SALT LAKE CITY—One day, about 74 million years ago, the situation got real. The oceans of the late Cretaceous were dominated by giant reptiles called mosasaurs; fossilized gut contents have shown that these apex predators enjoyed munching on everything from bony fish to, on occasion, smaller mosasaurs. But an unusual fossil described here today at the Society for Vertebrate Paleontology meeting tells a different story—of a mosasaur spat that didn’t end in anyone becoming lunch. The fossil, of a species called Mosasaurus missouriensis, was discovered in 2012 in a layer of shale by a mining company in southern Alberta in Canada. The reptile was about 6.5 meters long (the length of a pickup truck), with a skull that was slightly less than a meter long. And under one eye, this particular mosasaur’s bony skull also had a large hole where something had bitten into it. The animal survived the attack and the bone was healing around the lesion. But it couldn’t quite heal completely, because the biter had left behind a large tooth (circled). Analysis of the tooth by Takuya Konishi of the University of Cincinnati in Ohio and colleagues suggests that the mosasaur was attacked by one of its own kind and of similar size. Indeed, M. missouriensis had long, narrow teeth designed more for slicing than for crunching, and the animals were liable to lose teeth when chomping into hard substrates. The attack probably came from below, and the researchers say appears to have been a skirmish: They suggest that the fossil preserves an ancient competition between males or a mating behavior, rather than a predatory attack intended to kill and consume. That, they add, makes it the first evidence of a nonlethal mosasaur-on-mosasaur attack.