Introduction to Herrerasaurus
Found in the Late Triassic of the Ischigualasto Formation of northwestern Argentina, Herrerasaurus ischigualastensis is an early archosaur 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 redescribe Herrerasaurus 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 to saurichian dinosaurs, which had previously led to Herrerasuarus 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 to Herrerasaurus. These include the incompletely known Staurikosaurus pricei from southern Brazil and northwestern Argentina and Ischisaurus cattoi, which is very similar to Herrerasaurus and may even be the same species. The North American Chindesaurus briansmalli, from the Chinle Formation, may also be related.
Systematic relationships of Herrerasaurus 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 about Herrerasaurus and the Triassic World in lecture notes by Paul Olsen at Columbia University.