Ptyctodontida

From Palaeos

A trio of the ptyctodontid placoderm species, Ctenurella gladbachensis

Gnathostomata
|--Placodermi
|  `==Acanthothoraci (paraphyletic)
|     |--o--o--Romundina
|     |  |  `--Antiarchi
|     |  `--o--Radotina kosorensis
|     |     `--Rhenanida
|     `--o--"Radotina" prima
|        |--o--Ptyctodontida
|        |  `--Petalichthyida
|        `--Arthrodira
`--Eugnathostomata

The Ptyctodontids ("Beak-teeth") were a group of (largely) unarmoured 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 well-armored Acanthothoracids, and the holocephalids, the Ptyctodontids lived near the sea bottom and preyed on shellfish.

These unencumbered beasts were once thought to be among the most, if not the most, primitive placoderms, back in the old, old days when placoderms were thought of as being a spectrum of "shark-like and not so armored" to "shell lunchbox with a tail." With the reexamination of Acanthothoraci, the Ptyctodontids have been placed closer to the Arthrodires due to similarities of skull and mouth anatomies.

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 similiarities 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 similiar to sharks, and that of ptyctodontids were more similiar to those of other placoderms, and, most importantly, the holocephalids had true teeth, while the ptyctodonts had beak-like tooth-plates. Furthermore, of those ptyctodontids known to have scaly bodies (i.e., Cambellodus decepiens), the anatomy and histology of the scales are very different from the denticles of holocephalids.

The Ptyctodontids were the only known group of placoderms that were verifiably sexually dimorphic, in that the males had pelvic fins modified into clasping organs similiar, 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).

Despite the fact that they lacked the armor of their relatives, the Ptyctodonts were a very successful group, and were able to withstand competition from other, more heavily armored placoderm orders. In fact, when the Acanthothoraci (presumed to be their biggest competitors) went extinct prior to the Mid Devonian extinction event, the Ptyctodonts underwent a radiation that lasted from the Mid Devonian until the extinction of all placoderms at the close of the Devonian.

Characters: usually marine placoderm; adult size more than 20cm long. Look similar to Chimeroids. Armor back of head only. Mostly scaleless. Caudal fin much reduced. Very hard hypermineral "secondary dentine" on tooth plates. Claspers.

Note: I once described ptyctodonts as looking like an upper class Englishman dressed for dinner. I stand by that description. 020329.

Links

Mikko's Phylogeny Archive [1]

References

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

Palaeos com - Ptyctodontida

Acknowledgements

--Stanton 21:22, 24 September 2006 (PDT)