Paleontologists think that they may have identified the earliest known dinosaur, which was no bigger than a Labrador retriever and lived 243 million years ago, during the Triassic Period. This is 10 million years earlier than the previously oldest known dinosaurs, and could change researchers’ views of how they evolved.

The scientists published their findings in the journal Biology Letters. Some scientists, including the authors, caution that the fossils could also represent a close relative to dinosaurs.

The earliest dinosaurs were probably Eoraptor and Eodromaeus, which were true dinosaurs, and lived in what was Argentina 230 million years ago. In 2010, Sterling Nesbitt, a paleontologist at the University of Washington, Seattle, discovered a close relative to dinosaurs in Tanzania’s Manda Beds, a geological formation dated between 242 million and 245 million years ago. Asilisaurus isn’t a dinosaur, but a sister taxon, the closest that it could be without actually being a dinosaur.

Nyasasaurus was discovered in the 1930s by the paleontologist Alan Charig from London’s Natural History Museum. The fossils include an arm bone and several vertebrae, and were also discovered in the Manda Beds. Nesbitt and his colleagues took a closer look at them.

The scientists carried out a systematic comparison between the bones of dinosaurs and their closest relative species. The findings indicate that Nyasasaurus has a number of features characteristic of true dinosaurs. It had a broad crest of bone along the edge of its upper arm, which would have been attached to its chest muscles. The crest extends to more than 30% of the bone’s length, which is indicative of dinosaurs. Nyasasaurus had three vertebrae in its sacrum, whereas proto-dinosaur species had only two. A microscopic study of the arm bone showed that it had grown rapidly during the animal’s development, typical of dinosaurs, as well as mammals and birds.

This combination of features has led the scientists to think that Nyasasaurus was either a true dinosaur or the closest relative that has been discovered. By the time that Eoraptor and Eodromaeus appeared in Argentina, 10 million years later, they represented a diverse group that must have been evolving for millions of years. This could mean that dinosaur evolution must have begun a considerable time before that.

At that time, Africa was part of the supercontinent Pangaea, which included modern-day South America, Antarctica, and Australia. The scientists state that Nyasasaurus is probably, but not definitely a dinosaur, because the arm bone isn’t complete.

[via ScienceNOW]