mosasauridae
Mosasaurs (from Latin Mosa meaning the 'Meuse river', and Greek σαύρος sauros meaning 'lizard') are an extinct group of large marine reptiles. Their first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. Mosasaurs probably evolved from an extinct group of aquatic lizards known as aigialosaurs in the Early Cretaceous. During the last 20 million years of the Cretaceous period (Turonian-Maastrichtian ages), with the extinction of the ichthyosaurs and decline of plesiosaurs, mosasaurs became the dominant marine predators. They became extinct as a result of the K-T event at the end of the Cretaceous period, approximately 66 million years ago.
Size: The smallest-known mosasaur was Dallasaurus turneri, which was less than 1 metre (3.3 ft) long. Larger mosasaurs were more typical, with many species growing longer than 4 metres (13 ft). Mosasaurus hoffmannii, the largest known species may have reached up to 17 metres (56 ft) in length. A species of Prognathadon has been suspected of being the heaviest known mosasaur because of its abnormally bulky build and large skull
Coloration: The coloration of mosasaurs was unknown until 2014, when the findings of Johan Lindgren of Lund University and colleagues revealed the pigment melanin in the fossilized scales of a mosasaur. The studies revealed that mosasaurs were likely countershaded, with dark backs and light underbellies, much like a great white shark or leatherback sea turtle, the latter of which had fossilized ancestors for which color was also determined. The findings were described in the journal Nature.