Introduction to Herrerasaurus

Skull ofHerrerasaurus. Photo © Lou Tremblay.

Found in the LateTriassic of theIschigualasto Formation of northwestern Argentina,Herrerasaurus ischigualastensis is an earlyarchosaur and on the verge of being a dinosaur proper. The first specimen was found in 1958 by Victorino Herrera, for whom the fossil was named. This skeleton was incomplete, but the discovery of a complete skull in 1988 and additional fragments have provided enough information to make a complete reconstruction; this has also permitted Paul Sereno at the University of Chicago to redescribeHerrerasaurus properly in a series of papers published in the Journal of Vertebrate Paleontology. Material found thus far suggests that it was a large carnivore about three to four meters long.

Herrerasaurus has all but a few of the characters which define the dinosaurs, lacking only certain features of the hip and leg bones. The pelvic structure is similar tosaurichiandinosaurs, which had previously led toHerrerasuarus being classified in that group. This arrangement of hip bones, however, is ancestral in the archosaurs and not uniquely derived.

Also in the Middle to Late Triassic of South America, other dinosaur relatives have been found which may be closely related toHerrerasaurus. These include the incompletely knownStaurikosaurus pricei from southern Brazil and northwestern Argentina andIschisaurus cattoi, which is very similar toHerrerasaurus and may even be the same species. The North AmericanChindesaurus briansmalli, from the Chinle Formation, may also be related.

Systematic relationships ofHerrerasaurus and its relatives are far from certain. While some analyses suggest they are sister to the dinosaurs, others consider them saurischian or even theropods. The importance of this group is that they give us some idea of the time at which dinosaurs evolved (towards the end of the Triassic) and what the earliest dinosaurs would have looked like.

Read aboutHerrerasaurus and theTriassic World in lecture notes by Paul Olsen at Columbia University.

Sources:

• Carroll, R.L. 1988. Vertebrate Paleontology and Evolution. W.H. Freeman, NY.
• Sues, H.-D. 1990.Staurikosaurus and Herrerasauridae.In D.B. Weishampel et al. (eds.). The Dinosauria. University of California Press, Berkeley.
• Zago, M. (ed.). 1995. Ischigualasto Talampaya: Tiempo de dinosaurios (Time of dinosaurs). Manrique Zago ediciones, Buenos Aires.
• Novas, F.E. 1997. Herrerasauridae.In P.J. Currie and K. Padian (eds.). Encyclopedia of Dinosaurs. Academic Press.