Skulls of several crurotarsans, the “crocodile-line” archosaurs that were the main competitors of dinosaurs during the Late Triassic period
The “morphospace” for dinosaurs, crurotarsans and pterosaurs during the Triassic. This shows the range of body types, lifestyles, and diets present in a group of organisms. The crurotarsans occupy a clearly larger space
The dinosaurs got lucky. Before they finally came to dominate Earth life in the Jurassic period, they were perpetual also-rans to their crocodilian cousins. But then the climate gave them a helping hand.
Near the start of the Triassic period, 250 million years ago, the archosaurs, or “ruling reptiles”, split into two major groups: the dinosaurs and a group called crurotarsans, whose only living descendants are the crocodiles.
Palaeontologists had long thought that the more successful dinosaurs dominated the last 30 million years of the Triassic, but in recent years they have found that many of the fossils originally thought to be dinosaurs were actually similar-looking crurotarsans.
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Now new research challenges the idea that dinosaurs had some evolutionary advantage over their croc rivals.
Climate catastrophe
Analysing data for the 30 million years of the Triassic that the two groups lived side by side, Steve Brusatte and Mike Benton at the University of Bristol, UK, found they evolved at essentially the same rate. Moreover they also found that the crurotarsans also developed a much broader range of body types than the dinosaurs.
An observer comparing the two groups in the late Triassic would have expected the crurotarsans to eventually dominate, says Brusatte, now at the American Museum of Natural History in New York.
Yet all the crurotarsans bar the crocodiles were wiped out 200 million years ago, when rapid climate change caused a mass extinction. Dinosaurs “pretty much got lucky” and sailed through to dominate ecosystems for another 135 million years, he says.
It’s an example, Brusatte adds, of how evolution doesn’t really record “progress” – the winners may just have luck on their side.
Journal reference: Science (DOI: 10.1126/science.1161833)
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