MORPHOLOGY OF THE LATE CRETACEOUS CROCODYLOMORPH SHAMOSUCHUS DJADOCHTAENSIS AND A DISCUSSION OF NEOSUCHIAN PHYLOGENY AS RELATED TO THE ORIGIN OF EUSUCHIA

摘要

We describe a new specimen of the fossil crocodyliform taxon Shamosuchus djadochtaensis from the Late Cretaceous Djadokhta Formation. The new specimen consists of all almost complete skull found in association with postcranial material. Because it is considerably more complete than the holotype, the new specimen permits proper diagnosis of Shamosuchus djadochtaensis and offers new information for exploring its phylogenetic relationships. The phylogenetic analysis conducted here improves taxon sampling of neosuchian crocodyliforms with respect to previous approaches to crocodyliform systematics and reveals that Shamosuchus c djadochtaensis bears important information toward all understanding of the relationships of advanced neosuchians and the evolutionary origin of Eusuchia. Shamosuchus c djadochtaensis is found to be the sister group of Rugosuchus nonganensis. comprising an Asian clade diagnosed by the presence of a sagittal ridge oil the dorsal surface of the frontal. confluent openings for the exit of cranial nerves IX-XI, a posterior region of the palatine bar between suborbital fenestra that is flared posteriorly, and it longitudinal ridge oil the lateral surface of the angular. This chide is inferred to be the sister group of Eusuchia to the exclusion of Bernissartia fagesii and the Glen Rose form, based oil the absence of all acute anterior tip of the frontal that wedges between the nasals, the presence of rodlike neural spines in the posterior cervical vertebrae, procoelous cervical vertebrae, and the presence or hypapophyses in the three anteriormost dorsal vertebrae. Incorporating the new information into the phylogenetic analysis indicates the decoupled nature of the evolutionary history of procoely ill different regions of the vertebral column and the eusuchian type of palate. both traditionally considered its diagnostic of Eusuchia, All these features have complex evolutionary histories with several cases of convergences and reversals. Finally, it review of all the available evidence on the diversity of advanced neosuchians suggests this group achieved a worldwide distribution and a remarkable morphological diversity, pushing their evolutionary origins back to the Jurassic.