Paleontologists have long assumed that semiaquatic dinosaurs and land-based predators mostly kept their distance from each other, preferring to avoid conflict or interaction of any sort. But a new discovery by researchers at the University of Tennessee and Virginia Tech -- a tooth lodged in an ancient dino thighbone -- suggests these predators may have faced off regularly.

The tooth belonged to a semi-aquatic phytosaur, a large, long-snouted creature which resembled the crocodiles of today in size, appearance, and lifestyle. It was found lodged two-inches deep in the thighbone of a terrestrial rauisuchid, a hefty predator that stretched 25 feet long and rose four feet high at the hip.

The rauisuchid fossils showed that the specimen had survived the initial attack -- the tooth-impaled thighbone healed and sealed over. But a separate wound showed that a later phytosaur attack killed the land-based predator.

Though both predators hailed from the Late Triassic, ranging from about 235 to 200 million years ago, it was unexpected to find direct conflict between the two. The thighbone was originally unearthed in the southwestern United States.

"To find a phytosaur tooth in the bone of a rauisuchid is very surprising," Stephanie Drumheller, an earth and planetary sciences lecturer at Tennessee, said in a press release. "These rauisuchids were the largest predators in their environments. You might expect them to be the top predators as well, but here we have evidence of phytosaurs, who were smaller, semi-aquatic animals, potentially targeting and eating these big carnivores."

"This research will call for us to go back and look at some of the assumptions we've had in regard to the Late Triassic ecosystems," explained Drumheller's research colleague from Virginia Tech, Michelle Stocker. "The aquatic and terrestrial distinctions made were oversimplified, and I think we've made a case that the two spheres were intimately connected."

The research was published this week in the German-titled journal Naturwissenschaften, The Science of Nature.