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The “Morphotype 1″ tunnel complex: points marked “a” represent tunnels, and points marked “b” signify vertical shafts. From Colombi et al., 2012.

Dinosaurs never cease to surprise. Even though documentaries and paleoart regularly restore these creatures in lifelike poses, the fact is that ongoing investigations into dinosaur lives have revealed behaviors that we never could have expected from bones alone. Among the most recent finds is that dinosaurs were capable of digging into the ground for shelter. Burrows found in Australia and Montana show that some small, herbivorous dinosaurs dug out cozy little resting places in the cool earth.

But when did dinosaurs develop burrowing behavior? The distinctive trace fossils found so far are Cretaceous in age, over 100 million years after the first dinosaurs evolved. That’s why a new PLoS One paper by paleontologist Carina Colombi caught my eye. In the Triassic rock of Argentina’s Ischigualasto Basin, Columbi and coauthors report, there are large-diameter burrows created by vertebrates that lived approximately 230 million years ago. Archaic dinosaurs such as Eoraptor and Herrerasaurus roamed these habitats–could dinosaurs be responsible for the burrows?

Colombi and colleagues recognized three different burrow forms in the Triassic rock. Two distinct types–differentiated by their diameter and general shape–were “networks of tunnels and shafts” that the authors attributed to vertebrates. The third type showed a different pattern of “straight branches that intersect at oblique angles” created by the burrowing organism and the plant life. The geology and shapes of the burrows indicate that they were created by living organisms. The trick is figuring out what made the distinct tunnel types.

In the case of the first burrow type, Colombi and collaborators propose that the structures were made by small, carnivorous cynodonts–squat, hairy protomammals. In the other two cases, the identities of the burrow makers isn’t clear. The second type included vertical shafts that hint at a vertebrate culprit. Dinosaurs would have been too big, but, Colombi and coauthors suggest, other cynodonts or the bizarre, ancient cousins of crocodiles–such as aetosaurs or protosuchids–could have created the burrows. Unless remains of these animals are found associated with the burrows, it is impossible to be sure. Likewise, the third type of trace might represent the activities of animals that burrowed around plant roots, but there is no clear candidate for the trace-maker.

As far as we know now, Triassic dinosaurs didn’t burrow. Even though they were not giants, they were still too large to have made fossils reported in the new research. Still, I have to wonder if predatory dinosaurs such as Herrerasaurus, or omnivores like Eoraptor, dug poor little cynodonts out of their burrows much like the later deinonychosaurs scratched after hiding mammals. There’s no direct evidence for such interactions, but, if small animals often sheltered from heat and drought in cool tunnels, perhaps predators tried to nab prey resting in their hiding places. One thing is for sure, though: we’ve only just started to dig beyond the surface of Triassic life.

References:

Colombi, C., Fernández, E., Currie, B., Alcober, O., Martínez, R., Correa, G. 2012. Large-Diameter Burrows of the Triassic Ischigualasto Basin, NW Argentina: Paleoecological and Paleoenvironmental Implications. PLoS ONE 7,12: e50662. doi:10.1371/journal.pone.0050662