Former reptile: Gymnarthrus. Former reptile, former amphibian: Diadectes. Both from Case 1910.

Case 1910
described several skulls from what he presumed were Permian deposits in Archer County, Texas. Yes, they are Early Permian and home to many a Dimetrodon.

Among the several skulls
was Gymnarthrus willoughbyi (Fig. 1), known from a tiny 1.6cm skull. Case reported: “It was thought at first that both the basisphenoid and the parasphenoid process constituted the the parasphenoid bone and that the animal was an amphibian, but this is impossible… the animal approaches the intermediate form between the amphibians and reptiles.” Today we know Gymnarthrus to be one of the lizard mimics, the lepospondyl microsaurs. Case also wrote, “The nearest approach to this form is the small amphibian skull described by Broili as Cardiocephalous sternbergii, but this is described as having the skull complete, no parietal foramen, teeth regularly diminishing in size anteriorly but with cutting edges and lyra present.” I don’t know what lyra are in this context.

Figure 1. Gymarthrus willougbyi, drawn by Case 1910 on the left and von Huene 1913 on the right.

Figure 1. Gymarthrus willougbyi, drawn by Case 1910 on the left and von Huene 1913 on the right. These are apparently freehand sketches and, judging by the perspective implied by the large orbit on the right, sketched from two distances.

Carroll and Gaskill 1978
allied Gymnarthrus with Cardiocephalus, another microsaur.

Figure 2. Diadectes phaseolinus in situ, as originally illustrated and as reillustrated above according to phylogenetic bracketing.

Figure 2. Diadectes phaseolinus in situ, as originally illustrated and as reillustrated above according to phylogenetic bracketing. Case reported the tail was as long as the presacral portion of the column, but did not illustrate it that way for this specimen. No intercentra were present.

Case also identified Diadectes as a reptile
(order Cotylosauria), but later authors (and currently Wikipedia, taken from a PhD thesis by R Kissel 2010) considered it a reptile-like amphibian. The large reptile tree nests Diadectes as derived from Milleretta and all the “Diadectomorpha” listed in Wikipedia are reptiles. Limnoscelis, Orobates and Tseajaia do not nest with Diadectes in the large reptile tree, but bolosaurids and procolophonids do. So we’ve got some housecleaning to do at that node.

The interesting thing about this Diadectes specimen,
according to Case 1910, is the set of expanded dorsal ribs beneath the scapulae. He writes, “The ribs of the third, fourth and fifth vertebrae show a well defined articular end with a distinct neck. The bodies of these ribs are expanded into thin triangular plates, with the front edge straight and the posterior edge drawn out into a point which overlaps the succeeding rib; this forms a strong protection for the anterior thoracic region. The sixth, seventh and eighth [ribs] are overlain by thin, narrow, plates which continue backward the protection of the thoracic region to a point opposite the posterior end of the scapula.” Some, but not all Diadectes specimens have such expanded ribs.

Case presumed
that gastralia (his ‘abdominal ribs’  were present. They are not. Case notes “the animal was distinctly narrow chested, with the bones of the the girdle strongly interlocked. Diadectes had practically no neck.”

Based on the mounted skeleton, Case reiterated
“the suggestions previously made by the author that these animals are the nearest discovered forms to the ancestors of turtles.” That old hypothesis has not been confirmed by the large reptile tree, as noted earlier.

References
Carroll RL and Gaskill P 1978. The Order Microsauria. Memoirs of the American Philosophical Society 126:1-211 [J. Mueller/T. Liebrecht/T. Liebrecht]
Case EC 1910.
 New or little known reptiles and amphibians from the Permian (?) of Texas. Bulletin of the American Museum of Natural History 28 (17):163-181.
Huene FRF von and Gregory WK 1913. The skull elements of the Permian Tetrapoda in the American Museum of Natural History, New York. Bulletin of the AMNH ; v. 32, article 18.: 315-386.

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