Mosasaur
Mosasaurs (from Latin "Mosa" meaning the 'Meuse river', and Greek σαύρος "sauros" meaning 'lizard') are an extinct group of large marine reptiles containing 38 genera in total. 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 pliosaurs, mosasaurs became the dominant marine predators. They became extinct as a result of the K-Pg event at the end of the Cretaceous period, about 66 million years ago.
Tethysaurinae
Tethysaurines were primitive and comparatively small to medium-sized mosasaurs that lived during the earlier stages of mosasaur evolutionary history. "Tethysaurus" itself is approximately 3 metres long, whilst "Pannoniasaurus" might have reached lengths of 6 metres. They had a plesiopedal limb condition, meaning that they were not as well adapted to marine life as later mosasaurs and probably kept to shallow bodies of water. "Pannoniasaurus" is the only known mosasaur recovered from freshwater deposits.