Puzzling dinosaur was early plant eater

A new dinosaur with a very strange mosaic of physical features was revealed to the world by researchers in South America in 2015. Chilesaurus, as they named it, had a body that looked remarkably like a small carnivorous relative of T. rex or Velociraptor, but also a short, rounded skull with leaf-shaped teeth that belonged unmistakably to a plant eater. Those scientists deduced that it was a member of the largely carnivorous ‘theropod’ family that had evolved a subsequent penchant for vegetarianism.

But a pair of British researchers have now looked at this puzzling chimera again, comparing 450 anatomical characteristics in 76 early species of dinosaurs to come to quite a different conclusion. As they report today in Biology Letters, they believe the 145-million-year old species was in fact a very early member of the so-called ‘ornithischian’ group of herbivores that includes species such as Triceratops, Stegosaurus and Muttaburrasaurus.

The very earliest dinosaurs of all were relatively small carnivores that walked on their two hind legs, and it may be similarities to these ancestors, rather than later theropods, that account for the species’ unusual mix of features, the scientists claim.

“Chilesaurus is one of the most puzzling and intriguing dinosaurs ever discovered,” says study co-author Paul Barrett at the Natural History Museum in London. “Its weird mix of features places it in a key position in dinosaur evolution and helps to show how some of the really big splits between the major groups might have come about.”

This Late Jurassic species may now help experts understand how important features of the plant-eating ornithischians evolved, such as guts adapted to digest plant matter and teeth and jaws specialised for grinding vegetation.

The study is the most thorough yet attempt to understand how Chilesaurus fits into the dinosaur family tree, says Andy Farke, a palaeontologist at the Raymond M. Alf Museum of Paleontology in Claremont, California. “This movement of Chilesaurus across the dinosaur tree was initially pretty surprising … but seems quite plausible based on the animal’s bizarre anatomy,” he says. “We should now expect to find more dinosaurs like Chilesaurus within a currently empty 65-million-year space between it and the presumed origin of its direct ancestors.”

The first fossils of Chilesaurus diegosuarezi were found in 2004 in southern Chile by a seven-year-boy, Diego Suarez, after whom the species was named.