extinct aquatic lizards (lizard) that attained a high degree of adaptation to the marine environment and were distributed worldwide during the Cretaceous Period (144 million to 65 million years ago). The mosasaurs competed with other marine reptiles (reptile)—the plesiosaurs (plesiosaur) and ichthyosaurs (ichthyosaur)—for food, which consisted largely of ammonoids (ammonoid), fish, and cuttlefish. Many mosasaurs of the Late Cretaceous were large, exceeding 9 metres (30 feet) in length, but the most common forms were no larger than modern porpoises (porpoise).

Mosasaurs had snakelike bodies with large skulls and long snouts. Their limbs were modified into paddles having shorter limb bones and more numerous finger and toe bones than those of their ancestors. The tail region of the body was long, and its end was slightly downcurved in a manner similar to that of the early ichthyosaurs. The backbone consisted of more than 100 vertebrae. The structure of the skull was very similar to that of the modern monitor lizards (monitor), to which mosasaurs are related. The jaws bore many conical, slightly recurved teeth set in individual sockets. The jawbones are notable in that they were jointed near mid-length (as in some of the advanced monitors) and connected in front by ligaments only. This arrangement enabled the animals not only to open the mouth by lowering the mandible but also to extend the lower jaws sideways while feeding on large prey.