MEGARAPTORANS AS MEMBERS OF AN UNEXPECTED EVOLUTIONARY RADIATION OF TYRANT-REPTILES IN GONDWANA

NOVAS, FERNANDO EMILIO; AGNOLÍN, FEDERICO LISANDRO; EZCURRA, MARTÍN; CANALE, JUAN IGNACIO; PORFIRI, JUAN DOMINGO

Jornada; XXVI Jornadas Argentinas de Paleontología de Vertebrados; 2012

Megaraptor and its close relatives were poorly known carnivorous dinosaurs that inhabited South America, Australia, Asia, and possibly Africa, during Early to Late Cretaceous times. These theropods became relevant in the last years with the discovery of more complete skeletons. Recent phylogenetic analyses have viewed Megaraptor and its close relatives within a monophyletic group named as Neovenatoridae. This clade includes Megaraptor, Aerosteon, Orkoraptor, Chilantaisaurus, Fukuiraptor, Neovenator, and Australovenator. Neovenatorids were considered members of Allosauroidea, and particularly as the sister group of Carcharodontosauridae, as a clade of allosauroids that survived up to the end of the Cretaceous. However, we found anatomical information that supports that neovenatorids are a non-monophyletic group, and that Megaraptor and related genera are deeply nested within Coelurosauria and closely related to the Asiamerican Tyrannosauridae. Among coelurosaurian synapomorphies, these theropods share elongate metacarpals, ilium with enlarged fossa cuppedicus, distal end of tibia with a flat facet for the reception of the ascending process of the astragalus and gracile fibula and metatarsals. The Asian genus Fukuiraptor is recovered as the basalmost form of this new coelurosaurian clade. The phylogeny proposed here indicates that Neovenator is remotely related to Megaraptor and its kin, and indicates that this taxon is more closely related to Carcharodontosauridae, rather than with Coelurosauria. Chilantaisaurus from the Early Cretaceous of China is considered as an uncertain Coelurosauria. The newly recovered theropod clade considerably improves our knowledge about the scarcely documented basal radiation of coelurosaurs, filling a 15 MY gap in tyrannosauroid evolution.