Eoraptor lunensis (3D recon)

Eoraptor lunensis is roughly the same age as Herrerasaurus - 232 million years old - yet it already shows specialized features which indicate that it lies several branches up from the base of the dinosaurian family tree. The exact nature of these features suggest that Eoraptor lies at the base of the great Theropoda lineage - the theropod dinosaurs - which includes many celebrity species, such as Tyrannosaurus rex, Allosaurus, Deinonychus, and Velociraptor. Some of these became huge, although many remained small, like Eoraptor and Herrerasaurus. It is among these small theropod dinosaurs that we can now trace the ancestry of living birds. Although giant theropods like Tyrannosaurus went extinct at the end of the Cretaceous time period, the avian branch of the theropod family tree flew unscathed through whatever it was that caused the extinction of its larger cousins 66 million years ago. By CT scanning Eoraptor, we hope to learn more about the exact relationship among these earliest members of the avian family tree. 3D reconstruction of 274 0.5 mm CT slices, side view, pseudocolored. Specimen is 14 cm long. (Sample courtesy of Universidad Nacional de San Juan, Argentina and Dr. Oscar Alcober, University of Texas at Austin)