Enhydriodon

''Enhydriodon'' is an extinct genus of otters known from Africa, Pakistan, and India 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 (which also contains '' Sivaonyx'' and '' Vishnuonyx'') 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 (or third incisors) are canine-like and much larger than its other incisors (although shorter than its canines), 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 (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 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 (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.

''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 (albeit broken) 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. 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

''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 ''Enhydriodon'' was a result of a Miocene-Pleistocene trend that gave prehistoric lutrines bunodont teeth and large sizes compared to their extant relatives. It is classified as a member of the bunodont otters group, a categorical term commonly used by researchers that also includes ''Sivaonyx'', ''Paludolutra'', ''Vishnuonyx'', '' Torolutra'', '' Enhydritherium'', '' Djourabus'', '' Paralutra'', '' Tyrrhenolutra'', '' Siamogale'' and ''Enhydra''. 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. Despite sharing the feature of bunodont dentition, there are at least several clades of lutrines belonging to this category rather than one, so the term "bunodont otters" therefore is categorical rather than taxonomic and covers lutrines during the same periods with similar dentitions rather than one that directly defines their taxonomic state.

The following cladogram by Xiaoming Wang et al. in 2018 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 beginning from "''Paralutra jaegeri''"):

As shown in the above phylogeny, ''Enhydriodon'' shared a closer morphology with its other extinct relatives and ''Enhydra'' than the other extant lutrines 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'' as an isolated clade separate from typical members of Enhydriodontini ("''Paralutra''" ''jaegeri'' was proposed as an isolated clade from ''Siamogale'' as well). Regardless, they argued that ''Enhydra'' is closer to the clade composing of ''Enhydriodon'', ''Sivaonyx'', and ''Vishnuonyx'' than any other bunodont otter genus. The researchers explained that the acquisition of bunodont dentition occurred at least three times in the evolution of lutrines, reflected by the phylogeny tree's clades: in ''Sivaonyx''-''Enhydriodon''-''Enhydra'', in ''Paludolutra''-''Enhydritherium'', and in ''Siamogale''. Non-bunodont otters likely branched out separately 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

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. It is currently unknown whether the skulls' features of either species are well-representative of other species of ''Enhydriodon'', but the known ''E. dikikae'' and ''E. sivalensis'' skulls have somewhat different features from each other.

The ''E. sivalensis'' skull, identified as belonging to a fully-grown individual, is relatively well-preserved with identifiable temporal crests, frontal, maxillae, premaxillae, nasal, muzzle, and palatine bone parts. However, it has also suffered from wear and being slightly twisted clockwise. Most notably, the dental arch is complete, although the left M¹ and left I¹ are both missing and most of the teeth are broken from their crowns. It has a large brain case, a broad and short muzzle, and a large nasal opening. Outlines of the orbits around the skull's frontals can also be identified.

The broken skull belonging to ''E. dikikae'' contains a short and non-prognathic snout, parts of the orbits, a nearly complete upper dental arch that is missing both I¹s and a right I², and part of the lower jaw. The muzzle on the ''E. dikikae'' skull is short, a small anterior orbital border positioned just above the posterior side of the canine. The front part of the snout is identified as short, thereby comparable with the snout of ''Enhydra''. Although the evolution of bunodont otters like ''Enhydriodon'' is unclear, it is proposed that ''E. dikikae's'' short snout and very large canine size both clearly make the species different-looking and more evolutionarily derived (or evolutionarily recent) than ''E. sivalensis''.

Dentition

''Enhydriodon's'' dentition is well-defined by its extremely 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 generic 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 favour 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 to other bunodont lutrine genera where the upper incisor is known, its third incisors are only marginally larger than their first and second incisors. The right I¹ of a skull of E. sivalensis, for instance, measures in anteroposterior diameter (APD) and in transverse diameter (TD). The skull's right I² measures in APD and in TD. In comparison, the right I³ is the largest incisor of the holotype, with measurements of in APD and in TD (the canines are larger than the incisors, measuring in APD and in TD). The large I³ trait also 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. DIK-56's I³ tooth measures in mesiodistal width (MD) and in buccolingual width (BL) compared to its I² measurements of in MD and in BL. Like ''E. sivalensis'', the I³ is shorter than the canines, with C¹ measuring in MD plus in BL and C₁ measuring in MD and in BL.

Limbs

Postcranial remains of bunodont otters, including ''Enhydriodon'', are scarce, leaving too little information on the overall anatomies of many genera. The only known species of ''Enhydriodon'' with postcranial remains are ''E. hendeyi'', ''E. dikikae'', and ''E. omoensis''.

''E. hendeyi'' fossil remains include a fragmentary humerus, an ulna, two femurs, and an astragalus (also known as a talus bone). The femora of ''E. hendeyi'' are smaller than those of ''S. beyi'' and ''E. dikikae'' but also larger than those of ''Enydritherium'', ''Satherium'', and the extant African clawless otter. The astragalus is similar to ''E. omoensis'' but differs by the smaller head and thinner neck as well as a larger distal projection of the bone's tubercle. The talus bone's trochlea (grooved surface forming the joints of bones) is shallow and mediolaterally wide while its tubercle is projected in the approximate center, both of which produce a robust and deep groove (or furrow) of the tendons of the plantar flexion muscles for extension of the foot at the ankle compared to the African clawless otter.

The postcranial remains of ''E. dikikae'' are known by the proximal (upper part) left femur, distal (lower part) right femur, and a humerus. The proximal left femur is known by a large tubercle along the posterior area of the neck, middle-aligning lesser trochanter, and a similar position of the medial condyle of the femur. The femur specimens indicate large overall sizes of the femurs of ''E. dikikae'' compared with even the largest extant otters. The humerus (complete but weathered and cracked) is much longer and slightly robust compared to that of ''Sivaonyx beyi'', and its deltoid tuberosity is well-formed. The lateral supracondylar crest is longer compared to ''S. beyi'' while the medial epicondyle is not as prominent in size. The olecranon fossa is small and more circular compared to ''S. beyi''.

''E. omoensis'' is represented only by a single complete left femur which has a short neck and a round head that is oriented in a proximal direction (close to the center) rather than a medial direction (in the center), the former being shifted at 40° relative to the longitude of the diaphysis section of the bone. The lateromedial width of the epiphysis is narrow. The femur also has a large femoral head located on the ventromedial head (aligning to the middle underside of it), a greater trochanter that bends on the back and is lower than the femoral head, a short and deep trochanteric fossa, and a strong lesser trochanter that is centered more in the middle than on the ventral (or underside) and is thereby visible in a back view. The medial condyle of the femur is larger than the lateral condyle of the same bone. The intercondylar fossa of the femur is rectangular and wide.

Body Mass

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. Falconer's 1868 memoir described ''E. sivalensis'' as a lutrine the size of a panther. In 1932, Pilgrim diagnosed ''E. falconeri'' as being smaller than ''E. sivalensis'', although no size or weight estimates were offered for it by him. In 2007, 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 non-bunodont cousins. 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 have possibly been larger than it. ''E. hendeyi'' (then ''Sivaonyx hendeyi'') was estimated to be wolf-sized and around while''E. africanus'' and ''E. ekecaman'' are thought to be of similar sizes. ''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. This claim was made first by Willemsen from analogies of the diet of ''Enhydra'' (abalones and marine bivales) and '' Aonyx'' (freshwater crabs), but there is little palaeontological evidence to directly support this claim. Regardless, it is suggested that the thick enamel in the posterior dentition of Indian ''Enhydriodon'' species makes them more molluscivorous than cancrivorous (in contrast, Indian ''Sivaonyx'' species are suggested to have combined shearing functions of the carnassials with overall bunodont crowns to prey more on crustaceans, although bivalves could potentially have been secondary prey for it). The possibility of ''Enhydriodon'' preying on bivalves is supported by the presence of fossilized freshwater bivalve genera '' Parreysia'' and '' Lamellidens'' in the same locations as them, both of which are common throughout the entire Siwalik sedimentary column which spans from 15-2 mya, ranging with the presence of the Enhydriodontini tribe in the Indian subcontinent (India and Pakistan).

The larger ''Enhydriodon'' species in the African continent are suggested to have preyed upon a wider variety of foods in addition to their primary prey including softer prey despite their bunodont dentitions, making their potential diets distinct from those of their Indian subcontinental counterparts. One suggested type of prey was large fish with hard external coverings such as catfish. 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''.

Femora and dental remains of African ''Enhydriodon'' could possibly hint at a semiaquatic as well as terrestrial lifestyle, meaning that it could eaten both aquatic prey and terrestrial prey. The speculations of ''Enhydriodon's'' lifestyle, however, have been contradictory to each other, so there is, therefore, no majority consensus on it. In 2008, it was speculated that smaller African species of ''Enhydriodon'' based on their smaller femur sizes were more locomotor generalists similar to most mustelids while larger species were fully aquatic since their femur structures shared similarities to ''Enhydra''. However, the Omo and Hadar femoras' proximal ends pointed to a more aquatic nature than most lutrines, while their relative lengths resembled that of terrestrial generalist mustelids, including semiaquatic otters. ''Sivaonyx beyi'' of Chad, speculated to weigh to , had non-specialized limb proportions that implied generalist-terrestrial locomotion and poor aquatic adaptations. 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. The palaeobiologies and niche partitionings of ''E. ekecaman'' and ''E. cf. dikikae'' in Kanapoi, Kenya remain unclear as their fossil materials, uncovered in the 1960s, were not specifically pronounced beyond "Kanapoi", which future research would have to cover. 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

''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

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