Xenacanthus is a genus of prehistoric sharks. The first species of the genus lived in the Late Devonian, the genus surviving until the end of the Triassic, over 200 million years ago. Fossils of various species have been found worldwide. The species identified at Petrified Forest National Park is X. moorei.
Xenacanthus had a number of features that distinguished it from modern sharks. This freshwater shark was about 3 ft (1 m) in length. The dorsal fin was ribbon-like and ran the entire length of the back and round the tail, where it joined with the anal fin. A distinctive spine projected from the back of the head, and gives the genus its name. The teeth had an unusual V-shape, sometimes called pronged. Most likely Xenacanthus fed on small crustaceans and fish. Like many fossil sharks, Xenacanthus is mainly known because of fossilized teeth and spines.