Ptyctodontida

Ptyctodontida; Temporal range: Devonian PreꞒ Ꞓ O S D C P T J K Pg N
	Reconstruction of Rhamphodopsis threiplandi
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	†Placodermi
Order:	†Ptyctodontida; Gross, 1932
Family:	†Ptyctodontidae; Woodward, 1891
Genera
	Austroptyctodus; Campbellodus; Chelyophorus; Ctenurella; Deinodus; Destnoporella; Eczematolepis ; Goniosteus; Kimbryanodus; Materpiscis; Palaeomylus ; Ptyctodopsis; Ptyctodus ; Rhamphodopsis; Rhynchodus

The ptyctodontids ("folded-teeth") are

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

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.

The Ptyctodontida were the only known group of placoderms that were recognizably

Stensioella heintzi

to tell if the males of that species had claspers or not).

Because they had reduced armor, the ptyctodontids were once thought to be the most primitive of the placoderms. Indeed, there has been the idea that the placoderms had a gradient, of sorts, from the least armored, and most primitive forms, to the heavily armored, most advanced forms. During the 1980s and '90's, ptyctodont skulls were compared with skulls from other orders. From these analyses, this idea of a gradient from least armored to most armored in placoderms was discarded. Now, the ptyctodonts are regarded as the sister group of the Arthrodira and Phyllolepida.

Timeline of genera

Gallery

Fin spine of the ptyctodont, Gamphacanthus, showing color patterns; from the Middle Devonian of Wisconsin.
Tooth plates from the ptyctodont Ptyctodus ferox, from the Middle Devonian of Wisconsin. Note the grinding surfaces.
Beak-like tooth plate of a ptyctodont from the Middle Devonian of Wisconsin.

References

Long, John A. (1996): The Rise of Fishes: 500 Million Years of Evolution. Johns Hopkins University Press, Baltimore.
ISBN 0-8018-5438-5

Sepkoski, Jack (2002). "A compendium of fossil marine animal genera (Placodermi entry)". Bulletins of American Paleontology. 364: 560. Retrieved 2011-05-20.