Herrerasaurus
Staurikosaurus
? Chindesaurus
? Caseosaurus
Herrerasaurids are among the oldest known dinosaurs, appearing in the fossil record about 228 million years ago (mid-Triassic). They became extinct by the end of the Triassic period. Herrerasaurids were small to medium-sized carnivores generally resembling theropods, which may be their closest relatives. The best known representatives of this group are from South America (Brazil, Argentina), where they were first discovered in the 1960s. A nearly complete skeleton of Herrerasaurus ischigulastensis was discovered in Argentina in 1988. Less complete herrerasaurids have been found in North America, and they may have inhabited other continents as well.
Herrerasaurid anatomy is unusual and specialized, and they are not considered to be ancestral to any later dinosaur group. They often present a mixture of very primitive and derived traits. The acetabulum is only partly open, and there are only two sacral vertebrae, the lowest number among dinosaurs. The pubic bone has a derived structure, being rotated somewhat posteriorly and folded to create a superficially tetanuran-like terminal expansion, especially prominent in H. ischigulastensis. The hand is primitive in having five metacarpals and the third finger longer than the second, but theropod-like in having only three long fingers, with curved claws. Herrerasaurids also have a hinged mandible like theropods, but this may have evolved independently.
