Diego Pol, Alan H. Turner, Mark A. Norell
Bulletin of the American Museum of Natural History 2009 (324), 1-103, (30 June 2009) https://doi.org/10.1206/0003-0090-324.1.1
We describe a new specimen of the fossil crocodyliform taxon Shamosuchus djadochtaensis from the Late Cretaceous Djadokhta Formation. The new specimen consists of an 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 djadochtaensis bears important information toward an understanding of the relationships of advanced neosuchians and the evolutionary origin of Eusuchia. Shamosuchus djadochtaensis is found to be the sister group of Rugosuchus nonganensis, comprising an Asian clade diagnosed by the presence of a sagittal ridge on 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 a longitudinal ridge on the lateral surface of the angular. This clade is inferred to be the sister group of Eusuchia, to the exclusion of Bernissartia fagesii and the Glen Rose form, based on the absence of an 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 of 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 in different regions of the vertebral column and the eusuchian type of palate, both traditionally considered as diagnostic of Eusuchia. All these features have complex evolutionary histories with several cases of convergences and reversals. Finally, a 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.