Tylosaurus grew more than 45 feet (14 meters) long, making it the largest of the marine reptiles called mosasaurs. Like all mosasaurs, a long and muscular, vertically flattened tail powered Tylosaurus through the water, allowing it to ambush its prey with rapid bursts of acceleration. Paddle-like limbs helped steer the slim body covered in lizard-like scales through the water.
Preserved stomach contents indicate a diet heavy on fish, but seabirds, sharks, plesiosaurs, and other mosasaurs also failed to escape Tylosaurus's lethal grip. Though not a dinosaur, Tylosaurus lived alongside them and went extinct at around the same time. Many Tylosaurus remains have been found in Kansas, which was once covered by a large ocean called the Western Interior Seaway.