Ptyctodontida
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 Rhamphodopsis threiplandi
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Extinct (fossil)
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The Ptyctodontids ("Beak-teeth") were a group of unarmored placoderms. With their big heads, big eyes, and long bodies, the Ptyctodontids bore a strong resemblance to modern day chimaeras. Their armor was reduced to a pattern of small plates around the head and neck. Like the more advanced Acanthothoracids, and the holocephalids, the Ptyctodontids lived near the sea bottom and preyed on shellfish.
Some paleontologists have suggested that the Ptyctodontids were not actually placoderms, but actual holocephalids, or even were the ancestors of the holocephalids, 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 holocephalid had shagreen on their skin and ptyctodontids did not, that the armored plates and scales of holocephalids were made of dentine, and the armored plates and scales of ptyctodontids were made of bone, the anatomy of the craniums of holocephalids were more similar to sharks, and that of ptyctodontids were more similar to those of other placoderms, and, most importantly, the holocephalids had true teeth, while the ptyctodonts had beak-like tooth-plates.
The Ptyctodontids were the only known group of placoderms that were sexually dimorphic, in that the males had pelvic fins modified into clasping organs similar, if not identical to the clasping organs found in sharks, and chimaeras. The Ptyctodontids' claspers are another important piece of evidence for a relationship between the placoderms and the sharks and chimaeras. Paleontologists believe that the males of the ancestral placoderm had pelvic claspers, but the claspers were lost in the evolutionary development of each of the placoderm orders, save for the Ptyctodontids (there are too few whole specimens of the primitive Stensioella heintzi to tell if the males of that species had claspers or not).
Because they had reduced armor, the Ptyctodonts 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 1980's and '90's, ptyctodont skulls were compared with skulls from other orders. From these analysises, 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 Arthrodires and Phyllolepids.
[edit] External links
[edit] References
- Long, John A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The Johns Hopkins University Press, 1996. ISBN 0-8018-5438-5
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