Xiphactinus was one of the nastiest fish that ever lived. Xiphactinus was 20 feet long and had a mouth full of sharp teeth. Xiphactinus, however, sometimes bit off more than it can chew. In 1952, paleontologist George F. Sternberg found the fossil of a Xiphactinus with an entire Gillicus, a fish closely related to Xiphactinus, in the ribcage. Since the entire Gillicus was in the stomach of the Xiphactinus, it is likely that the Xiphactinus either chocked to death while swallowing the Gillicus or the Gillicus ruptured the stomach of the.