Mosasaurs were large, serpentine marine reptiles that lived during the Late Cretaceous period. They were not dinosaurs, but lepidosaurs, and their closest living relatives are snakes. Mosasaurs had a loose jaw and consumed large amounts of fish, and some species also ate sea urchins and mollusks. They moved through the water using only the waving motion of their tails.
Mosasaurs were marine reptiles that inhabited the warm, shallow continental seas of the Late Cretaceous. They lived approximately between 98 and 65 million years ago. Although 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 KT extinction that wiped out the dinosaurs, represented today by tuataras, lizards, snakes, and amphisbaenians. The closest living relatives of mosasaurs are snakes, although they evolved from aigialosaurs, the semi-aquatic ancestors of monitor lizards.
Mosasaurs were serpentine, air-breathing predators. In general, mosasaurs were huge. The smallest known was 3 m (10 ft) long, although the longest mosasaurs were more typical, with the longest known, Hainosaurus, reaching 17.5 m (57 ft). These were real sea monsters. Early in their existence, they would have competed with other marine reptiles such as ichthyosaurs and plesiosaurs, but became progressively more successful as the fishy surface-looking ichthyosaurs became extinct.
Mosasaurs had a basic body plan similar to monitor lizards, but streamlined for swimming, and were obviously 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 discovery was extremely well publicized and turned the attention of the thinking public to fossil animals.
Like snakes, mosasaurs had a loose jaw, which would have allowed them to gape and consume huge numbers of fish. Some species would have eaten sea urchins and molluscs, opening them 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 given the chance. Because they often consumed whole prey, intact fossils of seabirds, sharks, and fish have been found in their guts.
With limbs reduced, mosasaurs would have moved through the water using only the waving motion of their tails. This is in contrast to virtually every other marine reptile of the time, which had fins to help propel itself through the water. Mosasaurs would have moved more like a conger eel or a sea serpent.
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