Photo of Xiphactinus from Kansas by Spacini, used under Creative Commons license.
Teleost fish are known as far back as the Triassic, but they diversified mightily in the Cretaceous. This is the group that includes about 96% of all modern fishes, many of which began in the Cretaceous. More than 26,000 living species of fish are teleosts.
Teleosts have some skeletal differences that distinguish them from earlier fish, including a movable jaw and a spine that ends before the tail fin.
While the Cretaceous saw the expansion of many groups of fish alive today, including salmon, bass, and cod, on an individual species level there were plenty of comings and goings – new species appearing, while others went extinct, which is always happening. On the whole you’d probably be hard pressed to see any big differences between many Cretaceous teleosts and modern varieties. Xiphactinus, whose name means sword-fin, was one huge teleost that reached 20 feet in length. They were fanged predators who lived in the late Cretaceous seas of Kansas, where they were first discovered in the 1850s, and at other locations in North America as well as Europe, Australia, and South America. Many fossil specimens include the skeletons of large prey in the stomachs of Xiphactinus.
The name teleost is from the Greek meaning “complete” and “bone” – a nicely, completely, boned fish.
—Richard I. Gibson
Photo of Xiphactinus by Spacini, used under Creative Commons license.