Despite their name, and an outward appearance that somewhat resembled sharks, the Acanthodians were not sharks - rather had features of both bony fish (Osteichthyes) and cartilaginous fish (Chondrichthyes) - for example, they had a cartilaginous skeleton, but their fins had a bony base. Acanthodians are called "spiny", because all their fins, except the tail, were supported by bony spines.
Another interesting feature of Spiny Sharks is that their epidermis was covered with tiny rhomboid-shaped scales - although in some species particular scales are greatly enlarged, covering the top of the head, the shoulder girdle, or providing a bony flap over gill openings. Each order of Acanthodii had their own distinctive arrangement of scales, and as a result it is often possible to determine the relative age of sedimentary rocks by examining Acanthodii fossils and their scales.
Spiny Sharks Timeline:
Spiny Sharks were early jawed fish that lived between 440 and 250 million years ago
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