Polish paleontologists have reported a surprising fossil first - a Protoceratops dinosaur that apparently died in its own tracks about 80 million years ago in Mongolia writes Jeff Hecht of the New Scientist.
Footprints are some of the only fossils that show us how dinosaurs and other long-extinct animals behaved when they were alive. But animals are rarely fossilised together with their final footprints.
Invertebrates are an exception. Fossils of hard-shelled bottom crawlers such as horseshoe crabs and trilobites have been found at the ends of matching trackways.
But that's because a single deposition of sediment can kill the animal, and preserve impressions of both its shell and the tracks it made just before it was buried.
Such events are very rare for vertebrates living on land. Generally, different conditions are needed to preserve tracks and bones, so the two are rarely found in the same rock formations.
Matching skeletons with footprints is further complicated because the tracks are made by pads of soft tissue that cover the foot bones but are not fossilised - and by the fact that feet are often missing altogether from preserved skeletons.
Palaeontologists deal with the problem by giving separate species names to footprints and skeletons. Footprints are known as ichnospecies.
Often ichnologists can identify the type of animal that made the tracks, and sometimes they can even make a reasonable guess at the species. For instance, large predatory dinosaurs have large, distinctive three-toed feet.
Few species reached the gigantic size of Tyrannosaurus rex, so if giant three-toed impressions are found in areas that have already yielded T. rex bones, it's logical to link the two. But such clear matches are extremely rare.
Grzegorz Niedźwiedzki of the University of Warsaw calls his discovery of a plant-eating Protoceratops dead in its tracks "the holy grail of vertebrate ichnology".
The crucial footprint was far from obvious. A joint Polish-Mongolian team collected the slab containing skeleton from the Gobi desert in 1965.
Forty-five years later, Niedźwiedzki and a colleague were preparing the fossil for more detailed examination when they found an impression near the pelvis. Its shape and size closely matched the impression expected from the dinosaur's four-toed hind foot.
Protoceratops, about 1.6 to 1.8 metres long, was among the most common dinosaurs in Mongolia at the time, but its footprints have not been found before. Now we know what they looked like. Niedźwiedzki found no other tracks in the specimen in his lab, but says that other footprints may remain in the rock formation back in Mongolia.
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