Move over mama bear; dinosaurs just joined the ranks of animals with dens. The 2-meter-long Oryctodromeus may have raised its young in tunnels. The discovery of the 95-million-year-old burrow and skeletons could challenge long-held ideas about dinosaurs, including notions about their eventual extinction.
Lacking definitive evidence of burrows, paleontologists have assumed that dinosaurs lived above ground. But now, scientists report that they uncovered a jumble of dinosaur bones in southern Montana encased in a 2-meter-long fossilized tunnel. The burrow was recognizable because the sediment inside was distinct from the surrounding rock. The Oryctodromeus dinosaurs were trapped in an expanded area at the end of the burrow when it filled in with sediment during a flood. The dinosaurs closely match the size of the tunnel, and the absence of tooth marks on their bones indicates that predators didn't move the bodies, suggesting that the animals dug the passageway themselves and were inside when they died. The bones feature several possible adaptations for digging, including large shoulder bones to support powerful muscles, a shovel-like snout, and reinforced hips to help brace the body. The research appears this week in the Proceedings of the Royal Society.
Lead author David Varricchio of Montana State University in Bozeman and other paleontologists have previously found signs of adult dinosaurs caring for young, particularly babies. (ScienceNOW , 8 September 2004). Other discoveries of adjacent adult and juvenile bones have hinted at parental care, but those fossils could have been washed together. The large size of the juveniles--1.3 meters long--suggests that parenting continued for at least several months. That makes it some of the strongest evidence yet that dinosaurs tended their young, according to Varricchio.
Dinosaurs may have used burrows to escape harsh conditions as well, says Varricchio, which challenges the idea that the dinosaurs went extinct 65 million years ago because they were unable to find shelter from climatic upheaval caused by an asteroid impact or massive volcanic eruptions.
"It's really extraordinary to find direct evidence of behavior in the fossil record," says paleontologist Larry Witmer of Ohio University in Athens. "The fact that the large and small bones were found together is pretty good evidence that we have a family group." As for the hypotheses about the dinosaurs' extinction, the find "certainly throws a wrench in the works of the idea that no dinosaurs could have escaped in burrows," he says.