Enhydriodon Sivalensis Sketch Cranium

''Enhydriodon'' is an extinct genus of typically large otters that lived in what is now Kenya, Ethiopia, Uganda, Chad, South Africa, Pakistan, and India from the late Miocene up 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 while ''E. omoensis'' was described to be lion-sized, making them the largest mustelids described so far. Most species of the ''Enhydriodon'' genus 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.

Research history

''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. 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 has a similar derivation. According to Falconer, the Siwalik Hill fossils belonging to ''E. sivalensis'' were previously sorted under the name ''Amyxodon'' in 1835. Falconer calculated the dental formulas of '' Lutra'' and ''Enhydra'' as and , 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.

During the 19th and 20th centuries, more debated species of ''Enhydriodon'' such as ''E. campanii'' were introduced and more bunodont otter genera such as ''Sivaonyx'' and ''Vishnuonyx'' were described, creating 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''. 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 (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 fossil teeth being located in South Africa and the first described species from the continent of Africa. 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 largely reclassified into a new genus named ''Paludolutra'' (Hürzeler & Engesser, 1976), although the newer genus name remained relatively obscure in the paleontological record until Pickford used it in his studies of bunodont otters.

The taxonomies of individual otter species and genera continued to evolve into the 21st century as more prehistoric otter species were being discovered while paleontologists continually debated the assignment of species (to genera). For instance, ''Paludolutra'' was originally reclassiied as a sub-genus of ''Enhydriodon'' by Willemsen in 1999, but in 2005, Jorge Morales and Martin Pickford noted that the dental morphology of the ''Paludolutra'' genus was distinct enough to be its own genus (meaning that ''P. campanii'', ''P. lluecai'', and ''P. maremmana'' would no longer be sorted under ''Enhydriodon'' by technicality). Eventually, Paludolutra was considered a separate genus again. ''E. africanus'', ''E. ekecaman'', ''E. hendeyi'', and ''E. kamuhangirei'' were all initially classified into the ''Sivaonyx'' genus, with the latter 3 being sorted around their respective times of discovery. The reclassification of African fossil bunodont otters into the ''Sivaonyx'' genus had brought about continuous debate regarding the practicality of the differences between ''Enhydriodon'' and ''Sivaonyx'', with some researchers citing neutrality due to preferred focuses on researching the individual species instead of their genus placements. In 2022, the four species were eventually reclassified into the ''Enhydriodon'' genus. ''E. soriae'' was also initially sorted unto the Sivaonyx genus but was eventually assigned to the Enhydriodon genus, 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''. ''Enhydra'' was explicitly excluded from the ''Enhydriodontini'' tribe despite its similarities, and ''Paludolutra'' was later reclassified as a sister taxon to the tribe. In 2007, Pickford corrected the supposed species ''E. aethiopicus'' (previously described in 2004) by transferring it to ''Pseudocivetta ingens'', an extinct member of the Viverridae family.

In 2011, a team of researchers described ''E. dikikae'' based on its remains of a partial skull and femurs in the Lower Awash of Dikika, Ethiopia. It was described as having a notably heavier skull (albeit broken) than other ''Enhydriodon'' species or the modern sea otter. It was deemed as the largest species of ''Enhydriodon'' until another species whose remains were also found in Ethiopia, ''E. omoensis'', was described from the Lower Omo Valley in 2022.

Classification

''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 it being a primary source of comparisons to other bunodont otter genera. It is generally thought that the ''Enhydriodon'' was a result of a Miocene-Pleistocene trend that gave prehistoric otters bunodont teeth and large sizes compared to their extant relatives. 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):

As shown in the above phylogeny, ''Enhydriodon'' shared 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 ''Enhydra'' is a separate clade (''Paralutra jaegeri'' was proposed as a separate clade as well). Regardless, they argued that ''Enhydra'' is closer to the ''Enhydriodontini'' tribe (''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.

Description

Size

Some ''Enhydriodon'' species, particularly a few that had resided in Africa, are the largest known mustelids to have ever existed based on weight estimates, but their precise sizes and weights remain unknown given the lack of complete specimens in their fossil records. Some species like ''E. latipes(?)'' are poorly studied compared to others and therefore lack confirmed size or weight estimates. 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).

The two species of ''Enhydriodon'' native to the subcontinent of India had modest weight estimates, comparable with most other bunodont otter genera as well as extant otter genera. Dr. Hush Falconer's 1868 memoir described ''E. sivalensis'' as a lutrine animal the size of a panther. In 1932, Guy Pilgrim diagnosed ''E. falconeri'' as being smaller than ''E. sivalensis'', although no size or weight estimates were offered for it by him. In 2007, Martin Pickford estimated ''E. sivalensis'' to be the largest prehistoric otter in India, ranging from minimum to maximum in body weight, its skull possibly being wolf-sized. He also estimated the body of ''E. falconeri'' based on its lower m1 teeth dimensions to be similar to the African clawless otter (''A. capensis''), averaging to .

Africa's ''Enhydriodon'' species are estimated to be some of the largest species of otters to ever exist, reflecting on the Miocene-Pleistocene trend of bunodont otters growing larger than their nonbunodont cousins. Dr. Pickford described ''E. kamuhangirei'' of the Western Rift Valley, Uganda (at the time ''Sivaonyx kamuhangirei'') to possibly exceed in weight, making it the largest-known prehistoric otter at the time, although he mentioned that the undescribed fossil otters in Ethiopia (likely sorted later under ''E. dikikae'' and/or ''E. omoensis'') could've possibly been larger than it. ''E. africanus'', while also not estimated in total body mass or weight, is suggested to be likely larger than the modern African clawless otter based on its femur's overall length of being greater than the mean length of from four of the ''A. capensis'' specimens. ''E. hendeyi'' (then ''Sivaonyx hendeyi'' prior to 2022) was estimated to be wolf-sized and around . ''E. dikikae'' of Ethiopia is estimated to weigh 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. It is also said to potentially be "lion-sized," making it the largest mustelid species to ever exist.

Skull

There are currently only two known partial skulls that are attributed to ''Enhydriodon'': one of ''E. sivalensis'' of the Siwalik Hills and the other of ''E. dikikae'' of the Awash Valley.

Dentition

''Enhydriodon's'' dentition is well-defined by its broad, bunodont carnassials in the molars and premolars similar to the modern sea otter. The ''Enhydriodon'' and ''Sivaonyx'' species differences are usually attributed to dentition, so the premolar teeth or molar teeth fossils are examined to discern the two bunodont otter genera. The proposed genera differences (larger P⁴ hypocone, conical post-protocone cusps, and apparent lack of anterior upper premolars for ''Enhydriodon'') by tooth measurements have been difficult to prove due to the fragmentary nature of the fossils and relative inconsistencies of tooth measurements/dimensions by species. The reclassification of all "African ''Sivaonyx''" species other than ''S. beyi'' to ''Enhydriodon'' in 2022 has been attributed to " metaconid higher than the protoconid on M₁, presence of a carnassial notch and one or more cusps between the protocone and the hypocone on P⁴, and/or distolingual expansion on M¹."

''Enhydriodon'' as the latest-appearing genus is suggested to have the most bunodont dentition of the Enhydriodontini tribe, which includes the earliest-appearing ''Vishnuonyx'' and then ''Sivaonyx''. ''Enhydriodon's'' dentition suggests a near suppression of carnassial functions in favor of crushing as the predominant function. The I³ (or third upper incisor) of ''Enhydriodon'' is much larger than its I¹ (smallest incisor) and I², appearing larger and more canine-like in comparison to ''Paludolutra'' and ''Enhydra''. In comparison for most otters where the upper incisor is known, their third incisors are only marginally larger than their first and second incisors. The large I³ trait applies to ''E. dikikae'', which was described after Pickford's general description of the ''Enhydriodon'' genus as having a much larger I³ than I¹ - I² and being more conical in shape.

References

{{Taxonbar, from=Q27962593

Otters
Prehistoric mustelids
Miocene mustelids
Fossil taxa described in 1868
Miocene mammals of Africa
Pliocene mammals of Africa
Pleistocene mammals of Africa
Miocene mammals of Asia
Pliocene mammals of Asia
Taxa named by Hugh Falconer
Prehistoric carnivoran genera