Description: The Acanthodians are jaw-bearing fish who still are the subject of dispute over their systematic position. Most possessed highly-advanced, spindle-shaped bodies thought to have made them swift swimmers. The body was covered in small mosaic-like scales. They possessed small teeth which were typically confined to the lower jaw; some were toothless. The feature they all share in common is the fact that all fins other than the caudal are supported by massive spines formed of dentine. Indeed, the name Acanthodii is derived from the Greek word for spine. The oldest acanthodian lived during the late Ordovician. They reached their peak during the Devonian, and became extinct during the Great Dying of the end-Permian extinction. This well-preserved example is known as Mesacanthus, a name that means “middle spine”. While missing the distalmost portion it is quite detailed with a fine skull and many spines in evidence.