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

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