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Herrerasaurus ischigualastensis

Tallest? Longest? Heaviest? Fiercest? These are common questions among dinosaur lovers. But what's the oldest, and when did ordinary reptiles become dinosaurs? Tough questions and no easy answers. Herrerasaurus is one of two primitive dinosaurs that mark the transition to dinosaurs. 225 million years ago in the Late Triassic, these first dinosaurs were stalking food in the cycad, fern and conifer forests of what was to become modern South America. The dinosaurs' spread across the world was well underway and for the next 160 million years they dominated all life on land. At that time, all the world's land formed one giant super continent called Pangea. It was a warmer world, hot, tropical and without polar ice caps or flowers.

Herrerasaurs were meat-eating dinosaurs that walked on two legs and grew to lengths of 3-4 meters (10-12 ft.) while standing about 1 meter (3 ft.) tall at the back. They have a physical resemblance and posture close to the later allosaurs, tyrannosaurs, and other Theropods.

Taxonomy
Paleontologists divide dinosaurs into two major groups: saurichians (lizard-hipped) and ornithischian (bird-hipped). Both groups evolved from a yet-to-be discovered common ancestor. Current thinking has Herrerasaurus branching off earlier than the split between saurischians and orinthischians, but after dinosaurs separated from other reptiles. It has skeletal features common to both saurischians and ornithischians. The hip arrangement is similar to the ornithischians, but some of the bones making up the hip are shaped like the bones of saurischians. Its shin bones are shaped like those of sauropods, but its shoulder blades are closer to those of the ornithischians. Herrerasaurus' foot was built in a fashion that resembles that of the protosauropod Plateosaurus. Our limited knowledge and attempts to classify Herrerasaurus are based on material from only three partial skeletons found in Argentina.

Anatomy
As you look at the skull, take note of how it resembles other meat-eaters. A long narrow skull filled with typical theropod teeth: flattened, curved, serrated, and all the same shape. The large orbits for the eyes are set in an open, strut-like bone arrangement that made the skull lightweight and strong. The nasal openings are at the front of the snout.

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