Enhydriodon
Enhydriodon | |
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Enhydriodon omoensis right femur faced at different sides. | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Mammalia |
Order: | Carnivora |
Family: | Mustelidae |
Subfamily: | Lutrinae |
Tribe: | † Enhydriodontini
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Genus: | †Enhydriodon Falconer, 1868 |
Type species | |
†Enhydriodon sivalensis Falconer, 1868
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Other species | |
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Synonyms | |
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Enhydriodon is an
The exact sizes and lengths of Enhydriodon species are unknown given the lack of complete
Enhydriodon is known well by its advanced dentition, its broad,
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
During the 19th and 20th centuries, more species of Enhydriodon such as E. campanii were introduced and more lutrine genera with
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.[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 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.[5] 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.[9] 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.[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]
Erections and statuses 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.[12][8]
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.[8] 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.[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 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 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 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] 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.[16]
Classification
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"):[11]
Lutrinae
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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.[11] 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.[20]
Description
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.[21] 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. Falconer's 1868 memoir described E. sivalensis as a lutrine the size of a panther.[1] In 1932, 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, 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 non-bunodont cousins. 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 have possibly been larger than it.
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.[15]
The E. sivalensis skull, identified as belonging to a fully-grown individual, is relatively well-preserved with identifiable
The broken skull belonging to E. dikikae contains a short and
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.[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 to other bunodont lutrine genera where the upper incisor is known, its 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]
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.[13]
E. hendeyi fossil remains include a fragmentary
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
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.[14]
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
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.[23] 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.[24] 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.[15]
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.
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
With the overall lack of consensus on the lifestyle of African Enhydriodon species considered, a 2022 study on E. omoensis measured the
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.[14]
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
Other extinct members of extant and extinct mammalian families were found in the Nagri Formation and thereby existed with E. falconeri including bovids,
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
The carnivoran fossil records of the Tatrot Formation in India are scarce, but amongst the extinct members that existed with E. falconeri in the Pliocene were other lutrines, machairodontines, and hyaenids.
Amongst carnivoran taxa, Enhydriodon is the longest-lasting
Ethiopia
E. dikikae and E. omoensis were large lutrine species found in different locations within modern-day
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
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
Sidi Hakoma Submember 3 (SH-3) indicates the presence of woodlands and grasslands with more lakeside
The Hadar Formation represents many fossils of Australopithecus afarensis, most notably the partial skeleton known as "
Other Pliocene-age formations within Ethiopia show similar trends of great diversity in the Bovidae family from its 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. 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.[50][51][52] Fossil fish remains are also known from the Shungura Formation, namely the genera Polypterus, Sindacharax, Synodontis, Auchenoglanis, and Lates.[53]
Notes
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