Xiphactinus
Xiphactinus is an extinct genus of large predatory marine ray-finned fish that lived during the late Albian to the late Maastrichtian. The genus grew up to in length, and superficially resembled a gargantuan, fanged tarpon. It is a member of the extinct order Ichthyodectiformes, which represent close relatives of modern teleosts.
The species Portheus molossus described by Cope is a junior synonym of X. audax. Skeletal remains of Xiphactinus have come from the Carlile Shale and Greenhorn Limestone of Kansas, and Cretaceous formations all over the East Coast in the United States, as well as Europe, Australia, the Kanguk and Ashville Formations of Canada, La Luna Formation of Venezuela and the Salamanca Formation in Argentina.
Paleobiology
Species of Xiphactinus were voracious predatory fish. At least a dozen specimens of X. audax have been collected with the remains of large, undigested or partially digested prey in their stomachs. In particular, one fossil "Fish-Within-A-Fish" specimen was collected by George F. Sternberg with another, nearly perfectly preserved long ichthyodectid Gillicus arcuatus inside of it. The larger fish apparently died soon after eating its prey, most likely owing to the smaller prey's struggling and rupturing an organ as it was being swallowed. This fossil is on display at the Sternberg Museum of Natural History in Hays, Kansas.Like many other species in the Late Cretaceous oceans, a dead or injured individual was likely to be scavenged by sharks. The remains of a Xiphactinus were found within a large specimen of Cretoxyrhina collected by Charles H. Sternberg. The specimen is on display at the University of Kansas Museum of Natural History.
Like modern tarpons, Xiphactinus likely spent its juvenile stage of life in shallow seaway margins for protection and to utilize rich food resources, possibly rare in open marine water, though this needs confirmation due to the lack of shallow, nearshore deposits from the Western Interior Seaway. The teeth of the juvenile specimen indicate that the diet of Xiphactinus probably didn't change notably during its growth, implying that even the small specimens would have been fish-eating predators.