It appears that the shark's family tree has a new addition—a group of bony fishes called acanthodians.
After examining a fossil of a sharklike fish (Doliodus problematicus) that dated back to the lower Devonian period (about 400 million years ago), scientists at the American Museum of Natural History's Division of Paleontology realised that it bore some characteristics of a species of extinct bony fishes called acanthodians. These fishes had existed many years before the first sharks appeared.
For instance, the fossil had paired spines on its pectoral fins, as well as spikes that possibly lined its underbelly during its lifetime - two signs indicating its acanthodian ancestry. On the other hand, the fossil's other features (like its head and teeth) were very similar to a shark's.
All this points to the Doliodus problematicus as being a transitional species, literally linking ancient acanthodians to modern sharks, as John Maisey, the Museum's Herbert R. and Evelyn Axelrod Research Curator concluded, “The arrangement of these spines shows unequivocally that this fish was basically an acanthodian with a shark’s head, pectoral skeleton, and teeth.”