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‘Dawn Runner’ Fossil Sheds Light On Early Dinosaurs

January 14, 2011

Scientists have unveiled fossils of one of the earliest dinosaurs ever discovered — a petite, nimble carnivore from the late Triassic period some 230 million years ago.

Dubbed Eodromaeus, or “dawn runner”, the never-before seen species was a small, two-legged creature that may have been among the first dinosaurs to roam the Earth, making them likely ancestors of the famous Tyrannosaurus rex.

The scientists reconstructed the dinosaur from a nearly complete set of bones found in Ischigualasto, a well-explored rock formation in northwestern Argentina.  Many of the fossils were covered in iron encrustations, and required meticulous work under the microscope before casts could be constructed and a complete skeleton rebuilt.

Scientists say they discovered two near-complete dinosaur fossils adjacent to each other, something that helped shed light on the development of predatory dinosaurs known as theropods, which include the T. Rex.

“It really is the earliest look we have at the long line of meat eaters that would ultimately culminate in Tyrannosaurus rex near the end of the dinosaur era,” said Paul Sereno, a paleontologist at the University of Chicago and a National Geographic Explorer-in-Residence.

“Who could foretell what evolution had in store for the descendants of this pint-sized, fleet-footed predator?” he added

Fossils of small theropods are extremely rare.  This one is approximately six feet long, and had a long neck and tail, sharp claws and biting canine teeth.  The scientists believe it weighed around 10 to 15 pounds.

After examining its limbs, the scientists believe they have identified differences between the “dawn runner” and its contemporary, the Eoraptor, which they now believe belonged to the large, long-necked and four-legged sauropod lineage.  

Both species were roughly the same size and ran on two legs, the scientists said.  This suggests that the three main types of dinosaur — ornithischians, sauropodomorphs, and theropods — that lived during the late Triassic period had similar body types.

However, the newly discovered Eodromaeus’ skull resembled those of other theropods, while the plant-eating Eoraptor “had more sauropod-like features, including enlarged nostrils and an inset first lower tooth,” wrote the scientists in a report about the study.

An examination of the complete fossil record from the area revealed that early dinosaurs “were more common and diverse than previously thought,” the researchers said.

After recording thousands of fossils in the area, scientists believe that dinosaurs came to dominate the landscape slowly over the course of millions of years.

“Dinosaurs took their sweet time to dominate the scene,” said Ricardo Martinez of Argentina’s National University of San Juan, who led the current study.

“The story from this valley suggests that there was no single advantage or lucky break for dinosaurs but rather a long period of evolutionary experimentation in the shadow of other groups,” said Sereno.

The study was published Thursday in the journal Science.

Image 1: This reconstruction of Eodromaeus provides a look at the earliest stage in the evolution of the flesh-eating dinosaur lineage, called theropods, some 230 million years ago. “Dawn runner” features a scaled face for protection, saber-shaped upper teeth for snatching prey, draped neck skin for swallowing large prey and fringe of rudimentary, bristle-like feathers. (Copyright Mike Hettwer)

Image 2: Paul Sereno, Professor in Organismal Biology & Anatomy and National Geographic Explorer-in-Residence, dates Eodromaeus from the dawn of the dinosaur era, some 230 million years ago. (Copyright Mike Hettwer)

Image 3: Pint-sized Eodromaeus (“dawn runner”) weighed only 10 to 15 pounds and measured about 4 feet in length from snout to tail tip. It lies very close to the ancestor of all meat-eating dinosaurs, including Tyrannosaurus. (Copyright Todd Marshall)

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