EVEN though it happened 65 million years ago, we are pretty sure we know what happened to the last of the dinosaurs: they died when a 10-kilometre-wide asteroid struck what is now the Yucat谩n coast of Mexico.
Pretty sure, but not completely. A few palaeontologists insist that a few 鈥淟azarus鈥� species limped on after the impact, surviving for another million years or so.
Keith Rigby of the University of Notre Dame in Indiana is one. He has collected a large number of dinosaur teeth from Montana in sediments that formed about half a million years after the impact. The big question is whether the teeth came directly from living animals, or were 鈥渞eworked鈥� from older sediments. Rigby is convinced it is the former. 鈥淲e鈥檝e got some that are pristine,鈥� he says, with edges as sharp as the day the dinosaur shed them. Others are sceptical. 鈥淭here鈥檚 no proof that they are not reworked,鈥� says David Archibald of San Diego State University in California.
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Rigby has also reported dinosaur fossils, including intact nests with eggs and footprints, from post-impact sediments in the Nanxiong basin in China. Even his critics agree that some of the fossils are in post-impact sediments, but argue that these too are reworked. They point out that all the footprints and intact nests are below the impact layer; above it are only fragments of eggshell and bone.
Jim Fassett of the US Geological Survey, meanwhile, has reported finding a pristine dinosaur femur in rocks containing post-impact pollen in the San Juan basin of New Mexico. However, others have failed to confirm his pollen finding and so are not convinced that Fassett is right.
In the absence of unequivocal evidence of Lazarus dinosaurs, the last clear evidence of living dinosaurs is a series of tracks made by a family of duck-billed plant eaters in what is now Ludlow, Colorado. The footprints were found in a slab just below the layer marking the impact. 鈥淭he dinosaurs were alive and well, walking about at the very last minute,鈥� says Martin Lockley of Colorado University at Denver. But his timescale is geological: there are probably at least 1000 years between the footprint layer and the impact itself. Even so, Colorado is the last place we can say for sure that dinosaurs walked.
