Oryctodromeus, meaning “digging runner”, is a genus of ornithopod dinosaur from the Cenomanian age of the Middle Cretaceous Period (95 million years ago). Its remains were found in rocks of the Blackleaf Formation of southeastern Montana.
Oryctodromeus is known from a partial skeleton of one adult and two juveniles that were about 60% fully grown. This specimen was the first dinosaur to be published showing evidence of burrowing behavior. Its tail lacked the stiffness which would have made it easier for it to burrow. Adaptations in the jaws, forelimbs, and pelvis strengthen the idea that Oryctodromeus was a burrowing dinosaur.
It has been pointed out, however, that the forelimbs lack the modifications seen in most dedicated burrowing animals, like moles, echidnas, and wombats. Instead, the limbs were more comparable to that of diggers than animals that both run and burrow today, like aardwolves, cavies, and rabbits. It would seem that being bipedal, it would have had a more modified forelimb for burrowing as it would not have affected its ability to run.
Oryctodromeus shares similar burrowing features as the other hypsilophodonts Orodromeus and Zephyrosaurus, such as a broad snout. Also, the way they have been preserved seems to hold water that they were burrowing animals. This is not the first time that a hypsilophodont has been suggested as a burrower;
Robert Bakker has informally claimed since the 1990s that Drinker, from the late Jurassic of Wyoming, lived in burrows, but this has yet to be published.
Three Oryctodromeus individuals were found buried in the remains of an underground burrow measuring 6.5 feet long by 2.3 feet wide. The skeletons were densely packed together which indicates they died together in the burrow. The burrow is similar to those made by hyenas and puffins today. It was filled with sand, and the resulting sandstone stands out against the surrounding mudstone and claystone.
Other smaller cylindrical sections were found within the fossilized burrow which indicated smaller animals may have shared the burrow with the dinosaur. The main burrow fits the proportions of an adult Oryctodromeus, another indication that it was the digger of the burrow.
Oryctodromeus was a small, swift herbivore. Its full name, Oryctodromeus cubicularis, is translated as “digging runner of the lair”, in reference to its presumed lifestyle. The adult was 6.8 feet long and weighed about 50 to 70 pounds. Juveniles would have been about 4.3 feet long.
The presence of juveniles with the adult suggests parental care, and that at least one motivation for burrowing was to rear the juveniles. The size of the juveniles suggests an extended period of parental care.