Herrerasaurus ischigualastensis

(huh-RAYR-uh-SAW-rus ISH-ee-gwuh-las-TEN-sis)

Period: Triassic

Location: Ischigualasto Formation, Valle Fertil, San Juan, Argentina

Original in Collection: Museo de Ciencias naturales, Universidad Nacional de San Juan, Argentina

Reproduction in Collection: Texas Memorial Museum at Austin

Size: 13 feet long, 4 feet tall, 460 pounds

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Texas has some fragments of Herrerasaurus and a similar animal named Chindesaurus bryansmalli, which has the distinction of being the oldest known true dinosaur from Texas.  Sadly there are not enough fragments of this animal to mount, so we have substituted this specimen collected in Argentina.

During the Late Triassic, several similar species lived alongside Herrerasaurus.  Most of these animals were carnivores, a few were herbivores.  They were all slender bipeds, 10-20 feet long with a narrow skull.

The bone structures, used by scientists to classify true dinosaurs, are very primitive in this animal.  It is still being debated whether Herrerasaurus is a derived form of Archosaur thus not quite a dinosaur, or a basal theropod.  The evidence is leaning toward the latter.  That means earlier dinosaur fossils need to be found.

Herrerasaurus is a direct descendent of the Archosaurs.  One of the differences between animals like Dimetrodon and the archosaurs is that these animals have two holes behind the eye socket.  This is called the diapsid condition.  The holes were attachment locations for strong jaw muscles.  All the Archosaurs and their descendants have the two holes: lizard, snakes, crocodiles, dinosaurs, pterosaurs, and birds.  Dinosaurs have two additional holes, one in front of the eye hole and one in the jaw, making their skulls very light and their jaws very powerful.