The base of the theropod tree has been shaky despite major advances in understanding avian origins and the evolution of coelurosaurs. A part of the problem has been the poor Triassic and early Jurassic record of the theropods. A key to unlocking the phylogeny of early theropods was a spectacular find from the Early Jurassic fossil beds of Antartica, Cryolophosaurus. Finally, a long needed osteological study of Cryolophosaurus was conducted by ND Smith et al which offers and extraordinary view of theropod evolution. The early phylogenetics studies of theropods hinted suggested that there were two major clades within them the ceratosaurs and the tetanurans. Further, staurikosaurids and Eoraptor were considered basal-most clades of theropods. But this has been questioned over the years with newer analysis of the “ceratosaurs” and the discovery of new fossils like the well-preserved Zupaysaurus from South America and the fragmentary Dracovenator from South Africa. The new analysis by Smith et al incorporates data from Cryolophosaurus and improves the picture vastly.
In a nutshell it makes the following points:
– Eoraptor and the staurikosaurids are basal saurischians rather than theropods.
– The basal-most theropod clade is a coelophysoid clade comprised of forms like Liliensternus, Coelophysis, Syntarsus and Segisaurus.
–Zupaysaurus is a sister group of all other remain theropod groups.
-Of the remaining theropods the basal-most clade is the dilophosaurid clade formed by Dilophosaurus, Dracovenator, Cryolophosaurus, and “Dilophosaurus” sinensis from the Lufeng formation.
-The next clade which forms a sister group of the tetanurans is the neoceratosaurid clade which includes within it Ceratosaurus, Elaphrosaurus and the abelisauroids.
-The basal-most tetanuran clade unifies the two south American forms Condorraptor and Piatnitzkysaurus.
-The major clades within tetanurae are: spinosauroids (including spinosaurids proper and Eustreptospondylus, Torvosaurus and Afrovenator); allosauroids (including sinraptorids, carcharodontosaurids and Megaraptor, which emerges as a carcharodontosaurid); coelurosauria.
mAnasa-taraMgiNI 