Between 245 and 66 million years ago, dinosaurs ruled the Earth. Or so the saying goes. Avian dinosaurs are still with us in their glorious, feathery abundance, and even the varied, bizarre non-avian dinosaurs were not so monstrous that they went unchallenged. We celebrate dinosaurs for their apparent ferocity and power, but we often forget that they were part of prehistoric ecosystems that did not grant them the special status our imagination so often does. Case in point – some hapless dinosaurs wound up as meals for ancient crocs. We know because of mangled bones.
Not every dinosaur that ever lived made it into the fossil record. Dinosaur bodies had to wind up in environments where their remains could be buried. Crocs of the past helped accomplish this in their own carnivorous manner. As paleontologists Clint Boyd, Stephanie Drumheller, and Terry Gates write in a PLoS One study, published today, “Crocodyliforms serve as important taphonomic agents, accumulating and modifying vertebrate remains.” (A note on terminology: crocodyliforms include modern alligators, crocodiles, and gharials, as well as their close extinct relatives.) The predators are quite effective at destroying bodies, of course, but their habits also bring carcasses into environments where sediment is laid down. In such situations, paleontologists gain fossils within fossils – bones of prey and predatory behavior. Such is the case with small dinosaur bones, damaged by crocodyliform bites, cataloged and analyzed by Boyd, Drumheller, and Gates in the new paper.
The dinosaur bones in question were found within the exceptionally fossil-rich, 75 million year old rock of the southern Utah’s Kaiparowits Formation. I’ve been fortunate enough to hike over these badlands in search of fossils a few times. Fragments of Cretaceous bone are everywhere – vestiges of humid, lush ecosystems that thrived when southern Utah was on the western shore of a long-vanished seaway. The arid desert contains a prehistoric environment that more closely resembled the swamps of modern day Louisiana, albeit with dinosaurs and a greater diversity of aquatic ambush predators. There were some large crocodyliforms – such as the enormous Deinosuchus, capable of reaching 40 feet long – but the Kaiparowits has also yielded a slew of smaller forms that chomped on little dinosaurs.
Found at a site dubbed “Locality 303”, the dinosaur bones represent three juvenile ornithopods – small, beaked, bipedal herbivores that resembled the genus Orodromeus found in the Cretaceous rock of Montana. The Utah ornithopod does not yet have an official name, but the known remains of the dinosaur are still worth discussing because of croc-caused damage, including a femur with part of a crocodyliform tooth still embedded inside. CT scans of the bone showed that the tooth was already broken when the attacker bit the dinosaur, and after the strike the tooth broke a second time to snap off in the bone. The same femur also exhibits tooth pits, and a scapula – a shoulder blade – preserves pits that are attributable to a croc bite, rather than gnawing by mammals or the dining habits of a theropod dinosaur. Not all the bones found at Locality 303 were damaged in such a way, but the femur and scapula certainly put small crocs at the scene.
So what crocodyliform chomped on the dinosaurs? Boyd and colleagues estimate that the carnivore was about three feet long, but it’s hard to say whether the culprit was a small-bodied species or a juvenile of one of the giants. (Not to mention the fact that many Kaiparowits crocodyliforms are still being prepared and described, with no formal designation just yet.) Nor is it clear that a croc killed the dinosaurs. The bite marks record feeding behavior, but might not reflect predatory behavior. The ornithopods would have been within the appropriate prey size range for a three-foot croc – at least based on observations of how living alligators and crocodiles behave – but whether or not crocs actually caught these specific dinosaurs will probably remain unknown.
Despite the persistent mysteries about the fossil crocodyliform and its behavior, though, the toothmarked bones still provide insight into the world of the Kaiparowits. Little crocs – be they truly small-bodied species or the juveniles of larger predators – ate small dinosaurs. As Boyd, Drumheller, and Gates note, this might be a sign that the five different species of crocodyliform found in the Kaiparowits targeted different victims, allowing a high diversity of the ambush predators to coexist. Dinosaurs may have been among the most wonderful and charismatic Cretaceous creatures, but they still had to be watchful at the water’s edge.
Boyd, C., Drumheller, S., Gates, T. 2013. Crocodyliform Feeding Traces on Juvenile Ornithischian Dinosaurs from the Upper Cretaceous (Campanian) Kaiparowits Formation, Utah. PLoS ONE 8, 2: e57605. doi:10.1371/journal.pone.0057605