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Xiphactinus audax (from Latin and Greek for "audacious sword-ray") was a large, 15 to 20 foot long predatory bony fish that lived in the Niobraran Sea, in what is now North America, during the Late Cretaceous. When alive, the beast would have resembled a gargantuan, fanged tarpon. Skeletal remains of Xiphactinus have come from Kansas, Alabama, and Georgia.

Despite being preyed on by sharks and mosasaurs, X. audax was, nonetheless, a voracious predator in its own right. Numerous fossils have been found with large, undigested prey in their gullets or stomachs. In particular, one 15 foot fossil specimen was found by George F. Sternberg with another, perfectly preserved 6 foot long ichthyodectid Gillicus arcuatus, inside of it. The beast died immediately after eating its relative, most likely due to its prey struggling and rupturing an organ as it was being swallowed. This fossil can be seen at the Sternberg Museum of Natural History in Hays Kansas.

Virtually nothing is known about their larval or juvenile stages. The smallest fossil specimen of X. audax is a fragment of a premaxilla bone that came from an individual estimated to be about 12 inches long.