Platyhystrix: closer to Acheloma than to Cacops?

Platyhystrix was a dissorophid lepospondyl anamniote that had a dorsal sail (Figs. 1, 2 ), not quite like those  of its more famous reptilian/synapsid contemporaries, Dimetrodon and Edaphosaurus.

There must have been something in the air back then,
and those dorsal sails were there to catch it!

Figure 1. Platyhistrix skull reconstructed from slightly disassociated parts.

Figure 1. Platyhistrix skull reconstructed from slightly disassociated parts. And the Lewis and Vaughn 1965 dorsal sail, distinct from the others in figure 2. The skull here appears to have a confluent naris and antorbital finestra, as in Acheloma, but there are other bones missing there, too, like most of the maxilla.

Dissorophids are traditionally nested with
temnospondyls, but here, at the large reptile tree (LRT, now 959 taxa), they arise from a sister to the basal seymouriamorph, Utegenia and continue to be generally smaller taxa (< 60cm).

Figure 2. Other Platyhystrix specimens known chiefly from dorsal spines.

Figure 2. Other Platyhystrix specimens known chiefly from dorsal spines. That old skull from Williston 1911 is missing the central area, here imagined from the more complete specimen in figure 1.

Distinct from Acheloma
the skull of Platyhysterix does not appear to have giant palatal fangs, or such large marginal teeth. The jugal nearly separates the postorbital from the supratemporal. The postorbital is larger and much knobbier.

Like Acheloma
The rostrum may include a confluent nairs/antorbital fenestra, a constricted rostrum (in dorsal view), a naris of similar laterally wavy shape, robust premaxillary ascending processes, large tabulars and other traits relatively exclusive to these two.

A fair amount of reassembly
is required of the Platyhystrix skull. The random neural spine below the lower right jaw line allies the skull with specimens that also have long neural spines.

Figure 1. Acheloma dunni skull with a confluent antorbital fenestra and naris.

Figure 3. Acheloma dunni skull with a confluent antorbital fenestra and naris.

Wouldn’t it be interesting 
to see hatchlings and juveniles of Platyhystrix? It is widely considered, along with its double-armored kin, Dissorophus, to have been fully terrestrial. So, did these two have a swimming tadpole stage? And then develop spines and armor in adulthood? Or did they converge with reptiles, laying protected eggs on land, skipping the tadpole stage? Let’s keep an eye out for little finbacks.

References
Berman DS, Reisz RR and Fracasso MA 1981. Skull of the Lower Permian dissorophid amphibian Platyhystrix rugosus. Annals of the Carnegie Museum 50 (17):391-416.
Case EC 1911. Revision of the Amphibia and Pisces of the Permian of North America. Publ. Carnegie Inst. Washington 146:1-179.
Dilkes DW and Reisz R 1987. Trematops milleri identified as a junior synonym ofAcheloma cumminsi with a revision of the genus. American Museum Novitates 2902.
Lewis GE and Vaughn PP 1965. Early Permian vertebrates from the Cutler Formation of the Placerville area, Colorado, with a section on Footprints from the Cutler Formation by Donald Baird: U.S. Geol. Survey Prof. Paper 503-C, p. 1-50.
Williston SW 1911a. A new family of reptiles from the Permian of New Mexico. American Journal of Science 31:378-398.
Williston SW 1911b. American Permian vertebrates. University of Chicago Press: 145 pp.

wiki/Acheloma
wiki/Platyhystrix

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