PHYLOGENETIC ANALYSIS OF LATE TRIASSIC ? EARLY JURASSIC NEOTHEROPOD DINOSAURS: IMPLICATIONS FOR THE EARLY THEROPOD RADIATION

EZCURRA, M. D.

Raleight

Congreso; 72nd Annual Meeting of the Society of Vertebrate Paleontology; 2012

New discoveries and studies have improved our knowledge of early neotheropod dinosaursin the last decade. However, an updated and comprehensive phylogenetic analysis ofLate Triassic and Early Jurassic theropods is currently lacking. In order to assess thephylogenetic relationships of these taxa, a data matrix composed of 39 terminals and 633informative characters was compiled. In the most parsimonious trees recovered by theanalysis, Eodromaeus was found as the sister-group of Neotheropoda and Liliensternus,Procompsognathus, Lophostropheus, Gojirasaurus and a specimen previously identified as ajuvenile of Dracovenator, 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, Coelophysisrhodesiensis, Camposaurus and an unnamed Mexican coelophysoid. The position ofKayentavenator elysiae indicates that it should be considered a junior synonym of?Syntarsus? kayentakatae. The putative juvenile specimen of Dracovenator might actuallyrepresent 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 basalmember of Coelophysinae, and Coelophysis bauri as the sister-taxon of a clade composedof Camposaurus, Segisaurus and Coelophysis rhodesiensis. Outside Coelophysoidea,Zupaysaurus was found as the sister-taxon of a group including Dilophosauridae andAverostra. Dilophosauridae was composed of Dracovenator, Cryolophosaurus andDilophosaurus wetherilli, in agreement with some previous analyses. Within Averostra,Sarcosaurus woodi was recovered as a basal ceratosaur, probably representing the oldestmember of the clade. Optimization of femoral length under a maximum parsimony criterionrevealed a reduction of body size in Coelophysoidea and an overall increase in the lineageleading to Averostra. However, a conspicuous increase in body size is not documentedduring the Early Jurassic, contra to some prominent hypotheses of ?ecological release? fortheropods following the Triassic?Jurassic extinction. The results also indicate that basalcoelophysoids (i.e. those outside the ?Syntarsus? + Coelophysis clade) are currently themost abundantly sampled late Norian?Rhaetian theropods. However, following the Triassic?Jurassic extinction event, theropod assemblages are composed of derived coelophysoids,dilophosaurids and basal averostrans. Accordingly, this mass extinction appears to havehad a deep impact on the early evolutionary history of Theropoda, resulting in a shift of thetaxonomic content of the group across the Triassic?Jurassic boundary.