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DINOSAURS may have come in with a bang as well as gone out with one. Just below rocks that contain the earliest footprints of large meat-eating dinosaurs are the hallmarks of an asteroid impact and a mass extinction. The discovery suggests that the ecological stage was suddenly cleared at the beginning of the Jurassic period, leaving room for these giant beasts.

The first small dinosaurs evolved about 230 million years ago, and competed with many other reptiles until the Triassic period ended around 202 million years ago. In the Jurassic period that followed, most of the competitors vanished. Dinosaurs grew to gargantuan proportions, but why they became so dominant has remained an enigma.

The answer may lie in rocks at the Triassic-Jurassic boundary, in an area in north-eastern North America. As well as a few fossil bones, the rocks contain fossilised pollen and the footprints of animals that walked beside ancient lakes. The lake levels rose and fell as the climate cycled periodically due to variations in the Earth鈥檚 orbit, so these rocks can be dated particularly accurately, says Paul Olsen of the Lamont Doherty Earth Observatory of Columbia University, New York.

At the boundary between the periods, Olsen and his colleagues found a rock layer unusually rich in fern spores and iridium, an element found in asteroids. Ferns spread rapidly over devastated landscapes, and high counts of fern spores have been found in rocks dating from just after the impact that wiped out the dinosaurs 65 million years ago. Olsen concludes that there must have been another impact at the end of the Triassic period, just before the dinosaurs rose to prominence. But he has looked at only one area so far, so he can鈥檛 be sure if the impact would have had global effects.

The researchers studied fossil footprints at 80 sites and found that the transition from animals common in the Triassic to those of the Jurassic was particularly dramatic, taking only 50,000 years. 鈥淚n the late Triassic, there were lots of different footprints, representing many different reptile groups,鈥� Olsen told 快猫短视频. 鈥淵et at the start of the Jurassic all you see are dinosaurs, lizards and very small crocodiles.鈥�

The size of the dinosaurs leapt sharply as well: the largest Triassic dinosaur footprints were about 25 centimetres long, but in rocks only 50,000 years younger Olsen found tracks of the 6-metre-long predator Eubrontes giganteus, which was nearly twice the size of the largest Triassic dinosaur. Olsen thinks it was mainly the absence of competitors in the aftermath of the asteroid that enabled the predators to get so big so quickly.

An impact that devastated the existing vegetation could also explain why so many Jurassic dinosaurs were carnivorous. Plant-eating dinosaurs were common in the Triassic, but only meat eaters stalked the lake shore at the start of the Jurassic. With few plants around, dinosaurs had to live on whatever they could catch, says Olsen. Not until 100,000 years later did a few small plant eaters leave their footprints by the lake.