Opisthodontosaurus – not quite a captorhinid and definitely not a microsaur

Figure 1. Opisthodontosaurus (above) with missing bones in color. Black lines represent the referred specimen, OMNH 77470 scaled to fit the holotype, OMNH 77469, here in ghosted lines. Colors represent missing bones.

Figure 1. Opisthodontosaurus (above) with missing bones in color. Black lines represent the referred specimen, OMNH 77470 scaled to fit the holotype, OMNH
77469, here in ghosted lines. Colors represent missing bones. Note the concave maxilla ventral margin and the lower postorbital region compared to Cephalerpeton, its sister in the large reptile tree. These two and other taxa are sisters to captorhinids, but have narrower skulls.

A recent paper by Reisz et al. 2015 brings us a new basal reptile, Opisthodontosaurus carrolli (Fig. 1, Reisz et al. 2015; Artinskian, Early Permian ~289 mya), with teeth so robust it brought to mind a similar microsaur with thick posterior canines, Euryodus (Fig. 2).

Figure 1. Euryodus primus, a microsaur nesting between Scincosaurus and Micraroter. Note the odd posterior canine teeth.

Figure 2. Euryodus primus, a microsaur nesting between Scincosaurus and Micraroter. Note the odd posterior canine teeth.

Reisz et al. nested Opisthodontosaurus with Concordia (which it closely matches) and not far from Reiszhorhinus and Romeria primus. The large reptile tree duplicated these nestings, but recovered an excluded big-tooth taxon, Cephalerpeton (Fig. 1), closest to Opisthodontosaurus. I do not have data on another listed sister, Rhiodenticulatus, but will add it as soon as I am able to.

Reisz et al mentioned a depressed lateral dentary posterior to the tooth row. Since no large surangular was preserved and sister taxa have such a bone, it appears likely that  that depression received the missing surangular.

Like its sisters, Opisthodontosaurus is a basal lepidosaurmorph that nests with others that have a relatively narrower skull than outgroup taxa  including captorhinids and Thuringothyris. Tooth size varied a great deal in this clade.

Narrow-skulled sisters
to the captorhinids + cephalerpetontids, the larger Orobates and the smaller Milleretta, ultimately gave rise to the rest of the lepidosauromorphs, including limnoscelids, caseasaurs, diadectomorphs, pareiasaurs, turtles, lanthanosuchids, owenettids, kuehneosaurs and lepidosauriforms including pterosaurs.

Distinct from the microsaur Euryodus, Opisthodontosaurus had a taller squamosal, a greatly reduced supratemporal, a triangular postfrontal and postorbital along with a smaller basipterygoid with a more gracile cultriform process.

Thanks to Dr. Reisz for sending his paper this morning. This is a good discovery, well written and just missing one pertinent taxon.

References
Reisz RR et al. 2015. A new captorhinid reptile from the Lower Permian of Oklahoma showing remarkable dental and mandibular convergence with microsaurian tetrapods. The Science of Nature, October 2015, 102:50.

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