Orovenator – not a diapsid, but close

I love to find errors in my work.
Those help patch the structure of the large reptile tree to make it stronger and more parsimonious.

Case in point
Orovenator (Early Permian, 289 mya) looked like a basal younginid. And it nested that way in the large reptile tree with the diapsid reconstruction provided by Reisz, Modesto and Scott (2011). They considered it the oldest diapsid after the appearance of Petrolacosaurus and Araeoscelis. They based their nesting on putting together the holotype (basically a rostrum and mandible with a gracile forked jugal) with the larger referred specimen (a skull roof with a narrow parietal indicating upper temporal fenestra, Fig. 1).

Unfortunately,
when you scale the referred specimen to the size of the holotype, the bones they both have in common don’t quite match up. So, a new reconstruction rebuilt from the in situ specimen was created (Fig. 1) and rescored for only those traits.

Figure 1. Orovenator (holotype on right) along with the larger referred specimen (on left, and scaled down to the size of the holotype above right). Arrows point to mismatches.

Figure 1. Orovenator (holotype on right) along with the larger referred specimen (on left, and scaled down to the size of the holotype above right). Arrows point to mismatches. The referred specimen had upper themporal fenestrae. Analysis indicates that the holotype did not.

Analysis now indicates that Orovenator and Archaeovenator (Fig. 2) were sister taxa and the latter does not have upper temporal fenestra — but it’s on the line toward basal diapsids. So, credit goes to Reisz, Modesto and Scott (2011) for that. Unfortunately they did not mention Archaeovenator (Reisz and Dilkes 2003) in their analysis, but considered it a small synapsid. In the large reptile tree both are indeed synapsids, but outside the synapsid clade leading to therapsids. Orovenator and Archaeovenator were proto-diapsids.

Figure 2. Archaeovenator, a sister to Orovenator, is a protodiapsid.

Figure 2. Archaeovenator, a sister to Orovenator, is a protodiapsid.

Both of these taxa share a very large naris, a concave rostrum, a gracile jugal, a large orbit and tiny teeth. Longo-torsoed Archaeovenator provides clues as to the post-crania of Orovenator.

So what about that referred specimen?
It’s probably a large diapsid, a descendant of a member of the Eudibamus/ Petrolacosaurus clade both from the Early Permian and Late Carboniferous respectively.

This exercise 
adds further evidence to the dictum that it’s potentially dangerous to score taxa with more than one specimen. Building a chimaera can lead to nesting mistakes.

References
Reisz RR and Dilkes DW 2003. Archaeovenator hamiltonensis, a new varanopid from the upper carboniferous of Kansas. Canadian Journal of Earth Science 40: 667-678.
Reisz RR, Modesto SP and Scot DMT 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society, London B

wiki/Archaeovenator

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