A buried tooth has rewritten our thinking about how the apex predators of the Triassic interacted. Modern clashes between crocodiles and lions have nothing on what occurred where land met sea more than 200 million years ago. This discovery could help explain a mystery of the era's ecology.

Just as the extinction of the dinosaurs gave mammals a chance to flourish, a previous extinction event removed dominant species and allowed the dinosaurs to take over the planet. Before the Triassic-Jurassic extinction event, Rauisuchians topped the food chain in most terrestrial environments. Meanwhile, "Phytosaurs were thought to be dominant aquatic predators because of their large size and similarity to modern crocodylians," says Dr Michelle Stocker of Virginia Tech.

Paleontologists doubted these two giants interacted much.

In Naturwissenschaften, Stocker and Dr Stephanie Drumheller of the University of Tennessee report finding a phytosaur tooth lodged in the thigh bone of a rauisuchid they estimate at 8m long.

"Finding teeth embedded directly in fossil bone is very, very rare," Drumheller says. "This is the first time it's been identified among phytosaurs, and it gives us a smoking gun for interpreting this set of bite marks."

The tooth broke off in the battle and embedded 5cm deep into the bone, which then healed over, indicating the rauisuchid lived for a long time after surviving the attack. Other puncture marks can also be detected.

Chris Hayes, University of Tennessee. Artist's reconstruction of the battle.

The discovery is a reminder of the value of specimens waiting to be studied in museums, in this case the University of California Museum of Paleontology. "There are many bones that get dug up, not all are immediately processed, prepared, and studied. No one had recognized the importance of this specimen before but we were able to borrow it and make our study,” says co-author Dr Sterling Nesbitt of Virginia Tech. The team used CT scans and a 3-D printer to replicate the bone, concluding that the rauisuchid was attacked by a phytosaur on at least one other occasion.

The same paper also reports another specimen from the same 220-210 million year old formation that provides insight into feeding behavior. A second femur shows unhealed bite marks, which the authors write, indicates “the animal either did not survive the attack or was scavenged soon after death.” The shape and spacing of the marks indicate a phytosaur was responsible in this case as well.

The authors add, “These marks provide an opportunity to start exploring the seemingly unbalanced terrestrial ecosystems from the Late Triassic of North America, in which large carnivores far outnumber herbivores in terms of both abundance and diversity.”

“Both of the femora we examined came from some of the physically largest carnivorous species present at both locations. Yet they were targeted by other members of the region—specifically phytosaurs,” says Drumheller. “Thus, size cannot be the only factor in determining who is at the top of the food chain.”