Cretaceous North America,
Mooreville Chalk Formation
Mosasaurs were the dominant marine predators of their age, diversifying into a multitude of forms to rival the plesiosaurs, pliosaurs and sharks in Late Cretaceous seas.
Many of the more derived forms such as Tylosaurus and Mosasaurus grew to immense size, their overall morphology adapted to ambush large prey in an open-ocean environment. Yet some mosasaurs, like Clidastes seldom grew longer than two or three metres and retained an overall body plan similar to their earlier aigialosaur relatives from which they evolved.
Although now long extinct, mosasaurs are related closely to living monitor lizards and belong phylogeneticaly in the superfamily Varanoidea; a group of related reptiles including the extant Komodo dragon, the largest terrestrial lizard alive today.
Indeed, it is not uncommon to see Komodo dragons and other varanid reptiles swimming, with some species such as the Nile monitor spending a considerable portion of its life in water.
Mosasaurs, however, were fully aquatic animals, having evolved to spend their entire lives in water. A fossil of a basal mosasauroid found to contain advanced embryos in its abdomen implies that early in their evolutionary history mosasaurs were able to give birth to live young.
Clidastes is a basal member of the derived mosasaurinae, lacking the deep tail fluke seen in advanced members of the clade, and although not known for certain it probably hunted cephalopods and small fish in shallow surface water.
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