Eugnathostomata. New fossil finds suggests thelodonts as the closest relatives of the Gnathostomata.[3] It is believed that the jaws evolved from anterior gill support arches that had acquired a new role, being modified to pump water over the gills by opening and closing the mouth more effectively — the buccal pump mechanism.
The mouth could then grow bigger and wider, making it possible to capture larger prey. This close and open mechanism would with time become stronger and tougher, being transformed into real jaws. Newer research suggests that a branch of Placoderms most likely was the ancestors of present day gnathostomes. Late Ordovician-aged microfossils of what have been identified as scales of either acanthodians[7] or "shark-like fishes,"[8] may mark Gnathostomata's first appearance in the fossil record.