![]() 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. The acetabulum is only partly open, and there are only two sacral vertebrae, the lowest number among dinosaurs. They only superficially resemble theropods and often present a mixture of very primitive and derived traits. Herrerasaurid anatomy is unusual and specialized, and they are not considered to be ancestral to any later dinosaur group. Less complete possible herrerasaurids have been found in North America, and they may have inhabited other continents as well. A nearly complete skeleton of Herrerasaurus ischigualastensis was discovered in the Ischigualasto Formation in San Juan, Argentina, in 1988. The best known representatives of this group are from South America ( Brazil, Argentina), where they were first discovered in the 1930s in relation to Staurikosaurus and 1960s in relation to Herrerasaurus. Herrerasaurids were relatively small-sized dinosaurs, normally no more than 4 metres (13 ft) long, although the holotype specimen of " Frenguellisaurus ischigualastensis" (nowadays considered a synonym of Herrerasaurus ischigualastensis) is thought to have reached around 6 meters (20 ft) long. They are among the oldest known dinosaurs, first appearing in the fossil record around 233.23 million years ago (the Carnian stage of the Late Triassic), before becoming extinct by the end of the Carnian stage. ![]() Herrerasauridae is a family of carnivorous dinosaurs, possibly basal to either theropods or even all of saurischians, or even their own branching from Dracohors, separate from Dinosauria altogether. ↓ Possible Late Norian records if Chindesaurus and Caseosaurus are part of the family.
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