In a road cut near the Missouri River in central South Dakota, paleontologists uncovered signs of a fight to the death waged 73 million years ago in a shallow sea that covered much of what is now the Great Plains. These bones are evidence of combat between mosasaurs--vicious marine lizards of the Cretaceous Period. One mosasaur apparently ripped the head off a 16-foot-long rival. After the fight, sharks scavenged the victim’s carcass, leaving behind more than 3,000 teeth. The victor, though, left only one tooth, found embedded in the victim’s quadrate, a bone that connects the jaw to the skull. This serrated tooth--removed from the quadrate and shown here above the jaw--is typical of the species Mosasaurus conodon, as are the victim’s teeth. A wound from the fight--visible at the far right edge of the photo--scars the jaw. Below it are two healed wounds of an earlier contest the mosasaur survived. Gorden Bell, a paleontologist at the South Dakota School of Mines and Technology who studied the fossil, believes mosasaurs were territorial but not cannibalistic. They ate mosasaurs of different subfamilies, he says, but not their own kind. But they fought like the devil--and even killed each other, apparently.