Fossils from a family of the first known burrowing dinosaurs have been found by scientists in Montana. The fossils are from an adult and two juveniles. The bones are 95 million years old and were unearthed in a chamber at the end of a 2.1 meter long tunnel that was filled with sediment. The researchers say their discovery is the first definitive evidence that some dinosaurs dug dens and cared for their young in them.
"Burrowing also represents a mechanism by which small dinosaurs may have exploited the extreme environments of polar latitudes, deserts and high mountain areas," David Varricchio and colleagues write their paper.
The ability to burrow would have given it many advantages, said Anthony Martin, a co-author, such as surviving harsh climatic conditions or catastrophes such as volcanic eruptions -- rocks found in the sediment suggest there were active volcanos nearby. "Burrows have a nice way of evening out environmental conditions, like maintaining a stable temperature and humidity."
The researchers note that dinosaurs' snout, shoulder girdle and pelvis have structures that one would expect to see in an animal that dug into the ground. The burrow was a sloping twisting tunnel more than two metres (6.5 feet) long and about 70 centimetres (28 inches) wide. It is similar to burrows made today by striped hyenas and puffins.
"At first it seemed too big for the burrow," says Varricchio. However, modern mammals squeeze into tight places for protection, he reasons. "A tight fit precludes anyone meaner or bigger from getting in there," he said.
These burrowing dinosaurs represent a new species and has been given the scientific name Oryctodromeus cubicularis, which is a mix of Greek and Latin meaning "digging runner of the lair". They were hypsilophodonts, a group that includes the much larger duck-billed hadrosaurs. Based on the preserved vertebrae, the adult would have been about 2.1 meters (6.8 feet) from nose to tail, most of which was the tail itself. The whole animal probably weighed 22 to 32 kilograms, Varricchio estimates. So its body was approximately the size of a german shepard.
The tunnel ended in a chamber, where the skeletal remains of an adult and two juveniles were found. Unfortunately, the bones were disarticulated.
"It's not like they were sitting in the burrow and a flooding event filled the chamber with sediment and they were entombed. They must have died, undergone decay and then the burrow was filled."
The juveniles are more than half grown, suggesting that the parents cared for their young for a significant period.
This find is reported in the journal Proceedings of the Royal Society B, a journal of Britain's academy of sciences.
