknown in the other top two members. The Sidi Hakoma Submember 1 (SH-1), ranging from ~3.45 to 3.35 Ma, had similar fauna and thereby similar habitats to other members within the Hadar Formation but also likely included wetlands in certain regions. Taxa such as a species within the forest-dwelling Cephalophini tribe and five species of primates were recovered from the member, further indicating a large riverine forest with, predominantly, woodlands in the surrounding area. Aepyceros was the most abundant bovid, and SH-1 had the lowest proportion of grazing bovids at any sub-member of the Hadar Formation. The vegetation of SH-1 might have closely resembled those at the Guinea or Sudanese savannas that interdigitate with the central African rainforest, which creates habitat mosaics of grasslands, woodlands, and some forest belts. The ostracod assemblage of the Basal and Sidi Hakoma Members indicate sources of freshwater input, in which their shells also indicate only a three-month dry season, characteristic of the central African savannas. The single dry season, indicating a nine-month rainy season, is a distinctive factor of the Sidi Hakoma member from the modern climate in East Africa, which has a bimodal dry season format (two dry seasons) rather than a single one. The Sidi Hakoma Submember 2 (SH-2) is similar to SH-1 and is thought to have been associated with woodlands with some grassy plains, of which Aepycerotini was the most common. The Hadar Formation is also known for its representation of a great diversity of bovid species that represented most major tribes in Africa. The bovid tribes that were found in the formation included the Aepycerotini, Alcelaphini, Antilopini, Bovini, Caprini, Cephalophini, Hippotragini, Neotragini, Reduncini, and Tragelaphini. Other groups, represented by extinct species of extant or extinct genera, include giraffids, hippopotamuses, suids, canids, felids (machairodontines were the most common), hyaenids, other mustelids, viverrids, rhinoceroses, equids, catarrhines, deinotheres, and elephantids were all also found within the locality. Small mammal groups include bats, leporids, old world porcupines, murid rodents, spalacids, squirrels, and aardvarks. Fossil fish remains are also known from the Shungura Formation, namely the genera Polypterus, Sindacharax, Synodontis, Auchenoglanis, and Lates. ==Notes==