Ptyctodontida

Ptyctodontida Temporal range: Early Devonian–Late Devonian PreЄ Є O S D C P T J K Pg N
Reconstruction of Rhamphodopsis threiplandi
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Subphylum:	Vertebrata
Infraphylum:	Gnathostomata
Class:	Placodermi
Order:	Ptyctodontida Gross, 1932
Family:	Ptyctodontidae
Genera
Australoptyctodus Campbellodus Chelyophorus Ctenurella Deinodus Destnoporella Eczematolepis Goniosteus Materpiscis Palaeomylus Ptyctodopis Ptyctodus Rhamphodopsis Rhynchodus

Australoptyctodus
Campbellodus
Chelyophorus
Ctenurella
Deinodus
Destnoporella
Eczematolepis
Goniosteus
Materpiscis
Palaeomylus
Ptyctodopis
Ptyctodus
Rhamphodopsis
Rhynchodus

The ptyctodontids ("folded-teeth") are placoderms of the order Ptyctodontida, containing the family Ptyctodontidae. With their big heads, big eyes,, reduced armor 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.

On account of their radically reduced armor, some paleontologists have suggested that the Ptyctodontida were not actually placoderms, but actual holocephalians, some primitive group of elasmobranch fish, or even were the ancestors of the holocephalians, including the chimaeras. Thorough anatomical examinations of whole fossil specimens reveal that the profound similarities between these two groups are actually very superficial. The major differences between them were that holocephalians have shagreen on their skin and ptyctodontids did not, that the armored plates and scales of holocephalians are made of dentine, and the armored plates and scales of ptyctodontids were made of bone, the anatomy of the craniums of holocephalians is more similar to sharks, and that of ptyctodontids were more similar to those of other placoderms, and, most importantly, the holocephalians have true teeth, while the ptyctodonts had beak-like tooth-plates.