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Meet Eodromaeus, small predator from the dawn of the dinos

Weighing only 10-15 pounds and about 4 feet in length from snout to tail tip, pint-sized Eodromaeus (“dawn runner”) was discovered in 230-million-years old rocks in the foothills of the Andes and lies very close to the ancestor of all meat-eating dinosaurs including Tyrannosaurus.
[Illustration by Todd Marshall]

Researchers have discovered a new dinosaur, Eodramaeus, which lived during the dawn of the dinosaur era, about 230 million years ago.

The Eodramaeus' fossils were discovered in the Ischigualasto formation in northeastern Argentina, where paleontologists have found other important dinosaur fossils.

This small, two-legged runner from the late Triassic period is similar in some ways to another dinosaur that lived at time, Eoraptor. But, the dinosaurs also have significant differences that shed light on the early days of dinosaur evolution, a time we know relatively little about.

After comparing the two dinosaurs, Ricardo Martinez of Universidad Nacional de San Juan in San Juan, Argentina and his colleagues have concluded that Eodramaeus is an early ancestor of the theropod lineage, which includes the predatory dinosaurs.

And, Eoraptor, generally considered a theropod as well, is in fact an early ancestor of the sauropod lineage, which includes the giant, long-necked herbivores, the researchers say.

Eodramaeus had more theropod-like features in its skull, such as an opening near the end of the snout called the promaxillary fenestra, as well as other theropod-like features in its trunk pelvis and limbs, the authors report. Eoraptor not only lacked those features but also had more sauropod-like features, including enlarged nostrils and an inset first lower tooth.

Nonetheless, both species were less than 2 meters long and ran on two legs, and these general similarities suggest that the three principal groups of dinosaurs (the ornithischians, the sauropodomorphs and the theropods) did share an overall body plan in the late Triassic, before the dinosaurs rose to dominance in the early Jurassic, according to Dr. Martinez and his colleagues.


This research appears in the 14 January 2011 issue of the journal Science.