The 95-million-year-old skeletal remains of the diminutive dinosaur -- along with the bones of two juveniles -- were found tucked into a fossilized chamber at the end of a sediment-filled burrow in southwestern Montana.

"The discovery represents the first scientific evidence that some dinosaurs not only dug burrows but also cared extensively for their young inside their dens," says Anthony Martin, senior lecturer in Emory's Department of Environmental Studies, of the newly named species of dinosaur, Oryctodromeus cubicularis, meaning "digging runner of the lair."

The discovery is reported by Martin and his colleagues, David Varricchio, of Montana State University, Bozeman; and Yoshihiro Katsura of Gifu Prefectural Museum in Japan. The study was funded by the Jurassic Foundation and the Department of Earth Sciences at Montana State University.

"The presence of an adult and two juveniles within a denning chamber represents some of the best evidence for dinosaur parental care," Varricchio says. "The burrow likely protected the adult and young Oryctodromeus from predators and harsh environmental conditions. Burrowing behavior may have allowed other dinosaurs to survive in extreme environments such as polar regions and deserts, and questions some end-Cretaceous extinction hypotheses."

The study notes that the dimensions of the burrowing tunnel and its end chamber were only slightly larger than the skeletal remains of the adult O. cubicularis, making it difficult for relatively large predators to enter the tunnel. Through computational analysis, the researchers estimated that the herbivorous dinosaur weighed between 22 kg and 32 kg, was 2.1 m long (about seven feet), including a 1.25 m tail, and had a trunk breadth of 26 cm to 30 cm. The juveniles were about 55 to 65 percent the size of the adult.

Because the burrow was filled with sediment, the researchers hypothesize that the dinosaurs had drowned after water breeched a nearby riverbank and flooded their den. The sediment, says Martin, helped preserve all three skeletons as well the burrow structure.

The dinosaur's functional morphology gleaned from its skeleton also confirms that O. cubicularis was both a seasoned digger and an accomplished runner. Oryctodromeus possessed several physical traits suited for digging: a modified snout that could be used as a shovel; large bony attachments in the shoulder to accommodate powerful muscles; and a robustly built hip that allowed for bracing during digging. In contrast to many modern digging animals, the dinosaur had long hind limbs and was well adapted for running on two legs.

In addition to the three dinosaurs found, the team also uncovered fossil evidence of other burrowing animals, most likely invertebrates, which lived alongside O. cubicularis. The finding reinforces the idea that the dinosaur was a burrower.

"As we dug, we found five or six small burrows coming off the main one, filled with the same sediment, which convinced me that this was a dinosaur burrow," says Martin. "Burrowing vertebrates often live in the same environment with burrowing bees, wasps or beetles."

Martin says he and his colleagues will return to Montana to see if they can find more burrows as previously uncovered fossil evidence indicates that other species of herbivorous dinosaurs often lived in nesting colonies.

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