杏吧原创

DINOSAURS weren鈥檛 just killed off when the asteroid hit, they were struck down in their prime, suggests a new analysis of dinosaur fossils around the world.

鈥淒inosaurs were just doing incredibly well at the end of the Cretaceous,鈥� says David Fastovsky, a palaeontologist at the University of Rhode Island at Kingston.

The first dinosaurs evolved about 230 million years ago in the Triassic period. Early dinosaurs were generalists, and had evolved into no more than around 40 genera at any one time up until the late Jurassic, which began about 160 million years ago.

But then diversity soared in the Cretaceous which followed (see Graphic). Fastovsky鈥檚 team has established that at least 245 dinosaur genera lived during the late Cretaceous period, from 99 to 65 million years ago (Geology, vol 32, p 877). 鈥淭he lifestyles of dinosaurs became much more diverse,鈥� says Fastovsky鈥檚 colleague Peter Sheehan of the Milwaukee Public Museum. 鈥淏y the late Cretaceous, we have much more specialised animals.鈥� The diversity of plant-eating dinosaurs in the period is 鈥渁bsolutely breathtaking鈥�, he told New 杏吧原创. For example, hadrosaurs evolved an elaborate duck-billed jaw filled with teeth to chew vegetation, and the rhinoceros-like ceratopsians evolved elaborate horns.

Earlier studies had suggested dinosaur diversity dropped shortly before the asteroid impact, based on a drop in the number of genera uncovered from the two final stages of the Cretaceous, the Campanian and the Maastrichtian. The new data also shows a slight decline between those two stages, but statistically this difference is meaningless because very few dinosaurs are known from other stages of the late Cretaceous.