A REVIEW OF THE PHYLOGENETIC RELATIONSHIPS WITHIN AETOSAURIA (ARCHOSAURIA: PSEUDOSUCHIA) AND THE PARAPHYLY OF AETOSAURINAE

DESOJO, J. B.; EZCURRA, M. D.

San Juan

Congreso; IV Congreso Latinoamericano de Paleontología de Verterbados; 2011

Universidad Nacional de San Juan

The aetosaurs are a group of pseudosuchian archosaurs from the Late Triassic of South America, North America, Europe, India, and Africa. The most recent phylogenetic analyses recovered two main lineages of aetosaurs: aetosaurines and desmatosuchines (Parker 2007). Within the former clade, the North American and European typothoracisines were found as a monophyletic entity sister-group of an unnamed clade composed of globally distributed species. In the present contribution we review the aetosaur phylogenetic intrarelationships based on a modified version of Parker et al. (2008) data matrix. The analysis resulted in three most parsimonious trees, in which the late Carnian-early Norian South American Aetosauroides scagliai Casamiquela 1960 was found as the most basal aetosaur and sister-taxon of the stagonolepidids. The base of Stagonolepididae Lydekker, 1887 is represented by a polytomy composed of Neoaetosauroides Bonaparte, 1969, Coahomasuchus Heckert and Lucas 1999, Aetosaurus Fraas 1877, and more derived aetosaurs, supported by the presence of a synapomorphic ?slipper-shaped? mandible. In this context, aetosaurines are a paraphyletic group. In the next lesser inclusive node a trichotomy is found, which is represented by Stagonolepis robertsoni Agassiz, 1844, Calyptosuchus wellesi Long and Ballew, 1985, and more derived forms, which is characterized by the presence of paramedian dorsal osteoderms strongly flexed ventrally. A new Brazilian genus (CPE2 168) was recovered as the sister-taxon of a monophyletic clade composed of desmatosuchines and typothorasicines. This node is supported by the absence of cervical vertebrae with a ventral keel. This phylogenetic arrangement contrast with that published by Parker (2007) and Parker et al. (2008) in that the typothoracisines were found as more closely related to desmatosuchines than to the remaining aetosaurs. The Typothoracisinae plus Desmatosuchinae clade is supported by a synapomorphic presacral neural spine generally lower than the height of the centrum. The new phylogenetic hypothesis recovered here depicts a different scheme for the evolution of osteoderm ornamentation. In this regard, the presence of a radial pattern of ornamentation is optimized as a symplesiomorphic character of Aetosauria, which is retained by basal members of the clade (e.g. Neoaetosauroides, Coahomasuchus, Aetosaurus). The radial ornamentation seems to be plesiomorphic to the random and reticular patterns which are present in some desmatosuchines and typothorasicines, respectively.