Placodermi

Class of fish, now entirely extinct, having been most common during the Devonian period. Placoderms slightly resembled sharks or rays, but the head and the forepart of the trunk were covered by bony protective plates. Most were small, 10–40 cm/4–15.7 in long, but a few reached several metres.

There are six orders, classified mainly on the characteristics of the bony shielding plates: Arthrodira, mainly marine; Antiarchi, mainly freshwater; Rhenanida, mostly marine; Petalichthyda, exclusively marine; Phyllolepida, exclusively freshwater; and Ptyctodontida, marine and freshwater dwellers. Their position in evolution is not clear. They may have developed from the same ancestor as the sharks, skates, and rays, and their line may have died out, or the Ptyctodontida may be the ancestors of the Chimaera.