Crocodylus Depressifrons

''Asiatosuchus'' is an extinct genus of crocodyloid crocodilians that lived in Eurasia during the Paleogene. Many Paleogene crocodilians from Europe and Asia have been attributed to ''Asiatosuchus'' since the genus was named in 1940. These species have a generalized crocodilian morphology typified by flat, triangular skulls. The feature that traditionally united these species under the genus ''Asiatosuchus'' is a broad connection or symphysis between the two halves of the lower jaw. Recent studies of the evolutionary relationships of early crocodilians along with closer examinations of the morphology of fossil specimens suggest that only the first named species of ''Asiatosuchus'', ''A. grangeri'' from the Eocene of Mongolia, belongs in the genus. Most species are now regarded as ''nomina dubia'' or "dubious names", meaning that their type specimens lack the unique anatomical features necessary to justify their classification as distinct species. Other species such as ''"A." germanicus'' and ''"A." depressifrons'' are still considered valid species, but they do not form an evolutionary grouping with ''A. grangeri'' that would warrant them being placed together in the genus ''Asiatosuchus''.

Description

Like most other Paleogene crocodyloids, ''Asiatosuchus'' has a generalized crocodilian skull that is triangular in shape when viewed from above. ''Asiatosuchus'' species have teeth in the upper jaw that completely overlap the teeth in the lower jaw, giving them overbites. An overbite is a primitive feature among crocodyloids because modern crocodiles have teeth in the upper and lower jaws that interlock with each other with little overlap. ''Asiatosuchus'' can be distinguished from other early crocodyloids by its extended mandibular symphysis, the region where the two halves of the lower jaws connect. In many crocodyloids this joint is formed from two pairs of bones, the dentary bones and the splenial bones, but in ''Asiatosuchus'' it is only formed by the dentary bones. Based on largely complete skeletons of ''"A." germanicus'' and ''"A." depressifrons'', ''Asiatosuchus'' may have grown up to long.

Species

''A. grangeri''

''A. grangeri'', the type species of ''Asiatosuchus'', was named by paleontologist Charles Mook in 1940. It was named on the basis of a lower jaw and pieces of a skull from the Irdin Manha Formation of Inner Mongolia, China, which dates back to the Middle Eocene. These fossils were discovered by the American Museum of Natural History's Central Asiatic Expedition of 1930 near Erenhot. Mook named ''Asiatosuchus grangeri'' after Walter W. Granger, a vertebrate paleontologist with the American Museum of Natural History and a member of the expedition. Mook thought that ''Asiatosuchus grangeri'' was closely related to species of ''Crocodylus'' (modern crocodiles) but different in having 17 teeth in each half of the lower jaw and a splenial bone that does not form part of the mandibular symphysis.

''"A." germanicus''

Well-preserved remains of a crocodyloid were first described from Germany and France in 1966 and placed in a new species of ''Asiatosuchus'', ''A. germanicus.'' The German remains came from the Messel Pit quarry, a fossil site that has preserved many forms of life that inhabited a series of anoxic lakes and surrounding subtropical forests during the Eocene. Of all the species that have been assigned to ''Asiatosuchus'', ''"A." germanicus'' is known from the most complete material.

''"A." depressifrons''

''"Asiatosuchus" depressifrons ''was first named in 1855 as ''Crocodilus depressifrons.'' The naming of this new species was based on a skull found in the Sables du Castrais Formation, France that dates back to the Early Eocene. The skull was illustrated in the 1855 paper but it was not thoroughly described. The fossil has since become heavily pyritized, losing much of its original anatomical detail. After its naming, several other crocodilian fossils in European museum collections were labeled as ''C. depressifrons''. The species name ''depressifrons'' refers to the flattened shape of the frontal bone in the skull, a feature that is shared by all fossils attributed to the species. The fossils are also similar in having 6 pairs of teeth lining the symphysis at the tip of the lower jaw. As was the case for many other Paleogene crocodyloids, ''"A." depressifrons'' was originally placed in the still-living genus ''Crocodylus'' because the overall shape of its skull is similar to those of living crocodiles. Soon after Mook named ''Asiatosuchus grangeri'', ''C. depressifrons'' was reassigned to ''Asiatosuchus''.

Many new and much more complete fossils of ''"A." depressifrons'' have been found from Early Eocene deposits in Belgium. Together these specimens provide details on most of the skeleton. ''"A." depressifrons'' can be distinguished from all other species of ''Asiatosuchus'' by a combination of several characteristics including a large hole and a depressed area on the jugal bone of the skull, a frontal bone that does not touch the supratemporal fenestrae (two holes at the top of the skull behind the eye sockets), and a postorbital bone behind the eye socket that is visible when the skull is viewed from the side. Another distinguishing feature of ''"A." depressifrons'' is its lack of an overbite.

''"A." nanlingensis''

In 1964 Chinese paleontologist Yang Zhongjian named a new species of ''Asiatosuchus'', ''"A." nanlingensis'', based on fragmentary material from the Shanghu Formation in Nanxiong, Guangdong Province, China. Small coprolites (fossilized feces) were found alongside the type specimen of ''A. nanlingensis''. ''A. nanlingensis'' was discovered concurrently with the similar '' Eoalligator chunyii'', and a 2016 study proposed that they are synonymous. A subsequent paper argued otherwise, but other researchers agree with the initial assessment synonymizing ''"A." nanlingensis'' and ''"E." chunyii''. Since then, different analyses have recovered the species as a member of Mekosuchinae, or as basal to a clade including mekosuchines and Crocodylidae.

Other species

Two species of ''Asiatosuchus'' were named from Russia, ''A. zajsanicus'' in 1982 and ''A. volgensis'' in 1993. ''A. volgensis'' and ''A. zajsanicus'' were regarded as ''nomina dubia'' by Angielczyk and Gingerich (1998) because they are based on fossil specimens that preserve very little anatomical detail. ''A. zajsanicus'' was later reassigned to '' Dollosuchus'', a genus of tomistomine crocodilians, by Efimov (1988), but Brochu (2007) treated ''Dollosuchus'' as dubious.

Several crocodilian fossils from the Paleogene of North America have also been proposed to belong to ''Asiatosuchus''. In comparison to ''A. grangeri'', '' "Crocodylus" affinis'' from the Bridger Formation in Wyoming has a similarly shaped splenial bone in the lower jaw and frontal bone in the skull. Although ''"C." affinis'' is known from a complete skull, the skull material of ''A. grangeri'' is too fragmentary to support ''"C." affinis'' being classified within ''Asiatosuchus''.

The crocodyloid species ''"Crocodylus" monsvialensis'' was named from Early Oligocene deposits in Monteviale, Italy in 1914 and reassigned to ''Asiatosuchus'' in 1993, although subsequent authors questioned this referral and considered it synonymous with '' Diplocynodon ratelii''. ''"Crocodylus" vicetinus'' from the Middle Eocene locality of Monte Bolca, was assigned to ''Asiatosuchus'' sp. by Kotsakis et al. (2004) pending revision of the Mount Bolca crocodilian material.Kotsakis T, Delfino M, Piras P. 2004. Italian Cenozoic crocodilians: taxa, timing and biogeographic implications. Palaeogeography, Palaeoclimatology, Palaeoecology 210: 67–87.

A partial skeleton of a crocodyloid from the Sulaiman Mountains of Pakistan was tentatively attributed to ''Asiatosuchus''. The fossil was found in the Middle Eocene Drazinda Formation, a marine deposit which has also preserved the remains of archaeocete whales. The presence of a possible specimen of ''Asiatosuchus'' in marine deposits suggests that these crocodilians could have tolerated prolonged periods of time in the ocean, an ability that would have aided in the dispersal of early crocodyloids across Europe and Asia.

Phylogeny

Phylogenetic analyses of the evolutionary relationships of crocodilians place ''Asiatosuchus'' as a member of a clade or evolutionary grouping called Crocodyloidea, which includes living crocodiles and their extinct relatives. Recent phylogenetic analyses place ''Asiatosuchus'' as a basal ("primitive") member of this clade, close to the split between Crocodyloidea and Alligatoroidea, the group that includes living alligators, caimans, and their extinct relatives. Many of the species that are most closely related to species of ''Asiatosuchus'' were originally classified in the genus ''Crocodilus'' because they superficially resemble modern crocodiles. However, the majority of early crocodilians, even some early alligatoroids, resembled modern crocodiles because a triangular, crocodile-shaped head is a primitive condition for crocodilians.

Some phylogenetic analyses have placed ''"Asiatosuchus" germanicus'' as the sister taxon or closest relative of a group called Mekosuchinae. Mekosuchines are a group of crocodyloids from Australia and the South Pacific that are unusual in that they were highly specialized for life on land. If ''"A." germanicus'' is the sister taxon of Mekosuchinae, it may have been close to the ancestry of the group. The earliest known and most basal mekosuchine, ''Kambara'', lived during the same time as ''Asiatosuchus'', suggesting that ''Asiatosuchus'' or an ''Asiatosuchus''-like crocodyloid could have dispersed into Australia as the ancestor of mekosuchines. Despite the results of the phylogenetic analysis, ''"A." germanicus'' is an unlikely candidate for the ancestor of mekosuchines because it lived very far from Australia and the likelihood that it could have reached Australia from Europe is very low.

Most phylogenetic analyses do not support the idea that all species of ''Asiatosuchus'' belong to their own clade. Instead they find that ''Asiatosuchus ''species form a paraphyletic grouping, meaning that ''Asiatosuchus ''represents an evolutionary grade of successively more derived crocodyloids rather than its own separate lineage. Since a genus name is normally only applied to a monophyletic grouping by researchers who study prehistoric crocodilians, the type species ''A. grangeri'' is now considered the only valid species within ''Asiatosuchus''. The species ''"A." germanicus'' and ''"A. depressifrons'' are written in quotes because they do not belong to ''Asiatosuchus'' and have not yet been given different genus names.'' ''The relationships of other putative ''Asiatosuchus'' species are uncertain because only ''A. grangeri'', "''A." germanicus, ''and "''A." depressifrons'' have enough distinguishing features to be included in phylogenetic analyses.

Below is a cladogram from Delfino and Smith (2009) showing that ''Asiatosuchus ''species represent a non-monophyletic grouping. Delfino and Smith considered these relationships to have very weak support because only a few characteristics entered into the data matrix differed between ''Asiatosuchus ''species, and none differed between ''A. grangeri ''and ''"C." affinis.''

A 2018 tip dating study by Lee & Yates simultaneously using morphological, molecular (DNA sequencing), and stratigraphic (fossil age) data established the inter-relationships within Crocodilia, which was expanded upon in 2021 by Hekkala ''et al.'' using paleogenomics by extracting DNA from the extinct ''Voay''.

The below cladogram shows the results of the latest studies, which placed ''Asiatosuchus'' outside of Crocodyloidea, as more basal than Longirostres (the combined group of crocodiles and gavialids).

References

External links

''Asiatosuchus'' at the Paleobiology Database

{{Taxonbar, from=Q3282861, from2=Q20717084, from3=Q4806860

Crocodilia
Paleocene crocodylomorphs
Eocene crocodylomorphs
Prehistoric pseudosuchian genera