Maybe Araeoscelis DOES have a lateral temporal fenestra

Among basal diapsids,
Araeoscelis (Fig. 1, Williston 1910) has been the traditional outlier, closing up its lateral temporal fenestra shortly after gaining its upper temporal fenestra. Taking another look at the published drawings and moving the bones around a little, exposes a tiny lateral fenestra (Fig. 1). This is not traditional thinking, but also removes an odd autapomorphy.

Short reminder:
Araeoscelis is one sort of diapsid, the sort that ultimately led to dinos and birds. This entire clade is convergent with the diapsid configuration that developed in lepidosaurs according to the large reptile tree.

Figure 1. Araeoscelis fossi skull drawings from Reiz et al. 1984. Reconstructed in the middle.

Figure 1. Araeoscelis fossi skull drawings from Reiz et al. 1984. Reconstructed in the middle.

It’s worthwhile here to bring up Petrolacosaurus (Fig. 2) for comparison.

Figure 2. Petrolacosaurus is an earlier sister to Araeoscelis with a definite diapsid temporal configuration, but oddly the upper temporal fenestra is largely lateral in this taxon.

Figure 2. Petrolacosaurus is an earlier sister to Araeoscelis with a definite diapsid temporal configuration, but oddly the upper temporal fenestra is largely lateral in this taxon. The parietals are quite broad.

Note
in Petrolacosaurus the upper temporal fenestra is high on the lateral side of the skull and the jaw joint is in line with the jaw line, distinct from Araeoscelis. The new data shifts nothing in the large reptile tree.

IMHO,
the reduction of the lateral temporal fenestra in Araeoscelis.has something to do with the decent of the jaw joint and the blunting/thickening of the teeth. It was eating something that was tougher or crunchier than Petrolacosaurus preferred.

Araeoscelis is a terminal taxon, leaving no known descendant taxa.

References
Reisz RR, Berman DS and Scott D 1984. The anatomy and relationships of the lower Permian reptile Araeoscelis. Journal of Vertebrate Paleontology 4: 57-67.
Vaughn PP 1955. The Permian reptile Araeoscelis re-studied. Harvard Museum of Comparative Zoology, Bulletin 113:305-467.
Williston SW 1910. New Permian reptiles; rhachitomous vertebrae. Journal of Geology 18:585-600.
Williston SW 1913. The skulls of Araeoscelis and Casea, Permian reptiles. Journal of Geology 21:743-747.
wiki/Araeoscelis

What is Aphelosaurus? Something new and something old.

Aphelosaurus lutevensis (Gervais 1859, Falconnet and Steyer 2007, Early Permian 290 mya) has been known for a long, long time and was described as an enigmatic reptile from the Lower Permian. It is represented by a headless, neckless, tail-less but otherwise completely articulated skeleton. Falconnet and Steyer (2007) considered it an araeoscelidian, but not quite one, and the large reptile tree bears this out. Aphelosaurus nests at their base. That makes it a basal diapsid. Those tiny medial fingers and toes are dead giveaways.

Falconnet and Steyer (2007) considered Aphelosaurus a probable biped and a possible arboreal type. Outgroups were likely both, but also this clade strangely led to marine types, as noted earlier. They did not recognize Eudibamus as an early diapsid. I sure would like to see that specimen up close.

Aphelosaurus and tracing.

Figure 1. Click to enlarge. Aphelosaurus and tracing. sacrals in blue. Image from Steyer 2012.

Here, overlooked by previous workers, the most interesting aspect of Aphelosaurus is a posteriorly elongate ilium, displaced from the sacrals here to align with several caudals. Don’t get your hopes up. No sisters had more than two sacrals. In this case, it’s a toss-up.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Falconnet J and Steyer J-S 2007. Revision, osteology and locomotion of Aphelosaurus, an enigmatic reptile from the Lower Permian of France. Journal of Morphology (abstract of the 8th International Congress of Vertebrate Morphology, Paris, July 2007): 38.
Gervais P 1859. Zoologie et paléontologie française – 2e édition. Bertrand, Paris, 544 pp.
Steyer S 2012. Earth before the dinosaurs. Indiana University Press, 182 pp.