By Matt Kaplan
A small, toothy dinosaur known as Eoraptor, long considered to be the earliest member of the predatory branch of the dinosaur tree, is revealed to have traits that instead place it at the base of another dinosaur group that contains the largest of the plant-eaters.
The study, published in the January 14 issue of Science, revolves around the discovery of a new dinosaur species that lived round the same time as Eoraptor in the late Triassic, some 230 million years ago. Dubbed Eodromaeus murphi, it was discovered in the Ischigualasto Formation, a geological basin in northwestern Argentina that is riddled with some of the oldest dinosaur remains known. The fossil remains of Eodromaeus show features in the skull, forelimbs, hindlimbs and pelvic girdle that link it to the theropods, the group dominated by meat-eaters such as Tyrannosaurus rex and Velociraptor mongoliensis.
Tiny Eodromaeus is a two-legged active predator, judging from its jaws, "hands" and hind legs. The jaws are studded with saber-shaped teeth with fine serrations for cutting meat; the hands have long fingers and sharp claws for a powerful and deadly grasp; the legs have the elongated proportions of a fast runner. The end-half of the tail, in addition, is stiffened by overlapping structures that helped it to function as a balancing beam during swift turns. All of these features were inherited by later theropods.
"Eodromaeus gives us the earliest picture of this predatory line," says Paul Sereno, lead author of the new study, "but in the course of making this discovery we realized that Eoraptor was incorrectly placed at the base of the theropod family tree."
Dinosaurs can be divided into three distinctive groups, the predominantly meat-eating theropods, the long-necked plant-eating sauropodomorphs such as Diplodocus, and the highly varied plant-eating ornithischians such as Triceratops and Stegosaurus.
There are few doubts about the identity of the known early ornithischian dinosaurs, because their bones and teeth are so distinctive. This is not so for the earliest theropods and sauropodomorphs, which have many similar characteristics.
Many paleontologists have argued that Eoraptor was the earliest member of the theropods, but the new study suggests that this little dinosaur, toothy as it might appear, was in fact a basal sauropodomorph.
The argument for kicking Eoraptor out of the theropod lineage focuses on the fact that Eodromaeus and Eoraptor lived at the same time and that Eodromaeus has very marked theropod-like features, whereas Eoraptor not only lacks these but also has the inset lower first tooth and enlarged nostrils of a sauropodomorph.
The mistake is understandable, as the differences between the animals come down to very specific details that have only been picked up through comparison to one another. "If the two animals were to run by you with flesh on their bones, it would be a challenge to tell the difference," says Sereno.
The similarity is so pronounced that not all are convinced at this point. Paleontologist Mike Benton at the University of Bristol, UK, who was not involved with the study, says, "It's quite a shift to move Eoraptor from close affinity with theropods to sauropodomorphs, but I can't say whether I agree or disagree with this interpretation without seeing the material myself."
Yet Sereno is confident that consensus will grow for Eoraptor to be moved as more researchers have an opportunity to look at Eodromaeus and consider the evidence.
A different dawn
In addition to the discovery of Eodromaeus, the research team notes that the percentage of the total species diversity represented by dinosaurs in the Ischigualasto Formation does not increase during the Triassic. "Dinosaur diversity simply holds at around 10 percent relative to other species found, rather than showing any steady increases," says Sereno.
Moreover, as non-dinosaurian animals go extinct in the late Triassic, dinosaurs do not become more abundant.
This, Sereno and his colleagues propose, weakens a longstanding theory that dinosaurs were initially present in very low abundance during the Triassic and gradually became dominant on the landscape over time as other animals went extinct, opportunistically replacing them.
Although that idea may eventually be proved correct, Benton feels that it's too early to be certain. "Ecologically, Ischigualasto dinosaurs remain rare in the landscape. I'm not clear whether that weakens the opportunistic scenario," he says.
Sereno is quick to add that he agrees things are unclear. "I still back an opportunistic explanation for dinosaur dominance in the Jurassic. I just find it vexing that we are not seeing dinosaur diversity rising as other species disappear in Ischigualasto," he says.