Xenacanthus is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of prehistoric shark

Shark

Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s. The first species of the genus lived in the later Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 period, and they survived until the end of the Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

, 202 million years ago. Fossils of various species have been found worldwide.

Xenacanthus had a number of features that distinguished it from modern sharks. This freshwater

Freshwater

Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

 shark was about one meter (three feet) in length. The dorsal fin

Dorsal fin

A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

 was ribbon-like and ran the entire length of the back and round the tail, where it joined with the anal fin. This arrangement resembles that of modern conger eels, and Xenacanthus probably swam in a similar manner. A distinctive spine projected from the back of the head, and gives the genus its name. The teeth had an unusual V-shape, and it probably fed on small crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s, and heavily scaled palaeoniscid fishes.

As in all fossil sharks, Xenacanthus is mainly known because of fossilised teeth and spines.

Species

• Xenacanthus atriossis
• Xenacanthus compressus
• Xenacanthus decheni
• Xenacanthus denticulatus
• Xenacanthus erectus
• Xenacanthus gibbosus
• Xenacanthus gracilis
• Xenacanthus howsei
• Xenacanthus laevissimus
• Xenacanthus latus
• Xenacanthus luedernesis
• Xenacanthus moorei
• Xenacanthus ossiani
• Xenacanthus ovalis
• Xenacanthus parallelus
• Xenacanthus parvidens
• Xenacanthus robustus
• Xenacanthus serratus
• Xenacanthus slaughteri
• Xenacanthus taylori