Phylogenetic analysis of Late Triassic ? Early Jurassic neotheropod dinosaurs and their early biogeographic and evolutionary history

EZCURRA, M. D.

Berlin

Jornada; Centenary Meeting of the Paläontologische Gesellschaft; 2012

New discoveries and studies have improved our knowledge of early neotheropod dinosaurs in the last decade. However, updated and comprehensive phylogenetic analyses of Late Triassic and Early Jurassic theropods are currently lacking. In order to assess the phylogenetic relationships of these taxa, a data matrix composed of 39 terminals and 633 informative characters was compiled. In the most parsimonious trees recovered by the analysis, Liliensternus, Procompsognathus, Lophostropheus, Gojirasaurus and BP/1/5278 were recovered within a polytomy at the base of Coelophysoidea. The latter taxa were the sister-taxa of a clade composed of a ?Syntarsus? kayentakatae + Kayentavenator clade and a group including Segisaurus, Coelophysis bauri, Coelophysis rhodesiensis, Camposaurus, and an unnamed Mexican coelophysoid. The position of Kayentavenator elysiae indicates that it should be considered a junior synonym of ?Syntarsus? kayentakatae. The placement of BP/1/5278 does not agree with its proposed identification as a juvenile specimen of Dracovenator, and it may instead represent a distinct form of basal coelophysoid. After a posteriori pruning of wildcard taxa, Liliensternus was placed as the most basal coelophysoid, Lophostropheus as the most basal member of Coelophysinae, and Coelophysis bauri as the sister-taxon of a clade composed of Camposaurus, Segisaurus and Coelophysis rhodesiensis. Outside Coelophysoidea, Zupaysaurus was found as the sister-taxon of a group including Dilophosauridae and Averostra. Dilophosauridae was composed of Dracovenator, Cryolophosaurus and Dilophosaurus wetherilli. Within Averostra, Sarcosaurus woodi was recovered as a basal ceratosaur, probably representing the oldest member of the clade. Optimization of femoral length under a linear maximum parsimony criterion revealed a reduction of body size in Coelophysoidea and an overall increase in the lineage leading to Averostra. However, a conspicuous increase in body size is not documented during the Early Jurassic, contra to some prominent hypotheses of ?ecological release? for theropods following the Triassic-Jurassic extinction. The reconstructed body size evolution fits better a Browning Model (equivalent to a General Random Walk) rather than a Browning Model with trend. Thus, based on the available sample, the divergence in body size between coelophysoids and the clade including other neotheropods seems to have been mostly driven by passive events. The results also indicate that basal coelophysoids (i.e. those outside the ?Syntarsus? + Coelophysis clade) are currently the most abundantly sampled late Norian?Rhaetian theropods. However, following the Triassic?Jurassic extinction event, theropod assemblages are composed of derived coelophysoids, dilophosaurids and basal averostrans. Biogeographic analyses indicate that the Late Triassic neotheropod assemblages possessed a strong paleolatitudinal structure, but acquired a cosmopolitan distribution during the Early Jurassic. Accordingly, the Triassic-Jurassic boundary mass extinction appears to have had a deep impact on the early biogeographic and evolutionary history of Theropoda, resulting in a shift of the taxonomic content and biogeographical pattern of the group.