Mosasaurs were marine reptiles that inhabited the warm, shallow continental seas of the late Cretaceous period. They lived approximately between 98 and 65 million years ago. Though mosasaurs lived at the same time as the last dinosaurs, they are not dinosaurs but lepidosaurs, reptiles with overlapping scales. Lepidosaurs (but not mosasaurs) survived the K-T extinction that wiped out the dinosaurs, represented today by tuataras, lizards, snakes and amphisbaenians. The closest living relatives of mosasaurs are snakes, though they evolved from aigialosaurs, semi-aquatic ancestors of monitor lizards.
Mosasaurs were air-breathing serpentine predators. In general, mosasaurs were huge. The smallest known was 3 m (10 ft) in length, though longer mosasaurs were more typical, with the longest known, Hainosaurus, topping out at 17.5 m (57 ft). These were true sea monsters. Early on in their existence, they would have competed with other marine reptiles such as ichthyosaurs and plesiosaurs, but they became progressively more successful as the superficially fish-looking ichthyosaurs died out.
Mosasaurs had a basic body plan similar to monitor lizards, but streamlined for swimming, and of course they were much larger. Mosasaurs were one of many marine species that exploited the high sea levels and large continental seas of the late Cretaceous by adapting to the larger niche. The existence of mosasaurs has been known since a magnificent fossil was unearthed in 1780 in a Dutch limestone quarry. The finding was extremely well publicized, and turned the attention of the thinking public towards fossil animals.
Like snakes, mosasaurs had a loosely hinged jaw, which would have allowed them to open wide and consume huge numbers of fish. Some species would have consumed sea urchins and mollusks, cracking them open with their bulbous teeth, while larger, sharp-toothed species would have eaten other large marine reptiles and large fish. Like most reptiles, they probably ate their own kind if they had the chance. Because they often consumed prey whole, intact fossils of seabirds, sharks, and fish have been found in their guts.
With reduced limbs, mosasaurs would have moved through the water using the undulating motion of their tail only. This is in contrast to practically every other marine reptile of the time, which had fins to help push itself through the water. Mosasaurs would have moved more like a conger eel or sea snake.