Cretoxyrhina is was the toughest fish that lived in the western interior sea.  It ate fish, marine reptiles like Pleisosaurs, Moseosaurs and sea turtles.  They resemeble there modern counterpart, the great white shark.  Relying on there great sense of smell and hearing to find there prey.  There is only one creature that can kill a Cretoxyrhina, the Tylosaur.  A fully grown Tylosaur can devour a Cretoxyrhina.  So these sharks have to move to keep away from these predators.