Acanthodii
fishes , having features of both bony fish ( Osteichthyes) and cartilaginous fish (Chondrichthyes). They appeared in the early Silurian (430 mya) and lasted until the late Permian (250 mya). They are distinguished in two respects: they were the first known jawed vertebrates, and they had stout spines supporting their fins, fixed in place and non-movable (like a shark's dorsal fin).
There were three orders: Climatiiformes, Ischnacanthiformes and Acanthodiformes. Climatiiforma had shoulder armor and many small sharp spines, Ischnacathiforma with teeth fused to the jaw, and the Acanthodiforma were filter feeders, with no teeth in the jaw, but long gill rakers.
Almost all of them were small, slender fish with large eyes, heterocercal tails, with the caudal vertebrae supporting the top lobe of the tail fin, like a shark's tail has today. All had pairs of bony spines along the ventral mid-body line, that often supported a web of tissue between the spine and the body, creating a fin. Thus the "spiny shark" nickname. These distinctive spines give the class its name, from the Greek akanthos.
The scales of Acanthodii are unique and used in determining relative age of sedimentary rock. The scales are tiny, with a bulbus base, a neck, and a flat or slightly curved diamond-shaped crown.
References
Long, J.A. The Rise of Fishes: 500 Million Years of Evolution. John Hopkins Univ. Press. Baltimore and London. 1995.
