Around 160 million years ago, an enormous sauropod dinosaur trudged across an ancient marsh in what is now Xinjiang, China. It was not easy going. The eruption of a nearby volcano coated the area in a layer of ash which formed a thin surface over a morass of mud and volcanic debris, and as it walked it left deep holes which quickly filled in with sticky muck. As some of the smaller dinosaurs that lived in the area would find out, these in-filled footprints would soon become dinosaur death traps.
As reported by paleontologists David Eberth, Xu Xing and James Clark in the journal Palaios, the sauropod-made pits created circumstances in which small theropod dinosaurs would be more likely to be preserved. Once filled in with mud, it would have been hard to see the large holes, and when a small dinosaur stepped into them they would have had a very difficult time getting out. As we can tell from the layers of fossils preserved within the footprints, many of the dinosaurs became permanently stuck and died, piling one on top of the other (although, in some cases, trapped dinosaurs might have been able to gain better footing on the remains of previously trapped individuals and push themselves out).
These dinosaur death traps have been especially significant because at least two previously-unknown species of dinosaur have been recovered from them.
EBERTH, D., XING, X., & CLARK, J. (2010). DINOSAUR DEATH PITS FROM THE JURASSIC OF CHINA PALAIOS, 25 (2), 112-125 DOI: 10.2110/palo.2009.p09-028r