Asiatosuchus

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. ==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." 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. 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, 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. 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==