Enhydriodon

Enhydriodon is an extinct genus of mustelids known from Africa, Pakistan, and India that lived from the late Miocene to the early Pleistocene. It contains 9 confirmed species, 2 debated species, and at least a few other undescribed species from Africa. The genus belongs to the tribe Enhydriodontini (which also contains Sivaonyx and Vishnuonyx) in the otter subfamily Lutrinae. Enhydriodon means “otter tooth” in Ancient Greek and is a reference to its dentition rather than to the Enhydra genus.

Enhydriodon
Temporal range: Late Miocene to Early Pleistocene,
Enhydriodon omoensis right femur faced at different sides.
Scientific classification e
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. falconeri Pilgrim, 1931
  • † E. latipes? 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

The exact sizes and lengths of Enhydriodon species are unknown given the lack of complete fossils of it and most related fossil otters. 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 estimated to weight over 100 kg (220 lb), making them the largest known mustelids to exist, although a lack of complete specimens makes precise estimates impossible.

Enhydriodon is primarily studied for its dentition, its broad, bunodont carnassials seemingly emphasizing diets that would be consumed by crushing prey rather than shearing them like the modern sea otter and unlike most other extant otters. As such, it is classified as a member of the bunodont otters group, 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 unseen in extant and extinct otter genera. It is hypothesized that Indian species of Enhydriodon are semiaquatic and consumed bivalves because their bunodont dentitions would've 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've 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 disputes with similar bunodont otter genera like Sivaonyx and Paludolutra up to the modern day, although Paludolutra is today considered a distinct genus not closely related to Enhydriodon. Today, 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're closely related remains debated.

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 by Hugh Falconer when he erected the genus based on several craniums attributed to E. sivalensis in Siwalik Hills, India.[1][2] 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 has a similar derivation and is where the sea otter is classified. According to Falconer, the Siwalik Hill fossils belonging to E. sivalensis were previously classified under the name Amyxodon in an 1835 synopsis of the fossil genera in the Siwalik Hills that he and Proby Cautley found, in which the fossil taxon was considered to be a carnivoran of an unknown family and contained the sole species "A. sivalensis". As a result of the rename, Amyxodon had been considered a synonym of Enhydriodon despite its status as the older genus name. 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.[1][3]

During the 19th and 20th centuries, more species of Enhydriodon such as E. campanii were introduced and more otter 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, 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).[4] 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.[5] 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.[6] 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.[7][8]

The taxonomies of individual otter 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.[5] However, in January of 2005, Martin Pickford, Dolores Soria, and Jorge Morales 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.[9] In December of the same year, Morales and Pickford instead described Paludolutra as a distinct genus that might be related to Sivaonyx based on dentition convergences.[8] 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.[10][11] 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 of 2005.[12][8]

E. africanus, E. ekecaman, and E. hendeyi were reclassified into the Sivaonyx genus by Pickford and Morales in December of 2005, where they additionally described a new species Sivaonyx kamuhangirei.[8] 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 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 the Enhydriodon genus in a research paper by Camille Grohé et. al. E. soriae was also initially sorted unto Sivaonyx but was eventually assigned to the Enhydriodon, although its genus placement remains disputed.[13][14] 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 2017, Enhydra was explicitly excluded from the Enhydriodontini tribe despite its similarities, and Paludolutra was reclassified as a sister taxon to the tribe.[8][11] 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.[10]

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. [15] 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.[14]

Classification
Enhydriodon's closest extant relative, the sea otter. 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.[8] 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, a categorical term commonly used by researchers that also includes Sivaonyx, Paludolutra, Vishnuonyx, Torolutra, Enhydritherium, Djourabus, Paralutra, Tyrrhenolutra, Siamogale and Enhydra.[11][16][17] 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.[13][16] Despite sharing the feature of bunodont dentition, there are at least several clades of bunodont otters belonging to this category rather than one, making the term a categorical one of otters during the same periods with similar dentitions rather than one that directly defines their taxonomic state.[11]

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):[11]

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, 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. The researchers explained that the acquisition of bunodont dentition occurred at least three times in the evolution of otters, reflected by the phylogeny tree's clades: in Sivaonyx-Enhydriodon-Enhydra, in Paludolutra-Enhydritherium, and in Siamogale.[11] 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.[18]

Description

Size
Skeleton of Enhydritherium, a bunodont otter genus, in a bipedal position. Bunodont otters including Enhydra, Enhydritherium and Enhydriodon are typically estimated to be larger/heavier than nonbunodont otters.

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.[19] 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).[14]

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.[1] 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.[4] In 2007, Martin Pickford estimated E. sivalensis to be the largest prehistoric otter in India, ranging from 22 kg (49 lb) minimum to 25 kg (55 lb) 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 16 kg (35 lb).[10]

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 100 kg (220 lb) 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.[10] E. hendeyi (then Sivaonyx hendeyi prior to 2022) was estimated to be wolf-sized and around 40 kg (88 lb).[20] E. africanus and E. ekecaman are estimated to be of similar sizes to E. hendeyi.[14] E. dikikae of Ethiopia is estimated to weigh 100 kg (220 lb) minimum and 200 kg (440 lb) 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 25 cm (9.8 in).[15] E. omoensis was later estimated to weigh more than 200 kg (440 lb), 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.[14]

Dentition
Lower jaw dentitions of E. hendeyi (A-C) and E. africanus (D-F)

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.[10][15] The reclassification of all "African Sivaonyx" species other than S. beyi to Enhydriodon in 2022 has been attributed to "[a] 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."[14]

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 for most otters where the upper incisor is known, their third incisors are only marginally larger than their first and second incisors.[10] The right I1 of a skull of E. sivalensis, for instance, measures 3 mm (0.12 in) in anteroposterior diameter (APD) and 4.5 mm (0.18 in) in transverse diameter (TD). The skull's right I2 measures 5.2 mm (0.20 in) in APD and 5.5 mm (0.22 in) in TD. In comparison, the right I3 is the largest incisor of the holotype, with measurements of 10.5 mm (0.41 in) in APD and 8 mm (0.31 in) in TD (the canines are larger than the incisors, measuring 17.1 mm (0.67 in) in APD and 13.8 mm (0.54 in) in TD).[2] 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 12.4 mm (0.49 in) in mesiodistal width (MD) and 11.6 mm (0.46 in) in buccolingual width (BL) compared to its I2 measurements of 5.5 mm (0.22 in) in MD and 9.7 mm (0.38 in) in BL. Like E. sivalensis, the I3 is shorter than the canines, with C1 measuring 16.9 mm (0.67 in) in MD plus 15 mm (0.59 in) in BL and C1 measuring 19.5 mm (0.77 in) in MD and 15.3 mm (0.60 in) in BL.[15]

Measurements (mm & in) of the teeth of Enhydriodon falconeri and Enhydriodon sivalensis of the Indian Subcontinent (~ = estimated measure)[10]
Specimen Species Tooth Length Breadth Locality
NHM M 4847 E. falconeri P4 14.9 mm (0.59 in) 15.8 mm (0.62 in) Siwalik
NHM M 15397 E. falconeri M1 18.6 mm (0.73 in) 10.9 mm (0.43 in) Tatrot
GSI NRV 2/468 E. sivalensis I1 right 2.7 mm (0.11 in) 3.4 mm (0.13 in) Saketi
GSI NRV 2/468 E. sivalensis I2 right 4.6 mm (0.18 in) 4.5 mm (0.18 in) Saketi
GSI NRV 2/468 E. sivalensis I2 left 5.1 mm (0.20 in) 5.5 mm (0.22 in) Saketi
GSI NRV 2/468 E. sivalensis I3 root right 8 mm (0.31 in) 7 mm (0.28 in) Saketi
GSI NRV 2/468 E. sivalensis I3 root left 10 mm (0.39 in) 10 mm (0.39 in) Saketi
GSI NRV 2/468 E. sivalensis C1 root right 15 mm (0.59 in) 13.1 mm (0.52 in) Saketi
GSI NRV 2/468 E. sivalensis C1 root left 16 mm (0.63 in) 12.8 mm (0.50 in) Saketi
GSI NRV 2/468 E. sivalensis P3 right 10.2 mm (0.40 in) 9.6 mm (0.38 in) Saketi
GSI NRV 2/468 E. sivalensis P4 right 17.3 mm (0.68 in) 18.6 mm (0.73 in) Saketi
GSI NRV 2/468 E. sivalensis M1 right ~14.0 mm (0.55 in) ~20.3 mm (0.80 in) Saketi
NHM M 37153 E. sivalensis P4 right ~16.7 mm (0.66 in) ~17.2 mm (0.68 in) Siwalik
NHM M 37154 E. sivalensis P4 (left & right) 17.6 mm (0.69 in) 18.9 mm (0.74 in) Siwalik
NHM M 37155 E. sivalensis P4 left ~18.2 mm (0.72 in) 18.5 mm (0.73 in) Siwalik
GSI RCS 777A cast E. sivalensis P4 left 16.5 mm (0.65 in) 18.4 mm (0.72 in) Siwalik
GSI RCS 777A cast E. sivalensis P4 right 17.6 mm (0.69 in) 18.6 mm (0.73 in) Siwalik
GSI RCS 777A cast E. sivalensis M1 right 14.3 mm (0.56 in) 19.7 mm (0.78 in) Siwalik
GSI RCS 777A cast E. sivalensis M1 left 14.3 mm (0.56 in) 19.7 mm (0.78 in) Siwalik
GSI D 161 E. sivalensis M1 right 21.6 mm (0.85 in) 12.7 mm (0.50 in) Hasnot
IPSMG 1949.187 E. sivalensis M1 right ~22 mm (0.87 in) ~13 mm (0.51 in) Siwalik
Measurements (mm & in) of the lower teeth of Enhydriodon species of the African continent (~ = estimated measure)[14]
Specimen Species I3 P3 P4 M1 trigonid length, paraconid to metaconid M1 trigonid length, paraconid to protoconid M1 trigonid width M1 talonid width M2
B 426 E. omoensis 6.9 mm (0.27 in) x 7 mm (0.28 in) - - - - - - -
W 8-4S E. omoensis - 10.4 mm (0.41 in) x 8.1 mm (0.32 in) - - - - - -
L 2-10012 E. omoensis - - 18.2 mm (0.72 in) x 13.4 mm (0.53 in) - - - - -
B-376 E. omoensis - - 18.7 mm (0.74 in) x 13.8 mm (0.54 in) - - - - -
L 56-1 E. omoensis - - - 18.1 mm (0.71 in) 18.3 mm (0.72 in) 17.9 mm (0.70 in) 18.3 mm (0.72 in) -
OMO 18-1972-99 E. omoensis - - - ~31.7 mm (1.25 in) - - ~19.1 mm (0.75 in) -
L 2-148 E. omoensis - - - - - - - 10.1 mm (0.40 in) x 12.4 mm (0.49 in)
OMO 3/0 10084 Enhydriodon species - - - - - - - 10.1 mm (0.40 in) x 12.4 mm (0.49 in)
BAR 1984’05 E. soriae - - - - - - 10.6 -
KNM-LU 337 (holotype) E. soriae - - - - - - 10.5 -
SAM-PQL 50000A (holotype) E. hendeyi - ~7.7 mm (0.30 in) x ~5.2 mm (0.20 in) 12 mm × 9.4 mm (0.47 in × 0.37 in) - - - 13.9 mm (0.55 in) 8.1 mm (0.32 in) x 10.3 mm (0.41 in)
SAM-PQL-9138 E. hendeyi - ~7.8 mm (0.31 in) x ~4.6 mm (0.18 in) 13.8 mm (0.54 in) x 9.9 mm (0.39 in) - - - ~12.8 mm (0.50 in) ~9.2 mm (0.36 in) x ~7.2 mm (0.28 in)
KNM-KP 10034B (holotype) E. ekecaman - - - - - - 13.3 mm (0.52 in) -
BAR 720’03 E. ekecaman - - 11.1 mm (0.44 in) x 8.4 mm (0.33 in) - - 12.8 mm (0.50 in) - -
BAR 567’05 E. ekecaman - - - - - - 13 mm (0.51 in) -
unnumbered holotype E. kamuhangirei - - - - - - 15.9 mm (0.63 in) -
BSPG 1930 XI 1 (holotype) E. africanus - - 12 mm (0.47 in) x 9.1 mm (0.36 in) 14 mm (0.55 in) - - - -
KNM-ER 3110 (holotype) E. afman - - - 14.9 mm (0.59 in) - 15.8 mm (0.62 in) 16.5 mm (0.65 in) -
KNM-ER 3108 E. cf. afman - - - - - - - 11.8 mm (0.46 in) x 13.9 mm (0.55 in)
DIK-56-9 (holotype) E. dikikae - - 16.9 mm (0.67 in) x 11.9 mm (0.47 in) - - - ~20 mm (0.79 in) -
DIK-24-15 E. dikikae - - - - - - 16.2 mm (0.64 in) -
Measurements (mm & in) of the upper teeth of Enhydriodon species of the African continent (~ = estimated measure)[14]
Specimen Species P4 mesiodistal length P4 linguolabial length M1 mesiodistal length M1 linguolabial length
P 791-18 (holotype) E. omoensis ~25.8 mm (1.02 in) ~26.7 mm (1.05 in) - -
L 23-10021 E. omoensis - - 20.6 mm (0.81 in) 27.05 mm (1.065 in)
W 717 E. omoensis - - ~15.1 mm (0.59 in) ~12.6 mm (0.50 in)
BAR 1720’00 E. soriae 14.8 mm (0.58 in) 15 mm (0.59 in) - -
BAR 1082’01 E. soriae - - 12.2 mm (0.48 in) -
SAM-PQL 50000B (holotype) E. hendeyi 16.9 mm (0.67 in) 17.4 mm (0.69 in) - -
KNM-KP 10034A (holotype) E. ekecaman 16.5 mm (0.65 in) (labial) - - -
KNM-KP 10034C (holotype) E. ekecaman - - 15.8 mm (0.62 in) 19.8 mm (0.78 in)
BAR 566’05 E. ekecaman 14.7 mm (0.58 in) 17.5 mm (0.69 in) - -
BSPG 1930 XI 1 (holotype) E. africanus - - - -
DIK-56-9 (holotype) E. dikikae 20.5 mm (0.81 in) 22.7 mm (0.89 in) 21.4 mm (0.84 in) 25.9 mm (1.02 in)
KNM-KP 49887 E. cf dikikae 18.4 mm (0.72 in) 19.5 mm (0.77 in) 16.8 mm (0.66 in) ~23.6 mm (0.93 in)

Skull
Skull of Enhydra lutris. Its I3, while larger than its other incisors, is not hypertrophied in size unlike the Enhydriodon's I3.[10]

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.[15]

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.[2]

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 a 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 are 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 than E. sivalensis.[15]

Palaeobiology
A sea otter eating a clam, similar to suggested diets of certain Enhydriodon species

As fossil bunodont otter genera including Enhydriodon generally lack complete specimens, 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 in their diets that they commonly ate shellfish.[5] This claim is likely made 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 combine 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).[10]

The larger Enhydriodon species in the African continent are suggested to have different prey from their Indian subcontinental counterparts. It is suggested that they could've preyed upon a wider variety of foods in addition to their primary prey. Their bunodont dentition likely would not have limited genera like Enhydriodon from consuming softer prey. One suggested type of prey is large fish with hard external coverings such as catfish.[21] Several catfish genera were present in Africa starting from their first appearances during the late Miocene coinciding with the presence of Enhydriodon, including extant genera Clarotes, Bagrus, Auchenoglanis, and Chrysichthys and the extinct genus Nkondobagrus.[22] Crabs have been excluded as potential prey for African species of Enhydriodon species like E. dikikae 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. The overall lack of crabs in Africa and the large presence of catfish supports the claim that at least some African Enhydriodon species preyed on the latter, which were particularly abundant, easy targets for carnivores. Fast-swimming fish might've been unlikely to have been a regular source of its diet due to the emphasis on dentition meant to crush hard food in addition to large animals likely not catching having the ability to catch fast prey. Other armored prey, such as juvenile crocodiles, turtles, and ostrich eggs, are also suggested prey of E. dikikae.[15]

Femora and dental remains of African Enhydriodon could possibly hint at a semiaquatic plus terrestrial lifestyle, meaning that it could've 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 species of Enhydriodon (E. species from West Turkana, Kenya and E. hendeyi from Langebaanweg, South Africa) based on their smaller femur sizes were more locomotor generalists similar to most mustelids while larger species (E. species from Hadar, Ethiopia and a species eventually classified as E. omoensis from Omo, Ethiopia) were fully aquatic since their femur structures shared similarities to Enhydra. However, the Omo and Hadar femoras' proximal ends pointed out to a more aquatic nature than most lutrines, while their relative lengths resembled that of terrestrial generalist mustelids, including semiaquatic otters.[21][14] The same year however, Sivaonyx beyi of Chad, speculated to weigh 56.4 kg (124 lb) to 60.1 kg (132 lb), was thought to be a generalist locomotor whose niche was a terrestrial predator with poor aquatic adaptations based on its generalist limb proportions, implying that at least some large bunodont otters were possibly semi-aquatic or even terrestrial in lifestyle.[23] Because of the hypothesis that S. beyi was a terrestrial predator, E. dikikae is speculated to be mostly terrestrial based on its shared fossil location with both aquatic and terrestrial fauna at Dikika.[15] The diets 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.[24] 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.[25]

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 Enhydra and Pteronura 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. Since both the African clawless otter and Asian small-clawed otter are typically associated as less linked to water bodies and being the least aquatic of 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 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.[13]

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 (Enhydra lutris), 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 as a result contradict the 2008 assumption that the Omo Enhydriodon species was aquatic.[14]

It was also initially considered that the diet of Enhydriodon could've been 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 allow it to feed on carrion, including bones, in potentially a similar manner to hyeanas or bone-crushing mustelids.[14]

Palaeoecology

Pakistan and India
A restoration of Dinocrocuta gigantea, a species of percrocutinae 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 the hyaenodont Dissopsalis and the amphicyonids Amphicyon and Arctamphicyon. The early otter species also existed with various extinct carnivorous members of extant families during the late Miocene such as other mustelids (Sivaonyx, Promellivora, and Plesiogulo), ursids Agriotherium and Indarctos, felids (Mellivorodon, Felis, the Pseudaelurus cat Leptofelis, and machairodont felids Megantereon and Paramachairodus), hyaenids (percrocutinae hyaenids Dinocrocuta and Percrocuta, ictitheres Hyaenictitherium and Ictitherium, Lepthyaena, Lycyaena, and Adcrocuta), viverrids (Viverra, Aeluropsis, and Vishnuictis), and the mongoose Herpestes. It is suggested that the extinction of the amphicyonids and percrocutids left empty predatory niches that were quickly filled other hyaenid genera, which became highly diversified and coexisted with felids in the subcontinent.[26]

Other extinct members of extant and extinct mammalian families were found in the Nagri Formation and thereby existed with E. falconeri including bovids (Tragoportax, Miotragocerus, Selenoportax, Pachyportax, and Gazella), giraffids Giraffokeryx and Giraffa, anthracothere Merycopotamus, tragulines Dorcabune and Dorcatherium, the suid Propotamochoerus, the equid Hipparion, rhinoceroses (Chilotherium, Subchilotherium, and Brachypotherium), the chalicothere Chalicotherium, proboscideans Gomphotherium and Pentalophodon, the hominid ape Sivapithecus, and the rodent Rhizomys.[27] An extinct reptilian species of gharial, Gavialis lewisi (?), is reported from the Dhok Pathan Formation of Pakistan and is Pliocene in age.[28]

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 bovids Taurotragus and Elaschistoceros, the giraffid Bramatherium, cervids (Rucervus, Cervus, and Axis), anthracothere Microbunodon, suids (Tetraconodon, Listriodon, Hippopotamodon, and Sivachoerus), the equid Sivalhippus, the rhinoceros Aceratherium, proboscideans Choerolophodon and cf. Paratetralophodon, the old world monkey Cercopithecus, and the old world porcupine Hystrix.[29][30][31][32]

The arrival of Hipparionini equids as such Hipparion to Eurasia are representative of the major faunal turnovers and major extinction events that occurred in Eurasia during the late Miocene, although they aren't seen as a major cause of extinctions.

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.[33] 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. Hipparion equids had newly arrived to the Siwaliks by 10.7 Ma as immigrants from North America and became abundant, being representative of the start of the major faunal turnovers in the area, although the turnovers and extinctions did not start until 400 Kyr after their arrival. 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. Tragulids and suids both faced substantial declines during the period while bovids generally increased in relative abundance. The hominid ape Sivapithecus, deinotheres such as Deinotherium, and chalicotheres also faced extinction from rarity and declines in diversity within the late Miocene of the Siwaliks. Sivapithecus became extinct by around 8.4 Ma while deinotheres became locally extinct from the Indian subcontinent around the late Miocene as a result of the fragmentation of closed habitats in favour of open habitats that would eliminate food for C3 browsers and frugivores.[34][35][36] By 6.5 Ma, only one frugivorous mammal lineage and one browsing mammal lineage remained amongst herbivorous mammalian fauna, the remaining tragulids and suids seemingly incorporating C4 vegetation into their diets.[37]

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 the otter Amblonyx barryi, the machairodont cat Metailurus, and hyeana Lycyaena.[26] Herbivorous mammals found at the Tatrot Formation on the Potwar Plateau include the highly diverse bovids (Kobus, Antilope, Sivacobus, Vishnucobus, Gazella, Proamphibos, Sivoryx, and Leptobos), cervids (Cervus, Axis, and Rucervus), suids Hippohyus and Potamochoerus, proboscideans Stegodon and Elephas, the equid Eurygnathohippus, the anthracothere Merycopotamus, the hippopotamus Hexaprotodon, the giraffid Sivatherium, and the tragulid Vishnumeryx.[38][39][40] The porcupine Hystrix and the rabbit Alilepus were both relatively newer immigrants of the Indian subcontinent that had coexisted with the endemic bamboo rats (Rhizomys), their remains being found at the Dhok Pathan and Tatrot formations.[41] 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.[28][42][43][44]

Elephas 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.[45]

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 Canis and Sivacyon, mustelids (Mellivora, Sinictis, and Amblonyx), ursids Agriotherium and Melursus, felids (Megantereon, Panthera, Sivafelis, and Felis), hyaenids (Crocuta, Hyaenictis, and Pliocrocuta), and the viverrids Vishnuictis and Viverra.[26] Other mammalian genera found within the Pinjor Formation includes the hedgehog Chandysorex, the hominoid Homo (specifically Homo erectus(?)), the old world monkeys Theropithecus and Procynocephalus, rodents (Nesokia, Rattus, Hystrix, Mus, Cremnomys, Golunda, Dilatomys, Hadromys, Tatera, Rhizomys, and Bandicota), the lepoid Caprolagus, proboscideans Elephas and Stegodon, the horse Equus, the rhinoceroses Coelodonta(?) (=Punjabitherium(?)) and Rhinoceros, suids Potamochoerus and Sus, the deer Cervus, the giraffid Sivatherium, and bovids (Sivacapra, Damalops, Oryx, Sivacobus, Antilope, Hemibos, Bubalus, Leptobos, Bison, and Bos).[46]

Ethiopia
Geographical and stratigraphic distribution of Enhydriodon in East Africa by species

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.[15][14]

There are four members of the Dikika composite sequence as part of the Pliocene Hadar Formation, from base to top: the Basal, Sidi Hakoma, Denen Dora, and Kada Hadar members. All together, they are dated to ca. 3.5-2.9 Ma and are best known for the numerous remains of Australopithecus afarensis.[47] E. dikikae fossils are known from the formation's Basal and Sigi Hakoma members and are unknown in the other top two members.[15]

Based on methods of determining palaeoenvironments such as ecomorphological analysis, dental microwear of bovids, and carbon and oxygen isotopes of enamel, the Basal Member (BM) has the greatest abundance of bovids and suids in the Hadar Formation, suggesting that the environments of which they were present in were possibly woody grasslands as well as riverine forests. The Aepycerotini were common within the member, fitting with the tribe's preference for ecotonal habitats between grasslands and woodlands.[48]

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've 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.[48]

Sidi Hakoma Submember 3 (SH-3) indicates the presence of woodlands and grasslands with more lakeside wetlands compare to the previous sub-members, with increased presences of reduncine bovids and the highest abundance of tragelaphin bovids, which indicate either more closed habitats or wetlands. It also contains the largest micromammal assemblages of extant murid genera such as the extant Acomys, Golunda and Oenomys and the extinct Saidomys, of which Golunda is now extinct in Africa. Sidi Hakoma Submember 4 indicates wetland habitats that surround lakes within drier environments. A further increase of Reduncinae bovids and a decrease in alcelaphin bovids indicates said lakeshore environments and surrounding wetlands. The bovid abundance data suggests similar amounts of tree cover for SH-3 and SH-4 with the difference being that the latter was slightly drier than the former.[48]

Skeleton of Lucy, the most well-known Australopithecus afarensis fossil, at the National Museum of Ethiopia.

The Hadar Formation represents many fossils of Australopithecus afarensis, most notably the partial skeleton known as "Lucy". The aggregate time span of the species is at least 0.7 myr, from 3.7 Ma to 3.0 Ma.[49] 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 (Aepyceros), Alcelaphini (Damalborea and Parmularius), Antilopini (Gazella), Bovini (Ugandax and Pelorovis(?)), Caprini (Budorcas), Cephalophini, Hippotragini (Oryx), Neotragini (Raphicerus(?) and Madoqua), Reduncini (Kobus), and Tragelaphini (Tragelaphus). Artiodactyls outside the bovid family were present within the formation as well, namely the giraffids Giraffa and Sivatherium, Hippopotamus, and suids (Kolpochoerus, Notochoerus, and Nyanzachoerus). While a definitive list of carnivorans found within the Hadar Formation has yet to be made, confirmed genera that were found within the Hadar Formation include canids Canis and Nyctereutes, felids (Dinofelis, Leptailurus, Felis, Homotherium, and Panthera), hyaenids (Chasmaporthetes, Crocuta, Hyaena, Ikelohyena, and cf. Pliocrocuta), herpestids Herpestes and cf. Helogale, mustelids Mellivora and cf. Poecilogale, and the viverrid cf. Civettictis. Other mammals within the formation outside the artiodactyl and carnivoran families include a bat indet. (indeterminate), the leporid Lepus, the equid Eurygnathohippus, rhinoceroses Ceratotherium and Diceros, old world primates (Parapapio, Theropithecus, and Cercopithecoides), proboscideans (the deinothere Deinotherium and elephants Elephas, Loxodonta, and Mammuthus), old world porcupines Hystrix and Xenohystrix, murid rodents (Gerbilliscus, Acomys, Golunda, Oenomys, Praomys, Saidomys, Millardia, and Mus), the spalacid Tachyoryctes, a squirrel indet., and an aardvark species. Taxons within other classes are present within the Hadar Formation as well, such as birds (Plectropterus, Balearica, Anhinga, and Struthio) and reptiles (Crocodylus, Python, Varanus, and Bitis).[48][47]

Other Pliocene-age formations within Ethiopia show similar or exact trends of great diversity in the Bovidae family from the existence of multiple tribes along with suids, hippopatamids, cercopithecids, hominids, and equids of generally the same genera as the Hadar Formation. Most herbivores present in the Shungura Formation show either consistent C4 diets or had generally shifted from mixed C3-C4 diets to generally C4 diets as indicated from changes in dentition by formation member. These trends suggest that the African herbivores in the Pliocene were increasingly shifting to C4 herbivory as opposed to browsing and mixed feeding as a result of the increasing dominance of C4 grasslands in Africa. These changes are most notable at 2.8 Ma where bovids and suids both show dramatic increases in C4 dietary intakes based on stable isotope analyses of tooth enamels. There were a few exceptions, however, as Giraffidae and Deinotheriidae were both consistently C3 browsers within the formation while the bovid tribes Aepycerotini and Tragelaphini were predominantly mixed feeders with little change in diet. In the case of the latter family, Deinotherium was the only browsing proboscidean present in the Plio-Pleistocene of Africa and would disappear from the Turkana record at 1.6 Ma most likely from climatic changes further favouring C4 plant growth between 2 Ma and 1.5 Ma.[50][51][52] However, some genera that are unknown in the Hadar Formation make their first appearance datums in the Usno and/or Shungura Formations, such as the suid Metridiochoerus in the Usno Formation and the hominid Paranthropus in the Shungura Formation, Member C.[53][50] Fossil fish remains are also known from the Shungura Formation, namely the genera Polypterus, Sindacharax, Synodontis, Auchenoglanis, and Lates.[54]

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