Xiphactinus(zie-FAK-tin-nus) was a large predatory fish that lived in the cretacious sea about 75 million years ago and, like today's dolphins these chould jump clean out of the water. The usual lengh of a xiphactinus was around 20-30 feet long.
There is a famous fossil called "fish within a fish" and it is a fossil of a Xiphactinus with a 6 foot long fish in its gut. This was clearly its last meal, and most likely what killed it. Xiphactinus often fed on Hesperornis.