| Eogavialis | |
|---|---|
 | |
| Skulls from both species ofEogavialis | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauria |
| Clade: | Pseudosuchia |
| Clade: | Crocodylomorpha |
| Clade: | Metasuchia |
| Clade: | Neosuchia |
| Clade: | Eusuchia |
| Genus: | †Eogavialis Buffetaut 1982 |
| Species | |
| Synonyms[1] | |
Eogavialis is anextinctgenus ofeusuchiancrocodylomorph, usually regarded as agavialoidcrocodylian. It superficially resemblesTomistoma schlegelii, the extantfalse gharial, and consequently material from the genus was originally referred toTomistoma. Indeed, it was not until 1982 that the nameEogavialis was constructed after it was realised that the specimens were from a more basal form.[2]
The genus was firstdescribed byCharles William Andrews in 1901 when Andrews named a new species ofTomistoma,T. africanum, on the basis of a specimen found from an outcrop of theQasr el-Sagha Formation in Egypt, about 20 miles northwest ofFaiyum, dating back to thePriabonian stage of the lateEocene 37.2 to 33.9 million years ago. Other specimens were later found from theGebel Qatrani Formation, slightly younger than the Qasr el-Sagha dating back to theRupelian stage of the earlyOligocene 33.9 to 28.4 million years ago, and near the locality where the original specimen ofT. africanum was found in theFaiyum depression. A new species was also found from this locality and namedT. gavialoides by Andrews in 1905.
One of the first papers to identify the differences between these two species and others withinTomistoma was published in 1955 byJ. A. Kälin.[3] Other papers were written in the following decades that also questioned these species' relationships within Tomistominae.[4][5][6]Eric Buffetaut proposed the genus nameEogavialis in 1982 and reassigned bothT. africanum andT. gavialoides to it. In 2000, Brochu andGingerich argued thatT. gavialoides,T. kerunense andT. tenuirostre are all junior synonyms ofEogavialis africanum, since they're morphologically indistinguishable with the only difference in stratigraphic position.[1]
A third species was assigned toEogavialis in 2003 from material found in the 1990s from the lowerNawata Formation of theTurkana basin outcropping inLothagam,Kenya.[7] The strata from which the material was found dates back to the lateMiocene and earlyPliocene, around 11.61 to 2.59 million years ago. This extends the fossil range of the genus by approximately 17 million years. It was namedE. andrewsi for Charles Williams Andrews.[8] The holotype consists of a well preserved, nearly complete skull.
One reason whyEogavialis was initially placed withinTomistoma was due to the fact that thepremaxilla andnasal bones made contact with one another, a feature also seen inTomistoma. However, this characteristic has since been shown to be present in other extinct gavialids, meaning that premaxilla and nasal contact is aplesiomorphic trait of all tomistomines, includingbasal ones.Eogavialis also has a very similar cranial anatomy when compared toTomistoma, having the same proportions,rostral length, and tooth number, leading to the conclusion by some authors of papers published after 1982 thatEogavialis issynonymous withTomistoma.[9]
Tomistomines have been traditionally classified ascrocodiles. However, molecular analyses of the false gharial, the only living tomistomine, suggest that thesubfamily is actually withinGavialidae (along with the moderngharial of the subfamilyGavialinae) rather thanCrocodylinae.[10] The presence of a prominentcrista that runs along thepostorbital inEogavialis testifies to its position as a gavialid. Other characteristics such as a rectangular skull table, subcircular orbits with everted orbital rims, and a constricted antorbital area are also shared withEogavialis and other modern gavialids,[11][12] as seen in a well preserved skull ofE. africanum housed atYale (YPM 6263) and material from Kenya ofE. andrewsi.
Eogavialis has often been proposed to be non-tomistomine due to its lack of supposedly crocodylidsynapomorphies needed in order for a taxon to be placed within Tomistominae. The genus lacks the exposure of thevomer on thepalate that has been viewed as a characteristic of tomistomines.[13] The trend for a long, narrow rostrum developing progressively over time as seen inEogavialis has been used to suggest that the genus was a direct ancestor ofGavialis.Gryposuchus was once seen as phylogenetically betweenEogavialis andGavialis.[12]
Eogavialis africanum was included in the study on the phylogenetic relationships of putative fossil gavialoids published by Lee & Yates (2018). The authors considered it most likely thatE. africanum was not a gavialoid, or even a crocodylian, but rather a member of theclade of non-crocodylian eusuchians that also included the generaArgochampsa,Eosuchus,Eothoracosaurus andThoracosaurus.[14]
E. andrewsi was found influvial deposits within the Lower Nawata member of the Nawata Formation in Kenya. A broad, shallow, meandering river is thought to have existed at the time of deposition, suitable for an aquatic gavialid such asEogavialis. Evidence for a semideciduous treesavanna that may have surrounded the river is present in the lower beds, and a general trend in increased aridity can be seen in overlying beds in the member, suggesting a dry thornbush savanna environment.Fossils present from the strata that material fromE. andrewsi were found include those of numerousteleostfish such asosteoglossiformes andperciformes, manyturtles,crocodiles, and birds such asostriches, the enigmatic large birdEremopezus,anatids,rails, andowls, as well as manymammals representing both living and extinct taxa common inAfrica.
The area of the Gebel Qatrani Formation in the Faiyum Depression where most of the well-preserved specimens ofE. africanum andE. gavialoides were found was also deposited in a fluvialpaleoenvironment, although much older. Other fossils found from the formation include those of turtles, crocodiles,hyaenodontids, proboscideans such asPhiomia,Palaeomastodon, andMoeritherium, theEmbrithopodanArsinoitherium, numerous species ofhyraxes,artiodactyls, as well as some of the earliestsimianprimates such asApidium,Catopithecus,Oligopithecus, andAegyptopithecus.[15] Discoveries from this formation have added greatly to the understanding ofmammalian evolution in Africa. The presence of this type of fauna suggests a humid, tropical climate existed in Egypt during the Oligocene.
Much of the Gebel Qatrani consists of other deposits that represent both marine and non-marinesedimentary depositional environments.[16][17][18] Some specimens ofEogavialis are known from these strata as well,[19] suggesting that the genus may also have been adapted to a coastal marine habitat.[20] This differs from the mostly freshwater habitats inhabited by extantcrocodilians.
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