False trails

 作者:莫岂     |      日期:2019-03-07 06:17:15
By Matt Walker DINOSAUR footprints tell us less than we thought about the animals that made them. A geologist in Indiana has found that it’s almost impossible to tell which species made which prints, dashing hopes that tracks could be used to reveal how the dinosaurs lived. “If there were a way of saying unambiguously what species of dinosaur made which footprint, this would give you a very powerful technique for reconstructing the composition of the dinosaur fauna,” says James Farlow of Indiana University-Purdue University, Fort Wayne. But after studying the problem for two decades, he has reluctantly concluded that it can’t be done. “I went into this project looking for a magic key to assign footprints to their makers, but it’s eluded me,” he says. Farlow’s conclusion adds to growing evidence that dinosaur prints are being overinterpreted. For instance, palaeontologists had been excited about prints found in Greenland, dating back some 220 million years, which suggested that the animal making them had a reversed back toe—a feature not recorded in theropod dinosaurs of that time but common in their descendants, the birds. But this summer, Stephen Gatesy at Brown University in Rhode Island and his colleagues showed that the prints were made by a normal therapod walking on soft mud (Nature, vol 399, p 141). Gatesy identified the prints as coming from a species of meat-eating ceratosaur. But while Farlow agrees this is a reasonable assumption, his work shows that it’s impossible to be sure. “You could tell a print was made by a large theropod, but you would not know what group of theropods was responsible,” he says. Farlow started out by looking at bird prints. He made emus run across a smooth, soft field at his local zoo, and took measurements and casts of the prints they left behind, examining everything from the length of the print to individual toe components and angles. “I made the same observations that people usually make from dinosaur footprints.” He then moved on to other birds, building up a collection of hundreds of prints from over 40 species. Farlow then applied statistical techniques to see if the prints could be grouped according to the birds that made them. They couldn’t. “For instance, emu footprints look like very big bustard footprints and they are not very closely related,” says Farlow. He says this means it will never be possible to tell whether a dinosaur print was made by an adult of one type or a juvenile of another,