Dinosaurs overshadowed mammals for most of the Mesozoic, but evidence of actual dinosaur-mammal interactions are very rare. On the mammalian score, a specimen of the relatively large Cretaceous mammal Repenomamus robustus described in 2005 was found with the bones of baby dinosaurs in its stomach—it had apparently fed on young Psittacosaurus shortly before it died. A new set of fossils from southern Utah, though, evens the score for the dinosaurs.
In Utah's Grand Staircase-Escalante National Monument, within the 80-million-year-old rock of the Wahweap Formation, paleontologists have discovered evidence that small predatory dinosaurs dug down into the soil to reach the burrows of small mammals. As reported in the journal Geology, the vestiges of these events are left behind as traces within the rocks—scratches made by dinosaurs and dens used by mammals—and by looking at them together scientists can replay what might have happened during those Late Cretaceous days at the end of the Mesozoic era.
The first trace fossil type was made by a digging dinosaur, probably a maniraptoran similar in form to Deinonychus and Troodon. At first glance it doesn't look like much—just a lumpy bit of sandstone—but if you look carefully, a claw impression and numerous downward-arcing grooves can be seen. It appears that the dinosaur was repeatedly sticking its foot into the hole and raking out sediment, a behavior consistent with the idea that these dinosaurs probably did not use their arms to dig because their feathers would have gotten in the way or been damaged.
The second group of traces, found near the claw marks, preserve mammal burrows and dens. Networks of branching, winding burrows connect to bulbous chambers where mammals found refuge, and these underground structures are very similar to those made by small, social mammals living today. Based upon the close association of these structures with the claw marks, and especially the correspondence between deeper burrows and deeper dig marks made by dinosaurs, the researchers hypothesize that the predatory dinosaur was trying to get at the mammals.
Together the scratches and burrows tell of ancient interactions we could only previously infer on the basis of bones. It most have been terrifying for those small mammals, hearing the predatory dinosaur scratching deep into the ground in the hopes of catching them.
Edward L. Simpson, Hannah L. Hilbert-Wolf, Michael C. Wizevich, Sarah E. Tindall, Ben R. Fasinski, Lauren P. Storm and Mattathias D. Needle (2010). Predatory digging behavior by dinosaurs Geology, 38, 699-702 : 10.1130/G31019.1