Xiphactinus ( from Latin and Greek for `` sword-ray '' ) was a large , 4.5 to 5 m ( 15 to 20 feet ) long predatory bony fish that lived in the Western Interior Sea , over what is now the middle of North America , during the Late Cretaceous . Xiphactinus ( from Latin and Greek for `` sword-ray '' ) was a large , 4.5 to 6 m ( 15 to 20 feet ) long predatory bony fish that lived in the Western Interior Sea , over what is now the middle of North America , during the Late Cretaceous .