While reading through this issue of LIFE Magazine on Google books I ran across this article.
Text as follows:
"The fossil skeletons of a fish within a fish give 90 million-year-old record of a glutton"
"Ninety million years ago a 14-foot-long tyrant fish called Portheus swam through the great shallow sea that covered what today is Kansas. Like its distant modern relative the tarpon, Portheus was a fierce predator, armed with a bulldoglike lower jaw for lunging at prey from below. It came upon a 6-foot-long herring-type Ichthyodectes and proceeded to swallow it whole. This huge wriggling repast may have been too much for the ravenous Portheus. At any rate, it died with its belly full. Then it sank heavily into the sea ooze, where it became fossilized in the clay while the Cretaceous American Sea vanished about 70 million years ago. Uncovered under the dry land of a cattle ranch near Quinter, Kan., this skeleton-within-a-skeleton, vivid evidence of the reward of gluttony, is believed to be the most ancient fish-eat-fish record ever found intact."
The fossils were discovered by Walter Sorenson of the American Museum of Natural History.
It was a fast swimmer, as the forked tail and streamlined body show. Probably would have put up one hell of a fight on a 12 weight fly rod.