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.Everhart, M. J. 2002. New data on plesiosaur remains found as stomach contents of a Tylosaurus proriger (Squamata; Mosasauridae) from the Niobrara Formation of western Kansas. (Abstract) Transactions of the Kansas Academy of Science 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.Everhart, M. J. 2004. Late Cretaceous interaction between predators and prey. Evidence of feeding by two species of shark on a mosasaur. PalArch, vertebrate palaeontology series 1(1):1-7.
