Dinosaurs Lucky? Not Tougher? Not Fitter?

But Steve Brusatte, a doctoral student at Columbia University who is an affiliate of the American Museum of Natural History, and colleagues are challenging this idea with new fossil data and math. By comparing early dinosaurs to their competitors, the crurotarsan ancestors to crocodiles, they have found that dinosaurs were not "superior," as has long been thought. Rather, crurotarsans were the more successful group during the 30 million years they overlapped until the devastating mass extinction 200 million years ago, an event that dinosaurs weathered successfully.

"For a long time it was thought that there was something special about dinosaurs that helped them become more successful during the Triassic, the first 30 million years of their history, but this isn't true," says Brusatte. "If any of us were standing by during the Triassic and asked which group would rule the world for the next 130 million years, we would have identified the crurotarsans, not dinosaurs."

Both dinosaurs and crurotarsans evolved and filled some of the same niches after a massive extinction event that occurred at the end of the Permian (250 million years ago). Of the crurotarsan group, crocodilians are the only living members. But in the Triassic, crurotarsans were amazingly diverse—from giant carnivorous rauisuchians to long–snouted, flesh eating phytosaurs to herbivorous armored aetosaurs—and they have often been mistaken for dinosaurs in the fossil record, the animals that they probably competed with for the same resources. Both groups survived an extinction event 228 million years ago, but only a few crurotarsans—the crocodiles—squeaked through a period of rapid global warming at the end of the Triassic 200 million years ago. Dinosaurs faired better during the latter extinction: most types of dinosaurs survived until an asteroid ended their dominance 65 million years ago. It is because of this stroke of luck that dinosaurs were assumed to be the better competitors.

Brusatte and colleagues tested this assumption by measuring the evolution in both competing groups. Based on a database of 437 features of the skeletons of 64 species of dinosaurs and crurotarsans, as well as a new phylogenetic tree of these groups, they performed two calculations to look at the evolutionary pattern. The first measurement is of the disparity, or the known range of different body plans, of the two groups. Disparity is a reliable indicator of the different lifestyles, diets, and habitats of a group of animals. Remarkably, Brusatte and colleagues found that crurotarsans had twice the disparity of dinosaurs: They were exploring twice the range of body plans as early dinosaurs. "With this information, it's difficult to argue that dinosaurs were 'superior' during the Triassic. They just lucked out when the crurotarsans were hit hard at the end Triassic extinction," says Brusatte.

The team also measured the rate of evolution in both dinosaurs and crurotarsans to see if dinosaurs were diversifying into new species at higher rates, as may be expected if they had special abilities or were outcompeting their rivals. But the comparison showed that the two groups were evolving at the same rate over the 30 million years that they overlapped.

There's a second press release here. National Geographic covers it here. Our own faunal group in the blogosphere has Chinleana's post as well.

I have to wonder about this. We're starting to see a lot more smear for what we thought were originally very delineated faunal groups. As we are finding, this is turning out to be less and less true. Critters we thought went extinct by certain time frames seem to have merely retreated to refugia (*cough* dicynodonts *cough*) or just were not in the local area that we were finding fossils from.

Because we have so few locales that are rendering up fossils, I fear that we are drawing great and profound conclusions from so very little data. It may be that the the Permian Extinction in the terrestrial realm may be ratched down as far as the magnitude is concerned simply if another terrestrial site is discovered had less radical faunal transitions simply because, iirc, we have something on the order of three, yes, three locations that we are drawing such vast conclusions about for the PT Extinction. Something similar is happening for the TJ Extinction and other lesser ones.

Tread carefully, folks. Small sample sizes make fools of the best.