Theropod classification

Sunday, March 27, 2011

The Herrerasauridae are an early group represented by Herrerasaurus, which was discovered in a wonderful middle-late Triassic period fossil locality (the famous Ischigualasto Formation) in Argentina in the 1970s. Another herrerasaur is Staurikosaurus, which has been known since the 1960s from remains found in Brazil.

More recently (in 1993), another herrerasaur-like fossil was found in the same general area and named Eoraptor, or "dawn thief." It appears to be closely related to the herrerasaurs, but smaller in size and slightly older. Both Eoraptor and the herrerasaurs seem to have been small to medium-sized carnivores. These curious animals have some basic theropod characteristics, but lack others; in fact, they lack some dinosaurian characteristics as well.

The Herrerasauridae and Eoraptor may be the earliest group of theropods, or it is quite possible that they are not even theropods at all, but rather non-dinosaurs (dinosauromorphs) closely related to the ancestor of dinosaurs. The fact is, we don't know for sure. Experts in dinosaur systematics are currently embroiled in a controversy over the exact relationships of the Herrerasauridae to theropods and other dinosaurs.

A second group of theropods is the Ceratosauria, a more morphologically modified and diverse group which includes the UCMP's very own Dilophosaurus, one of the stars of the novel and movie Jurassic Park. Segisaurus is a small, mysterious theropod known from only one specimen, which is housed in the collections of the UCMP. Recent discoveries have revealed that ceratosaurs formed a more diverse group than was previously expected....

The last, and by far the largest group of theropods is the Tetanurae, consisting of two major clades (sister taxa), the Carnosauria and the Coelurosauria. Some early tetanurines such as Megalosaurus fall outside of this dichotomy, but most are poorly known.

The carnosaurs were the huge, fierce predators such as Allosaurus (shown at the top of this page chasing Dryosaurus, an ornithischian dinosaur), and recent headline-makers like the gigantic Carcharodontosaurus and Giganotosaurus, both of which seemed to have reached or exceeded the size of T. rex, making them the largest terrestrial bipeds ever to have terrorized the terrestrial realm.

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