eoraptor

Student Ricardo Martinez found the fossilized Eoraptor less than a mile from the place where, in 1988, Sereno discovered a nearly complete skeleton of the oldest known dinosaur, Herrerasaurus.

Careful examination of the fossil made it clear that Eoraptor represents a very early stage in dinosaur evolution.

Because it is the most primitive dinosaur ever found, Eoraptor provides an unprecedented look backward into the earliest evolution of these beasts from an ancestral form that spawned all later dinosaurs.

The Eoraptor and Herrerasaurus skeletons found by Sereno and his colleagues come from the Ischigualasto Provincial Park in northwestern Argentina.

As dinosaurs go, Eoraptor was tiny, reaching only a meter long.

According to Sereno, Eoraptor's skull was particularly primitive, displaying almost none of the specialized adaptations that characterize the three main dinosaur subgroups: the meat-eating theropods, which include Tyrannosaurus;the gargantuan sauropods, which include brontosaurus; and the herbivorous ornithischians, which include Triceratops.

Even Herrerasaurus, a contemporary of the 225-million-year-old Eoraptor, seems specialized in comparison.

Other parts of the Eoraptor anatomy also come close to that of the ancestral dinosaur, although the new fossil does show some specialization.

During the dinosaurs' heyday, long after Eoraptor had vanished from Earth, the ornithischians, sauropods, and theropods each evolved specialized bodies suited to their modes of life.

To visualize that ancestral dinosaur, many paleontologists have looked to a small reptile called Lagosuchus, which lived in the Ischigualasto region in the mid-Triassic period, 10 million years before Eoraptor. While not a dinosaur, Lagosuchus had evolved some features similar to those of the later beasts.

According to Sereno, the primitive nature of the Eoraptor skull supports the theory that dinosaurs arose from a common ancestor and only later developed specializations that would split them into ornithischians and saurischians.

On the basis of Herrerasaurusand other finds, researchers have pictured the earliest dinosaurs as small, bipedal carnivores-a description that matches Eoraptor. This dinosaur itself could not have been the common ancestor because it had already evolved specializations that place it within the theropod subgroup.

Even paleontologists prepared for finding small dinosaurs might have been surprised by the tiny size of Eoraptor, says Sues.

From months of prospecting in the region, Sereno knows that dinosaur skeletons are among the rarest fossils in Ischigualasto's rocks dating from the time of Eoraptor. This indicates that Eoraptorand the other dinosaurs alive at the time played a relatively small role in the ecology of the region.

The Eoraptor and Herrerasaurus skeletons show that dinosaurs had developed many specialized anatomical features early, long before the reptiles became common.