In this week’s Nature, researchers say they’ve analyzed a near-complete skeleton of one of the closest relatives to early dinosaurs, a silesaur called Asilisaurus. The fossil is more than 240 million years old, which is ten million years older than the earliest known fossils of true dinosaurs. The finding of this dino relative therefore suggests that dinosaurs emerged earlier than we previously believed, and it throws another surprise into the debate over their origins.

From the remains of 14 different individuals, the scientists managed to piece together what a whole skeleton looked like. However, the finished product didn’t look quite like they expected. After studying the bones for 3 years, the team concludes that Asilisaurus was about the size of a Labrador retriever. The animal walked on four legs, and the shape of its teeth suggests that it ate plants and maybe a little meat.[ScienceNOW]. That conflicted with the expectation of study coauthor Randy Irmis, who said the team would’ve thought small carnivores, and not mostly plant eaters walking on four legs, were the closest relatives to the dinosaurs.

Indeed, that question remains open. According to the Nature editor’s summary, Asilisaurus is an early member of the Ornithodira line, the “avian” group that broke off from the crocodile group during the time before dinosaur emergence. What does that mean for the dinosaur ancestry? The balance of opinion has alternated between more reptilian ancestors, which walked on all fours, and two-legged animals that had bird-shaped bodies but couldn’t fly. Recently, the idea of two-legged dino ancestors had been winning out, but the new find yanks the trend back toward quadrupeds [ScienceNOW].

Paul Barrett of London’s Natural History Museum says: “The creatures share a lot of features with dinosaurs,” he said. “They show us an intermediate step between more primitive reptiles and the more specialised dinosaurs” [BBC News]. While dinos hung around for 165 million years or so, the silesaurs like Asilisaurus lived only 45 million years before extinction. However, since silesaurs and true dinosaurs diverged from a common ancestor, the two groups should have existed during the same time frame [National Geographic]. Thus, the earliest emerging dinos might stretch back even to the time frame of this Asilisaurus, more than 240 million years ago.

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Images: Sterling Nesbitt, Marlene Hill Donnelly / Field Museum