XENACANTHUS

long-surviving but now extinct genus of freshwater sharks. Xenacanthus survived from the end of the Devonian Period, some 360 million years ago, to about the end of the Triassic Period, 208 million years ago. Xenacanthus had a slim, elongated body with a low dorsal fin that extended down most of it, almost merging with the triangular, pointed tail. From the back of the skull, a long, sharp, movable spine projected.