From: Michael Balter, Science AAAS
Published December 5, 2012 08:22 AM

The Earliest Known Dino?

A team of paleontologists thinks it may have identified the earliest known dinosaur—a creature no bigger than a Labrador retriever that lived about 243 million years ago. That's at least 10 million years earlier than the oldest known dinos and could change researchers' views of how they evolved. But some scientists, including the study's authors, caution that the fossils could instead represent a close dino relative.

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Tracing back the earliest dinosaurs has not been easy. Fossils that old tend to be fragmentary, and researchers don't always agree about their evolutionary pedigree. Paleontologists do agree, however, that pint-sized specimens found in Argentina and dated to 230 million years ago—with names like Eoraptor and Eodromaeus—are true dinosaurs. And in 2010, a team led by Sterling Nesbitt, a paleontologist at the University of Washington, Seattle, reported in Nature the discovery of a close dinosaur relative in Tanzania's Manda Beds, a geological formation dated to between about 242 million and 245 million years ago. That specimen, called Asilisaurus, is not a dinosaur, but belongs to a so-called "sister taxon"—that is, the closest it can be to a dinosaur without actually being one.

That made Nesbitt and his colleagues take a closer look at what else has been found in the Manda Beds. One set of fossils, including an arm bone and several vertebrae, had been discovered in the 1930s and studied for decades by Alan Charig, a famed paleontologist at London's Natural History Museum. Before he died in 1997, Charig named the specimens Nyasasaurus, but he never published his conclusions about whether it was a dinosaur.

For the new study, which also includes Nyasasaurus fossils housed in the South African Museum in Cape Town, Nesbitt's team carried out a systematic comparison of the bones with those of other dinosaurs and their relatives. The researchers, who report their findings today in Biology Letters, find a number of features characteristic of true dinos. For example, Nyasasaurus has a broad crest of bone along the edge of its upper arm, to which the animal's chest muscles would have attached; this crest appears to extend more than 30% of the bone's length, a telltale dino feature. Nyasasaurus also has three vertebrae in its sacrum, the part of the spine that is attached to the pelvis, whereas dino ancestors only had two. And a microscopic study of the arm bone, carried out by team member Sarah Werning of the University of California, Berkeley, shows that it had grown very rapidly during the animal's development, also typical of dinosaurs as well as later mammals and birds.

Article continues at Science AAAS

Image credit: Natural History Museum, London/Mark Witton

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