The ptyctodontids ("folded-teeth") are the paraphyletic order Ptyctodontida of unarmored placoderms, containing the family Ptyctodontidae and, by extension, all modern gnathostomes such as chondrichthyes, bony fish and tetrapods. With their big heads, big eyes, and long bodies, the ptyctodontids bore a superficial resemblance to modern day chimaeras (Holocephali). Their armor was reduced to a pattern of small plates around the head and neck. Like the extinct and related acanthothoracids, and the living and unrelated holocephalians, most of the ptyctodontids are thought to have lived near the sea bottom and preyed on shellfish.