dinosaurs may have arrived with a bang, as well as gone out with one. 快猫短视频s have found the hallmarks of a meteorite impact and mass extinction in rocks just below strata containing the earliest footprints of large meat-eating dinosaurs.
The finding of high levels of iridium metal and fossilised fern spores suggests that a sudden extinction cleared the ecological stage, leaving room for meat-eating dinosaurs to grow suddenly larger. A subsequent, massive meteorite impact about 65 million years ago resulted in the extinction of the creatures.
Dinosaurs evolved about 230 million years ago and competed with many other reptiles until the Triassic period ended about 202 million years ago. Then most of the competitors vanished and dinosaurs grew to their characteristically monstrous proportions in the Jurassic period that followed.
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The key to how the dinosaurs came to dominate the land in this way may now have been discovered in the sedimentary rocks laid down over the Triassic-Jurassic boundary in what is now northeastern North America.
Devastated landscape
The rocks contain few fossil bones, but they preserve both fossil pollen and the footprints of animals that walked beside ancient lakes. Lake levels rose and fell with periodic climate cycles, so the rocks can be finely dated, says Paul Olsen of the Lamont Doherty Earth Observatory of Columbia University.
Surveying 80 sites, Olsen鈥檚 group found that fossil footprints changed from typical Triassic to typical Jurassic groupings in a period of just 50,000 years. In between lay the boundary between the periods, defined by a change in pollen type, and including the layer rich in iridium and fern spores.
The fern spores are indicative because ferns spread rapidly over devastated landscapes 鈥� sharp peaks of spores also occurred just after the final, cataclysmic impact 65 million years ago.
Eat anything
The faunal change was also sharp. 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. Yet at the start of the Jurassic 鈥渁ll you see are dinosaurs, lizards and very small, fully terrestrial crocodiles鈥�.
And the size of the dinosaurs jumps sharply. Just 50,000 years after the start of the Jurassic, there are tracks of Eubrontes giganteus, a six-metre long predator that Olsen says was nearly twice as massive as the biggest Triassic dinosaur.
The meat eaters survived the disaster probably because of their adaptable diets, says Olsen. It is typical for a 鈥渄ecimated ecosystem鈥� to become dominated by animals that can survive on whatever they can find, he says.
Instead of hunting plant-eaters, 鈥渢hey鈥檙e primarily hunting other carnivores and things in the water,鈥� such as fish. Not until about 100,000 years after the extinction did a few small plant-eaters start leaving their footprints by the lakes.
Michael Benton of the University of Bristol agrees that the rapid change in seen in the dinosaurs suggests 鈥渋t was more of a catastrophic event than people had thought鈥�. However, he warns that Olsen鈥檚 group has studied only one area, while iridium-rich deposits from the impact 65 million years ago have been found at 200 different sites.
Journal reference: Science (vol 296, p 1305)