Video: Dinosaur reconstruction

Eodromaeus never had to worry about that predator (Image: Mike Hettwer)

Triassic Park was a more diverse place than we thought, according to the latest finds from the 230-million-year-old Ischigualasto formation in north-eastern Argentina. Dinosaurs make up one-third of all vertebrate genera found in the fossil beds, and all three major dinosaur groups had already appeared – not bad for a time when the beasts were thought to be rare.

The newly recognised diversity is surprising because the fauna contains the oldest known dinosaurs, thought to have lived just a few million years after the first tiny ones appeared.

The Argentine deposits have been famed as the home of the world’s oldest true dinosaurs since the discovery in 1989 of Herrerasaurus. The petite 1.2-metre-long Eoraptor followed a few years later; both were identified as very primitive two-legged predatory dinosaurs called theropods, a group which ultimately gave rise to Tyrannosaurus rex and modern birds.


Now Ricardo Martinez of the National University of San Juan, Argentina, and colleagues have discovered another Triassic dinosaur in the deposits – a 1.2-metre-long predator that they have named Eodromaeus. They have also found that Eoraptor was not a predator at all, but a plant-eating sauropodomorph – a lineage that eventually evolved into the gigantic long-necked sauropod dinosaurs.

Close cousins

That reclassification is not surprising, says Sterling Nesbitt of the University of Washington in Seattle, who was not involved in the study. Because the various groups of dinosaurs separated only a few million years earlier, “only a few features differentiate them”, he says.

In this case, key evidence came from a close examination of the teeth, which revealed Eoraptor had the same spatulate, or leaf-shaped, teeth as the giant sauropods. Eodromaeus, by contrast, had a predator’s hooked teeth for tearing through flesh, says Paul Sereno of the University of Chicago, a co-author of the study.

The dinosaurs of Ischigualasto had already evolved features important for their later success, whether as predators or plant-eaters, says Sereno. That makes what happened next surprising: the dinosaurs’ march to dominance of vertebrate fauna apparently stalled during the remaining 30 million years of the Triassic.

“Why didn’t they take over in 1 to 5 million years?” Sereno wonders – that’s what would be expected in the traditional view of evolution as progress in adaptation.

Opportunity knocks

He thinks what happened to dinosaurs during the late Triassic was similar to what happened to mammals during the reign of the later dinosaurs. Triassic dinosaurs couldn’t displace the more abundant vertebrate groups – the rhynchosaurs and cynodonts – that dominated their world. Instead, they became victors by accident, expanding into niches left vacant by a major extinction event at the end of the Triassic.

Steve Brusatte of the American Museum of Natural History in New York City welcomes the “very important” find, but warns against drawing major evolutionary conclusions. He points out that huge holes remain in the Triassic fossil record – the next-oldest well-preserved dinosaur fossils are about 15 million years younger. “You have to be careful in drawing big pictures from one site,” he warns.

Journal reference: Science, DOI: 10.1126/science.1198467