Pictures: "Nasty" Little Predator From Dinosaur Dawn Found

Hold Me Closer, Tiny Dinosaur

Photograph courtesy Mike Hettwer

Needle-like teeth spike a full-size replica of the skull of the new Eodromaeus dinosaur, held by University of Chicago paleontologist Paul Sereno, also a National Geographic explorer-in-residence. (The National Geographic Society owns National Geographic News.)

Eodromaeus lived alongside—and now appears to have, in a sense, taken the place of—a very similar dinosaur species, Eoraptor (picture), Sereno said. "If you went back 230 million years ago and one of these creatures flitted by, you'd have to wonder which one it was."

Sereno and his team once thought Eoraptor was an ancestor of meat-eating dinosaurs. But due to recent analysis of Eoraptor fossils, as well as the discovery of Eodromaeus, he now thinks Eoraptor was an ancestor of the giant, long-necked, plant-eating dinosaurs called sauropods.

"That's the beauty of dinosaur origins," Sereno said. "Who could predict that these 10- to 15-pound [4.5- to 7-kilogram] creatures—both looking quite similar but eating different things—would end up evolving into things  as disparate as Diplodocus and Tyrannosaurus?"

The reclassification of Eoraptor makes sense, agreed Hans-Dieter Sues, a dinosaur expert at the National Museum of Natural History in Washington, D.C., who was not involved in the study.

"One thing that everyone noticed when Eoraptor was first discovered was that the back teeth were very odd-looking for a theropod," said Sues, also a contributing editor to the National Geographic News Watch blog. "It had these little leaf-shaped teeth in the back, and those are teeth you don't really find in theropods."

(Related: "Smallest Meat-Eating Dinosaur in North America Discovered.")

Published January 13, 2011

Complete Set

Photograph courtesy Mike Hettwer

Nearly all of the bones of Eodromaeus (pictured in a replica skeleton) have been accounted for—considered remarkable for such a small creature.

Based on its fossils, scientists think Eodromaeus, like its theropods descendents, stood and ran on two legs and had sharp teeth and grasping claws, which the new dinosaur used to snatch the young of other reptiles.

University of Maryland Paleontologist Thomas Holtz agreed that Eodromaeus is likely an early theropod ancestor.

"I think they've got a good case for it here," said Holtz, who wasn't part of the new study. "In terms of characteristics, it does seem to be very, very low in the theropod family tree."

It's not too surprising that Eodromaeus and Eoraptor looked very similar, he added. Both shared a common ancestor only about ten million years before, which in evolutionary terms is not a very long time.

"The closer we get to the common ancestor, the less time they've had to diverge, so they look a lot more like each other," he said. "If you go back far enough, eventually they're the same creature."

(Pictures: Oldest Dinosaur Embryos Show "Big Surprises."

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Published January 13, 2011

Valley of the Dawn Runner

Photograph courtesy Ricardo Martinez

The desolate Valley of the Moon in northwestern Argentina, where fossils of Eodromaeus and Eoraptor were found, was filled with lush forests 230 million years ago, according to the study co-author Sereno. "It was a gorgeous environment."

(Related: "New T. Rex Cousin Suggests Dinosaurs Arose in South America.")

Eodromaeus and Eoraptor shared this Triassic paradise with various other groups of reptiles, including parrot-beaked reptiles that were distantly related to dinosaurs, protomammals, and a number of large crocodile-like creatures.

Studying the shared traits that Eodromaeus and Eoraptor could help scientists paint a picture of the unknown last common ancestor of all dinosaurs, the University of Maryland's Holtz said.

That dinosaur Eve, Holtz said, "was probably bipedal, its hands may have already been adapted for grasping ... and its diet may not have been strictly meat or plants." By contrast, "the first dinosaurs may have been omnivores."

The new Eodromaeus dinosaur fossils are detailed and analyzed in this week's issue of the journal Science.

Published January 13, 2011