Xenacanthus

Name:

Xenacanthus (Greek for "foreign spike"); pronounced ZEE-nah-CAN-thuss

Habitat:

Oceans worldwide

Historical Period:

Late Carboniferous-Early Permian (310-290 million years ago)

Size and Weight:

About 2 feet long and 10-20 pounds

Diet:

Marine animals

Distinguishing Characteristics:

Slender, eel-shaped body; spine jutting from back of head

About Xenacanthus:

As prehistoric sharks go, Xenacanthus was the runt of the aquatic litter--the numerous species of this genus measured only about two feet long, and had a very un-shark-like body plan more reminiscent of an eel. The most distinctive thing about Xenacanthus was the single spike protruding from the back of its skull, which some paleontologists speculate carried poison--not to paralyze its prey, but to deter larger predators. For a prehistoric shark, Xenacanthus is very well represented in the fossil record, because its jaws and cranium were made of solid bone rather than easily degraded cartilage, as in other sharks.

Source : http://dinosaurs.about.com