Primitive Heterostracan Armored Fish Fossil

Description: The Heterostracii are a primitive group of jawless fish. Most had a pair of plates, one dorsal and one ventral shield. The plates are typically made up of scales which have fused together as seen here. The scales of heterostracans are histologically distinct from other vertebrates, having three layers composed of dentine and aspidine, an acellular bony tissue unique to this class. The middle layer was honeycombed with tiny spaces called "cancella." The group went extinct at the end of the Devonian.