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Mosasaurs were so well adapted to this environment that they gave birth to live young, rather than returning to the shore to lay eggs as sea turtles do.
The smallest-known mosasaur was Dallasaurus turneri, which was less than 1 m (3.3 ft) long.
Mosasaurs had a body shape similar to those of modern-day monitor lizards (varanids), but were more elongated and streamlined for swimming.
Their limb bones were reduced in length and their paddles were formed by webbing between their long finger and toe bones.
Currently, the largest publicly exhibited mosasaur skeleton in the world is on display at the Canadian Fossil Discovery Centre in Morden, Manitoba.
The specimen, nicknamed "Bruce", is just over 13 m (43 ft) long.
Whether this behaviour was common across all size classes of mosasaurs is not clear.
This lack may be due to the delicate nature of the scales, which nearly eliminates the possibility of preservation, in addition to the preservation sediment types and the marine conditions under which the preservation occurred.However, new evidence suggests that many advanced mosasaurs had large, crescent-shaped flukes on the ends of their tails, similar to those of sharks and some ichthyosaurs.Rather than use snake-like undulations, their bodies probably remained stiff to reduce drag through the water, while their tails provided strong propulsion.Until the discovery of several mosasaur specimens with remarkably well-preserved scale imprints from late Maastrichtian deposits of the Muwaqqar Chalk Marl Formation of Harrana in Jordan, knowledge of the nature of mosasaur integument was mainly based on very few accounts describing early mosasaur fossils dating back to the upper Santonian–lower Campanian, such as the famous Tylosaurus specimen (KUVP-1075) from Gove County, Kansas.Material from Jordan has shown that the bodies of mosasaurs, as well as the membranes between their fingers and toes, were covered with small, overlapping, diamond-shaped scales resembling those of snakes.Early reconstructions showed mosasaurs with dorsal crests running the length of their bodies, which were based on misidentified remains of tracheal cartilage.By the time this error was discovered, depicting mosasaurs with such crests in artwork had already become a trend.Their tails were broad, and supplied their locomotive power.Until recently, mosasaurs were assumed to have swum in a method similar to the one used today by conger eels and sea snakes, undulating their entire bodies from side to side.Holes have been found in fossil shells of some ammonites, mainly Pachydiscus and Placenticeras.These were once interpreted as a result of limpets attaching themselves to the ammonites, but the triangular shape of the holes, their size, and their presence on both sides of the shells, corresponding to upper and lower jaws, is evidence of the bite of medium-sized mosasaurs.