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.
Paleobiota of the Niobrara Formation
Mosasaurs are the most common marine reptiles in the Niobrara Chalk and the most successful ones in the sea at the time. Four different genre representing the four different subfamilies of Mosasauridae: the Tylosaurinae, Plioplatecarpinae, Mosasaurinae, and Halisaurinae, were present in Niobrara. They were the dominant carnivorous marine reptiles and ate cephalopods, fish, turtles, pterosaurs, birds, and even plesiosaurs. There is evidence of them consuming other smaller mosasaurs. Despite this, mosasaurs often fell prey to some of the large sharks at the time, such as "Cretoxyrhina".