Name Meaning: Paired Thorns
Geologic Era: Devonian
Location Found: England
Size: Under 1 meter in length
Range: Modern Europe and North America
Extinction: Middle Devonian (350 million years ago)
This little fish is fairly typical of an acanthodian, more commonly called “spiny sharks.” The name is a bit misleading, as these animals were not really sharks, which are cartilagenous fishes. That being said, acanthodians were related to true sharks, and can probably be considered either a sister group or an ancestor to modern cartilagenous fish. This specimen in somewhat curled up on itself, but the namesake spines are still visible. The fish is facing belly outward, so the dorsal spine (if present) is probably either crushed beyond recognition or buried in the rocky matrix.
Acanthodians came in a variety of forms, but a constant is the stiff spines on the pectoral, dorsal, and sometimes pelvic area. These spines probably supported a fin-like structure, which would have been fairly rigid. While probably not the most mobile creatures, the spines could have served as a deterrent from predators, as spines would increase the risk of the Diplacanthus becoming lodged in the predator’s throat. Acanthodians like Diplacanthus evolved in saltwater, but eventually spread to fresh water, where they were quite successful.
The end of the acanthodians came with the dominance of the true bony fishes and true cartilagenous fishes. The last of the acanthodians died out in the Permian Era. Diplacanthus itself became extinct much earlier, in the Devonian. No living acanthodians exists today, though their distant descendants in the form of bony fish and sharks still persist.
Acanthodian Reconstruction: http://spinops.blogspot.com/2012/01/diplacanthus-crassissimus.htm