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{{Short description|Extinct genus of carnivorans}}
{{good article}}
{{Automatic taxobox
| fossil_range = Late [[Miocene]] to Early [[Pleistocene]], {{fossilrange|10.1|2}}
| image = Enhydriodon
| image_caption = Lower jaw dentitions of ''Enhydriodon
| taxon = Enhydriodon
| authority = [[Hugh Falconer|Falconer]], 1868
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| type_species_authority = Falconer, 1868
| subdivision_ranks = Other species
| subdivision = {{species list
}}
| synonyms =*''Amyxodon'' <small>Falconer & Cautley, 1835</small>▼
| synonyms = {{collapsible list|bullets = true|title=<small>Genus synonymy</small>
| ''Amyxodon'' <small>Falconer & [[Proby Cautley|Cautley]], 1835</small>
}}
{{collapsible list|bullets = true
|title=<small>Synonyms of ''E. sivalensis''</small>
}}
{{collapsible list|bullets = true
|title=<small>Synonyms of ''E. ekecaman''</small>
| ''E. pattersoni'' <small>[[R. J. G. Savage|Savage]], 1978</small>
}}
}}
'''''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
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 {{cvt|100|kg}}. Given these weight estimates, the three species likely reached sizes comparable to extant [[bear]]s or [[lion]]s, making them the largest known mustelids to exist, although a lack of complete specimens makes precise estimates impossible.
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.
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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.<ref>{{cite journal|last=Repenning|first=Charles A.|year=1976|title=Enhydra and Enhydriodon From The Pacific Coast of North America|journal=Journal of Research of the U.S. Geological Survey.|volume=4|issue=3|pages=305–315}}</ref> 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.<ref>{{cite journal|last1=Hürzeler|first1=Johannes|last2=Engesser|first2=Burkart|year=1976|title=Les faunes de mammifères néogènes du Bassin de Baccinello (Grosseto, Italie)|language=French|journal=Comptes Rendus de l'Académie des Sciences de Paris|volume=283|pages=333–336}}</ref><ref name ="Enhydriodontini">{{cite journal|last1=Morales|first1=Jorge|last2=Pickford|first2=Martin|year=2005|title=Giant bunodont Lutrinae from the Mio-Pliocene of Kenya and Uganda|journal=Estudios Geológicos|language=Spanish|volume=61|issue=3|pages=233–245|doi=10.3989/egeol.05613-666|doi-access=free}}</ref>
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.<ref name="Lutrinae"/> 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.<ref>{{cite journal|last1=Pickford|first1=Martin|last2=Soria|first2=Dolore|last3=Morales|first3=Jorge|year=2005|title=Carnivores from the Late Miocene and Basal Pliocene of the Tugen Hills, Kenya|journal=Revista de la Sociedad Geológica de España|volume=18|issue=1–2|pages=39–61}}</ref> 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.<ref name ="Enhydriodontini"/> In 2007, the two palaeontologists reaffirmed that the dental [[morphology (biology)|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'',
===
[[File:Enhydriodon Omoensis Femur Views.png|thumb|''E. omoensis'' right femur faced at different sides]]
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.<ref>{{cite journal|last=Werdelin|first=Lars|year=2003|title=Carnivores from the Kanapoi Hominid site, Turkana Basin, northern Kenya|journal=Contributions in Science|volume=498|pages=115–132|doi=10.5962/p.214388 |s2cid=82943977 |doi-access=free}}</ref><ref name ="Enhydriodontini"/>
''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''.<ref name ="Enhydriodontini"/> 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.<ref name="giantmustelid">{{cite journal|last1=Valenciano|first1=Alberto|last2=Govender|first2=Romala|year=2020|title=New insights into the giant mustelids (Mammalia, Carnivora, Mustelidae) from Langebaanweg fossil site (West Coast Fossil Park, South Africa, early Pliocene)|journal=PeerJ|volume=8 |pages=e9221 |doi=10.7717/peerj.9221|pmid=32547866|pmc=7271888 |doi-access=free }}</ref><ref name="Omoe">{{cite journal|last1=Grohé|first1=Camille|last2=Uno|first2=Kevin|last3=Boisserie|first3=Jean-Renaud|year=2022|title=Lutrinae Bonaparte, 1838 (Carnivora, Mustelidae) from the Plio-Pleistocene of the Lower Omo Valley, southwestern Ethiopia: systematics and new insights into the paleoecology and paleobiogeography of the Turkana otters|language=French|journal=Comptes Rendus Palevol|volume=30|issue=30 |pages=684–693|doi=10.5852/cr-palevol2022v21a30|s2cid=252106648 |doi-access=free}}</ref> 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'',
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.<ref name="Dikikae">{{cite journal|doi=10.1080/02724634.2011.550356 | volume=31 | title=Enhydriodon dikikae, sp. nov. (Carnivora: Mammalia), a gigantic otter from the Pliocene of Dikika, Lower Awash, Ethiopia | journal=Journal of Vertebrate Paleontology | year=2011 | pages=447–453| last1=Geraads | first1=Denis | last2=Alemseged | first2=Zeresenay | last3=Bobe | first3=René | last4=Reed | first4=Denné | issue=2 | bibcode=2011JVPal..31..447G | s2cid=84797296 }}</ref> 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.<ref name="Omoe"/> 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''.<ref>{{cite conference|last1=Valenciano|first1=Alberto|last2=Morales|first2=Jorge|last3=Pickford|first3=Martin|year=2022|title="Lutra" libyca from the Mio/Pliocene of Wadi Natrun (Egypt): a new systematics proposal|conference=The 2nd International Symposium on Vertebrate Paleontology 20th-22nd September 2022|pages=31–32|url=https://www.researchgate.net/publication/364210301}}</ref>
===Classification===
[[File:Adult Sea Otter in Morro Bay.jpg|thumb|''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.<ref name ="Enhydriodontini"/> 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''.<ref name="Siamo"/><ref name="Vishnuonyx">{{cite journal|last1=Kargopoulos|first1=Nikolaos|last2=Valenciano|first2=Alberto|last3=Kampouridis|first3=Panagiotis|last4=Lechner|first4=Thomas|last5=Böhme|first5=Madelaine|year=2021|title=New early late Miocene species of Vishnuonyx (Carnivora, Lutrinae) from the hominid locality of Hammerschmiede, Bavaria, Germany|journal=Journal of Vertebrate Paleontology|volume=41|issue=3|doi=10.1080/02724634.2021.1948858|bibcode=2021JVPal..41E8858K |s2cid=240538139 }}</ref><ref>{{cite web|title=Enhydritherium terraenovae|date=30 March 2017 |url=https://www.floridamuseum.ufl.edu/florida-vertebrate-fossils/species/enhydritherium-terraenovae/|publisher=Florida Museum of Natural History|accessdate=10 October 2022|language=en}}</ref> 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.{{efn|Bunodont dentition refers to
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''"):<ref name="Siamo"/>
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==Description==
===Body Mass===▼
[[File:Enhydritherium terraenovae.jpg|thumb|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 non-bunodont otters.]] Some ''Enhydriodon'' species, particularly a few that had resided in Africa, are the [[Largest prehistoric animals#Carnivores (Carnivora)|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.<ref name="latipes">{{cite journal|last=Koufos|first=George D.|year=2011|title=The Miocene carnivore assemblage of Greece|journal=Estudios Geológicos|language=Spanish|volume=67|issue=2|pages=296–297|doi=10.3989/egeol.40560.190|doi-access=free}}</ref> 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).<ref name="Omoe"/>▼
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 [[jaguar|panther]].<ref name="Siva"/> In 1932, Pilgrim diagnosed ''E. falconeri'' as being smaller than ''E. sivalensis'', although no size or weight estimates were offered for it by him.<ref name="Falconeri"/> In 2007, Pickford estimated ''E. sivalensis'' to be the largest prehistoric otter in India, ranging from {{cvt|22|kg}} minimum to {{cvt|25|kg}} maximum in body weight, its skull possibly being wolf-sized. He also estimated the body of ''E. falconeri'' based on its lower M<sub>1</sub> teeth dimensions to be similar to the [[African clawless otter]] (''A. capensis''), averaging to {{cvt|16|kg}}.<ref name="Pickford"/>▼
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 {{cvt|100|kg}} 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.<ref name="Pickford"/> ''E. hendeyi'' (then ''Sivaonyx hendeyi'') was estimated to be wolf-sized and around {{cvt|40|kg}} while''E. africanus'' and ''E. ekecaman'' are thought to be of similar sizes.<ref>{{cite news|author1= Alberto Valenciano Vaquero|author2 = Romala Govender|url = https://theconversation.com/gigantic-wolverines-otters-the-size-of-wolves-fossils-offer-fresh-insights-into-the-past-140752|title = Gigantic wolverines, otters the size of wolves: fossils offer fresh insights into the past|publisher = The Conversation|date = 21 June 2020|access-date = 27 November 2022}}</ref><ref name="Omoe"/> ''E. dikikae'' of Ethiopia was estimated to have weighed {{cvt|100|kg}} minimum and {{cvt|200|kg}} 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 {{cvt|25|cm}}.<ref name="Dikikae"/> ''E. omoensis'' was later estimated to weigh more than {{cvt|200|kg}}, 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.<ref name="Omoe"/>▼
===Skull===
[[Image:Enhydra lutris 10zz.jpg|thumb|left|Skull of ''[[Enhydra lutris]]''. Its I<sup>3</sup>, while larger than its other incisors, is not hypertrophied in size unlike the ''Enhydriodon's'' I<sup>3</sup>.<ref name="Pickford"/>]]
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===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 P<sup>4</sup> 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.<ref name="Pickford"/><ref name="Dikikae"/> 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 M<sub>1</sub>, presence of a carnassial notch and one or more cusps between the protocone and the hypocone on P<sup>4</sup>, and/or distolingual expansion on M<sup>1</sup>."<ref name="Omoe"/>
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=== Limbs ===
[[File:Sivaonyx beyi Enhydriodon (Sivaonyx) hendeyi Aonyx capensis femora.jpg|thumb
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''.<ref name="giantmustelid"/>
''E. hendeyi'' fossil remains include a fragmentary [[humerus]], an [[ulna]], two [[femurs]], and an [[Talus bone|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 (bone)|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 [[Anatomical terms of motion|plantar flexion]] muscles for extension of the foot at the ankle compared to the African clawless otter.<ref name="giantmustelid"/>
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 [[humerus|lateral supracondylar crest]] is longer compared to ''S. beyi'' while the [[medial epicondyle of the humerus|medial epicondyle]] is not as prominent in size. The [[olecranon fossa]] is small and more circular compared to ''S. beyi''.<ref name="Dikikae"/>
''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 femur|lateral condyle]] of the same bone. The [[intercondylar fossa of femur|intercondylar fossa]] of the femur is rectangular and wide.<ref name="Omoe"/>
▲===Body Mass===
▲[[File:Enhydritherium terraenovae.jpg|thumb|left|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 non-bunodont otters.]] Some ''Enhydriodon'' species, particularly a few that had resided in Africa, are the [[Largest prehistoric animals#Carnivores (Carnivora)|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.<ref name="latipes">{{cite journal|last=Koufos|first=George D.|year=2011|title=The Miocene carnivore assemblage of Greece|journal=Estudios Geológicos|language=Spanish|volume=67|issue=2|pages=296–297|doi=10.3989/egeol.40560.190|doi-access=free}}</ref> 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).<ref name="Omoe"/>
▲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 [[jaguar|panther]].<ref name="Siva"/> In 1932, Pilgrim diagnosed ''E. falconeri'' as being smaller than ''E. sivalensis'', although no size or weight estimates were offered for it by him.<ref name="Falconeri"/> In 2007, Pickford estimated ''E. sivalensis'' to be the largest prehistoric otter in India, ranging from {{cvt|22|kg}} minimum to {{cvt|25|kg}} maximum in body weight, its skull possibly being wolf-sized. He also estimated the body of ''E. falconeri'' based on its lower M<sub>1</sub> teeth dimensions to be similar to the [[African clawless otter]] (''A. capensis''), averaging to {{cvt|16|kg}}.<ref name="Pickford"/>
▲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 {{cvt|100|kg}} 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.<ref name="Pickford"/> ''E. hendeyi'' (then ''Sivaonyx hendeyi'') was estimated to be wolf-sized and around {{cvt|40|kg}} while''E. africanus'' and ''E. ekecaman'' are thought to be of similar sizes.<ref>{{cite news|author1= Alberto Valenciano Vaquero|author2 = Romala Govender|url = https://theconversation.com/gigantic-wolverines-otters-the-size-of-wolves-fossils-offer-fresh-insights-into-the-past-140752|title = Gigantic wolverines, otters the size of wolves: fossils offer fresh insights into the past|publisher = The Conversation|date = 21 June 2020|access-date = 27 November 2022}}</ref><ref name="Omoe"/> ''E. dikikae'' of Ethiopia was estimated to have weighed {{cvt|100|kg}} minimum and {{cvt|200|kg}} 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 {{cvt|25|cm}}.<ref name="Dikikae"/> ''E. omoensis'' was later estimated to weigh more than {{cvt|200|kg}}, 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.<ref name="Omoe"/>
==Palaeobiology==
[[Image:Sea Otter enjoying a clam (33086393883).jpg|thumb|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 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]].<ref name="Lutrinae"/> 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 [[molluscivore|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).<ref name="Pickford"/>
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]].<ref name="femur">{{cite journal|last=Lewis|first=Margaret E.|year=2008|title=The femur of extinct bunodont otters in Africa (Carnivora, Mustelidae, Lutrinae)|language=French|journal=Comptes Rendus Palevol|volume=7|issue=8|pages=607–627|doi=10.1016/j.crpv.2008.09.010|bibcode=2008CRPal...7..607L }}</ref> 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]]''.<ref>{{cite journal|last=Stewart|first=Kathlyn M|year=2001|title=The freshwater fish of Neogene Africa (Miocene–Pleistocene): systematics and biogeography|journal=Fish and Fisheries|volume=2|issue=3|pages=177–230|doi=10.1046/j.1467-2960.2001.00052.x|bibcode=2001AqFF....2..177S }}</ref> 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''.<ref name="Dikikae"/>
[[File:Enhydriodon sivalensis restoration.jpg|thumb|
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.<ref name="femur"/><ref name="Omoe"/> ''Sivaonyx beyi'' of [[Chad]], speculated to weigh {{cvt|56.4|kg}} to {{cvt|60.1|kg}}, had non-specialized limb proportions that implied generalist-terrestrial locomotion and poor aquatic adaptations.<ref>{{cite journal|last1=Peigné|first1=Stéphane|last2=De Bonis|first2=Louis|last3=Likius|first3=Andossa|last4=Mackaye|first4=Hassane Taïsso|last5=Vignaud|first5=Patrick|last6=Brunet|first6=Michel|year=2008|title=Late Miocene Carnivora from Chad: Lutrinae (Mustelidae)|journal=Zoological Journal of the Linnean Society|volume=152|issue=4|pages=793–846|doi=10.1111/j.1096-3642.2008.00377.x|url=https://zenodo.org/record/5442989 }}</ref> 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.<ref name="Dikikae"/> The palaeobiologies and [[niche differentiation|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.<ref>{{cite journal|last1=Werdelin|first1=Lars|last2=Manthi|first2=Fredrick Kyalo|year=2012|title=Carnivora from the Kanapoi hominin site, northern Kenya|journal=Journal of African Earth Sciences|volume=64|pages=1–8|doi=10.1016/j.jafrearsci.2011.11.003|bibcode=2012JAfES..64....1W }}</ref> 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.<ref>{{cite journal|last1=Werdelin|first1=Lars|last2=Lewis|first2=Margaret E.|year=2020|title=A contextual review of the Carnivora of Kanapoi|journal=Journal of Human Evolution|volume=140|page=102334 |doi=10.1016/j.jhevol.2017.05.001|pmid=28625408 |bibcode=2020JHumE.14002334W |s2cid=23285088 |url=http://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-2412 }}</ref>
''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 [[fossorial|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''.<ref name="giantmustelid"/>
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''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 [[Hyainailourinae|hyainailourine]] [[hyaenodont]]s and [[amphicyonid]]s. The early otter species also existed with various extinct carnivorous members of extant families during the late Miocene representing other [[mustelids]], ursids, felids ([[felinae|felines]] and [[machairodontine]]s), hyaenids ([[Percrocutidae|percrocutinae]] hyaenids [[Ictitheriinae|ictitheres]], and hyaenines), viverrids, and [[herpestid]]s. It is suggested that the extinction of the amphicyonids and percrocutids left empty predatory niches that were quickly filled by other hyaenid genera, which became highly diversified and coexisted with felids in the subcontinent.<ref name="siwaliks">{{cite journal|last1=Jasinski|first1=Steven E.|last2=Abbas|first2=Sayyed Ghyour|last3=Mahmood|first3=Khalid|last4=Babar|first4=Muhammad Adeeb|last5=Khan|first5=Muhammad Akbar|year=2022|title=New Carnivoran(Mammalia: Carnivora) specimens from the Siwaliks of Pakistan and India and their faunal and evolutionary implications|journal=Historical Biology: An International Journal of Paleobiology|volume=35 |issue=11 |pages=1–36|doi=10.1080/08912963.2022.2138376|s2cid=253346978 }}</ref>
Other extinct members of extant and extinct mammalian families were found in the Nagri Formation and thereby existed with ''E. falconeri'' including bovids, [[giraffid]]s, [[anthracothere]]s, [[tragulid]]s, [[suidae|suids]], [[
[[Image:Hipparion sp 87348.jpg|thumb|right|The arrival of [[Hipparionini]] equids such as ''[[Hipparion]]'' to Eurasia are representative of major Eurasian faunal turnovers of the late Miocene]]
The transition from the middle Miocene to the late Miocene reflected a period in which the evergreen to deciduous tropical forests once covering a large part of the Indian subcontinent shrank and were replaced by [[grasslands]] because of global cooling, drier conditions, and the intensification of Asian [[monsoons]].<ref>{{cite book|last1=Wang|first1=Xiaoming|last2=Flynn|first2=Lawrence|last3=Fortelius|first3=Mikael|editor-first1=Xiaoming |editor-first2=Lawrence J. |editor-first3=Mikael |editor-last1=Wang |editor-last2=Flynn |editor-last3=Fortelius |date=2013|title=Fossil Mammals of Asia: Neogene Biostratigraphy and Chronology|language=english|publisher=Columbia University Press|chapter=Chapter 17: Indian Neogene Siwalik Mammalian Biostratigraphy: An Overview|pages=423–444|doi=10.7312/wang15012-017|isbn=9780231520829 }}</ref> A change from the Nagri [[floodplains]] to the Dhok Pathan floodplains suggests less draining in the fluvial system of the latter compared to the former with Dhok Pathan's smaller [[rivers]] having more seasonal flow than before. This reflects the general trend of late Miocene [[climate forcing]] resulting in more seasonality, bringing about large faunal turnovers. The drier and more seasonal climates along with fluvial changes gradually brought about larger, open [[woodland
[[File:Elephant threesome (51340109244).jpg|thumb|left|''[[Elephas]]'' was a typical grazer of C<sub>4</sub> plants from the Pliocene-Pleistocene. It adapted its diets to mixed feeding of C<sub>3</sub> plants by middle Pleistocene while ''[[Stegodon]]'' was a consistent C<sub>4</sub> browser that failed to adapt and went extinct.<ref>{{cite journal|last1=Ma|first1=Jiao|last2=Wang|first2=Yuan|last3=Jin|first3=Changzhu|last4=Hu|first4=Yaowu|last5=Bocherens|first5=Hervé|year=2019|title=Ecological flexibility and differential survival of Pleistocene Stegodon orientalis and Elephas maximus in mainland southeast Asia revealed by stable isotope (C, O) analysis|journal=Quaternary Science Reviews|volume=212|pages=33–44|doi=10.1016/j.quascirev.2019.03.021|bibcode=2019QSRv..212...33M |s2cid=135056116 }}</ref>]]
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''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 [[Awash Valley|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.<ref name="Dikikae"/><ref name="Omoe"/>
There are four [[stratigraphic unit#member|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]]''.<ref name="Hadar">{{cite journal|last1=Geraads|first1=Denis|last2=Alemseged|first2=Zeresenay|last3=Bobe|first3=René|last4=Reed|first4=Denné|year=2015|title=Pliocene Carnivora (Mammalia) from the Hadar Formation at Dikika, Lower Awash Valley, Ethiopia|journal=Journal of African Earth Sciences|volume=107|pages=28–35|doi=10.1016/j.jafrearsci.2015.03.020|bibcode=2015JAfES.107...28G }}</ref> ''E. dikikae'' fossils are known from the formation's Basal and Sigi Hakoma members and are unknown in the other top two members.<ref name="Dikikae"/>
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 (taxon)|tribe's]] preference for [[ecotone|ecotonal]] habitats between grasslands and woodlands.<ref name="Hakoma">{{cite book |last1=Campisano |first1=Christopher J. |title=African Paleoecology and Human Evolution |last2=Rowan |first2=John |last3=Reed |first3=Kaye E. |author-link3=Kaye Reed |publisher=Cambridge University Press |year=2022 |editor-last1=Reynolds |editor-first1=Sally C. |
The Sidi Hakoma Submember 1 (SH-1), ranging from ~3.45 to 3.35 Ma, had similar fauna and thereby similar habitats to other members within the Hadar Formation but also likely included wetlands in certain regions. Taxa such as a species within the forest-dwelling [[Cephalophini]] tribe and five species of primates were recovered from the member, further indicating a large riverine forest with, predominantly, woodlands in the surrounding area. ''[[Aepyceros]]'' was the most abundant bovid, and SH-1 had the lowest proportion of grazing bovids at any sub-member of the Hadar Formation. The vegetation of SH-1 might have closely resembled those at the Guinea or Sudanese [[savannas]] that interdigitate with the central African [[rainforest]], which creates habitat mosaics of grasslands, woodlands, and some forest belts. The [[ostracod]] assemblage of the Basal and Sidi Hakoma Members indicate sources of freshwater input, in which their shells also indicate only a three-month dry season, characteristic of the central African savannas. The single dry season, indicating a nine-month rainy season, is a distinctive factor of the Sidi Hakoma member from the modern climate in East Africa, which has a bimodal dry season format (two dry seasons) rather than a single one. The Sidi Hakoma Submember 2 (SH-2) is similar to SH-1 and is thought to have been associated with woodlands with some grassy plains, of which ''Aepycerotini'' was the most common.<ref name="Hakoma"/>
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Sidi Hakoma Submember 3 (SH-3) indicates the presence of woodlands and grasslands with more lakeside [[wetlands]] compared to the previous sub-members, with increased presences of [[Reduncinae|reduncine bovids]] and the highest abundance of [[Tragelaphini|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 [[Alcelaphinae|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.<ref name="Hakoma"/>
[[File: Lucy (Australopithecus) National Museum of Ethiopia.jpg|thumb|left|upright=0.7|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 (Australopithecus)|Lucy]]". The aggregate time span of the species is at least 0.7 myr, from 3.7 Ma to 3.0 Ma.<ref>{{cite journal|last1=Alemseged|first1=Zeresenay|last2=Wynn|first2=Jonathan G.|last3=Kimbel|first3=William H.|last4=Reed|first4=Denné|last5=Geraads|first5=Denis|last6=Bobe|first6=René|year=2005|title=A new hominin from the Basal Member of the Hadar Formation, Dikika, Ethiopia, and its geological context|journal=Journal of Human Evolution|volume=49|issue=4|pages=499–514|doi=10.1016/j.jhevol.2005.06.001|pmid=16026815 |bibcode=2005JHumE..49..499A }}</ref> The Hadar Formation is also known for its representation of a great diversity of bovid species that represented most major tribes in Africa. The bovid tribes that were found in the formation included the Aepycerotini, Alcelaphini, [[Antilopini]], [[Bovini]], [[Caprini]], Cephalophini, [[Hippotragini]], [[Neotragini]], Reduncini, and Tragelaphini. Other groups, represented by extinct species of extant or extinct genera, include giraffids, hippopotamuses, suids, canids, felids (machairodontines were the most common), hyaenids, other mustelids, viverrids, rhinoceroses, equids, [[catarrhines]], deinotheres, and elephantids were all also found within the locality. Small mammal groups include [[bat]]s, leporids, old world porcupines, murid rodents, [[spalacid]]s, [[squirrels]], and [[aardvark]]s.<ref name="Hakoma"/><ref name="Hadar"/>
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 C<sub>4</sub> diets or had generally shifted from mixed C<sub>3</sub>-C<sub>4</sub> diets to generally C<sub>4</sub> diets as indicated from changes in dentition by formation member. These trends suggest that the African herbivores in the Pliocene were increasingly shifting to C<sub>4</sub> herbivory as opposed to browsing and mixed feeding as a result of the increasing dominance of C<sub>4</sub> grasslands in Africa. There were a few exceptions, however, as Giraffidae and Deinotheriidae were both consistently C<sub>3</sub> browsers within the formation while the bovid tribes Aepycerotini and Tragelaphini were predominantly mixed feeders with little change in diet.<ref name="herbivory">{{cite journal|last1=Bobe|first1=René|year=2011|title=Fossil Mammals and Paleoenvironments in the Omo-Turkana Basin|journal=Evolutionary Anthropology|volume=20|issue=6|pages=254–263|doi=10.1002/evan.20330|pmid=22170694 |s2cid=205826374 }}</ref><ref>{{cite journal|last1=Negash|first1=Enquye W.|last2=Alemseged|first2=Zeresenay|last3=Bobe|first3=René|last4=Grine|first4=Frederick|last5=Sponheimer|first5=Matt|last6=G. Wynn|first6=Jonathan|year=2020|title=Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia|journal=Proceedings of the National Academy of Sciences|volume=117|issue=36|pages=21921–21927|doi=10.1073/pnas.2006982117|pmid=32839326 |pmc=7486712 |bibcode=2020PNAS..11721921N |doi-access=free }}</ref><ref>{{cite journal|last1=Bibi|first1=Faysal|last2=Souron|first2=Antoine|last3=Bocherens|first3=Hervé|last4=Uno|first4=Kevin|last5=Boisserie|first5=ean-Renaud|year=2013|title=Ecological change in the lower Omo Valley around 2.8 Ma|journal=Biology Letters|volume=9|issue=1|pages=1–4|doi=10.1098/rsbl.2012.0890|pmid=23234862 |pmc=3565503 |s2cid=10791621 }}</ref> Fossil fish remains are also known from the Shungura Formation, namely the genera ''[[Polypterus]]'', ''[[Sindacharax]]'', ''[[Synodontis]]'', ''[[Auchenoglanis]]'', and ''[[Lates]]''.<ref>{{cite journal|last1=Stewart|first1=Kathlyn M.|last2=Murray|first2=Alison|year=2008|title=Fish remains from the Plio-Pleistocene Shungura Formation, Omo River basin, Ethiopia|journal=Geobios|volume=41|issue=2|pages=283–295|doi=10.1016/j.geobios.2007.06.004|bibcode=2008Geobi..41..283S }}</ref>
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