The spiny base, Acanthodes (1848)

Phylum : Chordata Class : Acanthodii Order : Acanthodiformes Genus : Acanthodes Species : A. guizhouensis

For the signature genus of the acanthodians, or spiny sharks, Acanthodes was only moderately spiky, sporting a mere half a dozen sharp spines (one for each of its fins). Still, these spines served their purpose in repelling the larger marine predators of Acanthodes' freshwater environment, where this prehistoric fish filter-fed for plankton and other small organisms.

Despite its designation as a type of shark, Acanthodes had no teeth. This unusual fact can be explained by the moderate "missing link" status of this middle Permian genus, which shared characteristics of both cartilaginous and bony fish (that is, fish, like sharks, with skeletons made of cartilage and those with skeletons made of bone). A recent study of Acanthodes' brain case shows that this shark (or a closely related species) may have been ancestral to human beings, since it was the immediate precursor of the bony fishes from which terrestrial vertebrates evolved.