Megacephalosaurus

Megacephalosaurus (; "great-headed lizard") is an extinct genus of short-necked pliosaur that inhabited the Western Interior Seaway of North America about 94 to 93 million years ago during the Turonian stage of the Late Cretaceous, containing the single species M. eulerti. It is named after its large head, which is the largest of any plesiosaur in the continent and measures up to in length. Megacephalosaurus was one of the largest marine reptiles of its time with an estimated length of . Its long snout and consistently sized teeth suggest that it preferred a diet of smaller-sized prey.

Remains representing the pliosaur include two fossil skulls, three ribs, and a neural arch. The fossils have been found in deposits of the Carlile Shale and Greenhorn Limestone in Kansas and elsewhere in the midwestern United States. First discovered in 1950, they were originally thought to have been giant fossils of a closely related pliosaur known as Brachauchenius lucasi. But by 2013, paleontologists understood that such fossils belonged to a distinct animal. Megacephalosaurus was among the last known pliosaurs.