Pre-Diapsids. The Opening Act.

The Traditional View 
Diapsids were derived from the Protorothyridae (or Captorhinomorpha), close to Paleothyris (Carroll 1969). Petrolacosaurus is the earliest known diapsid. The diapsid configuration was not preceded by any temporal fenestration. Petrolacosaurus nests at the base of the Neodiapsida, which includes most other reptiles with temporal fenestration other than synapsids and bolosaurids.

The Heretical View
Diapsids were derived from basal synapsids close to Aerosaurus (a synapsid) and Protorothyris (a protosynapsid). Heleosaurus is the most primitive known protodiapsid (it nests outside the Synapsida). Eudibamus and Spinoaequalis are the most primitive known diapsids (two pairs of temporal openings). The diapsid configuration was preceded by lateral temporal fenestration. Click here to see the list of reptiles that succeeded  Eudibamus. Lepidosaurs and their sisters are not included on that list.

The Origin of the Protodiapsida
Petrolacosaurus has been the poster child for the origin of the Diapsida and it’s a good example. However, what evolved before the Diapsida has been largely ignored or overlooked.

Basal Protodiapsida to scale.

Figure 1. Basal Protodiapsida to scale. Diapsids in yellow. The synapsid Aerosaurus is grey.

Basal Archosauromorpha with a focus on the Protodiapsida

Figure 2. Click to enlarge. Basal Archosauromorpha with a focus on the Protodiapsida beginning with Heleosaurus.

Heleosaurus
Heleosaurus was considered an indeterminate diapsid by Broom (1907) and Carroll (1976), but Reisz and Modesto (2007) determined it was a varanopid synapsid. Here, with the benefit of new data on the skull (Botha, Brink and Modesto 2009) Heleosaurus nested just outside of the Synapsida at the base of a previously unrecognized clade, the Protodiapsida. Distinct from its predecessors, the skull was longer and the more cervicals were added. The suborbital jugal was more gracile. The pelvis was relatively larger. The limbs were longer. At 270 million years of age, the sole specimen of Heleosaurus is 40 million years younger than its phylogenetic descendants, indicating a long ghost lineage.

The Reduction of the Lateral Temporal Fenestra
The next two taxa, Archaeovenator (306 mya) and Mesenosaurus (266 mya) spanned that 40 million year gap. They had a smaller lateral temporal fenesatra, reduced by an advancing squamosal. Together with Heleosaurus, these two formed a clade.

The Milleropsis Detour
The temporal region of Milleropsis (Gow 1972, 290 mya) deviated from the basic skull pattern of its sisters. The lower temporal bar was absent, convergent with owenettids and squamates. The whip-like tail was incredibly long and the pelvis that anchored it was robust. Not enough is known of this taxon, but it appears able to run bipedally given available data.

Eudibamus
The most primitive diapsid may be Eudibamus (Berman et al. 2000, 290 mya), which was originally considered a sister to BolosaurusThe crushed skull does bear a strong resemblance. The teeth were blunt. The squamosal expanded further anteriorly to reduce the lateral temporal fenestra and the upper temporal fenestra first appeared. As in Milleropsis, the tail was whiplike. With an enlarged hind limb and short torso, Berman et al. (2000) considered Eudibamus an early biped. The proximal fingers and toes were greatly reduced, as in sister taxa, but more so.

Spinoaequalis.

Figure 3. Spinoaequalis. This reconstruction finds tiny upper temporal fenestrae.

Spinoaequalis 
Spinoaequalis (deBraga and Reisz 1995, Fig. 3) had small upper temporal fenestrae and a shorter temporal area. The skull is otherwise a good match for Petrolacosaurus. The tail of Spinoaequalis was distinct from all sisters in having high neural spines and deep chevrons. Spinoaequalis does not nest as a sister to Hovasaurus, a diapsid with a similar deep tail.

Petrolacosaurus
In Petrolacosaurus (Lane 1945, Reisz 1977) we find further reduction in the lateral temporal fenestra and further expansion of the upper temporal fenestra. The fingers and toes were asymmetrical, but less so than in Eudibamus and Spinoaequalis. The neck was further elongated. Interestingly, in Petrolacosaurus (Fig. 1) the two temporal fenestra are visible in lateral view.

Araeoscelis
Another araeoscelid, Araeoscelis (Williston 1910, Reisz, Berman and Scott 1984), completely infilled the lateral temporal fenestra but kept the upper temporal fenestra, which is unlike the vast majority of the other phylogenetic successors of Petrolacosaurus. On the other hand, Mesosaurus infilled the upper temporal fenestra and largely infilled the lower one, leaving only a small opening in some specimens. Restoration is difficult on many others due to crushing and scattering.

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
Botha-Brink J and Modesto SP 2009.Anatomy and Relationships of the Middle Permian Varanopid Heleosaurus scholtzi Based on a Social Aggregation from the Karoo Basin of South Africa. Journal of Vertebrate Paleontology 29(2):389-400.
Berman, DS, Reisz RR, Scott D, Henrici AC, Sumida SS and Martens T 2000. Early Permian bipedal reptile. Science 290: 969-972.
Broom R 1907. On some new fossil reptiles from the Karroo beds of Victoria West, South Africa. Transactions of the South African Philosophical Society 18:31–42.
Carroll R L 1969. A middle Pennsylvanian captorhinomorph, and the interrelationships of primitive reptiles: Journal of Paleontology, 43:1151-170.
Carroll RL 1976. Eosuchians and the origin of archosaurs; pp. 58–79 in C. S. Churcher (ed.), Athlon: Essays on Paleontology in Honour of Loris Shano Russell. Miscellaneous Publications of the Royal Ontario Museum, Toronto.
deBraga M and Reisz RR 1995. A new diapsid reptile from the uppermost Carboniferous (Stephanian) of Kansas. Palaeontology 38 (1): 199–212. palass-pub.pdf
Gow CE. 1972. The osteology and relationships of the Millerettidae (Reptilia: Cotylosauria). Journal of Zoology, London 167:219-264.
Lane HH 1945. New Mid-Pennsylvanian Reptiles from Kansas. Transactions of the Kansas Academy of Science 47(3):381-390.
Reisz RR 1977. Petrolacosaurus, the Oldest Known Diapsid Reptile. Science, 196:1091-1093. DOI: 10.1126/science.196.4294.1091
Reisz RR and Modesto SP 2007. Heleosaurus scholtzi from the Permian of South Africa: a varanopid synapsid, not a diapsid reptile.
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.
Rieppel O and deBraga M 1996. Turtles as diapsid reptiles. Nature 384:453-454.
Vaughn PP 1955. The Permian reptile Araeoscelis re-studied. Harvard Museum of Comparative Zoology, Bulletin 113:305-467.

wiki/Araeoscelis
wiki/Archaeovenator
wiki/Eudibamus
wiki/Heleosaurus
wiki/Mesenosaurus
wiki/Milleropsis
wiki/Petrolacosaurus
wiki/Spinoaequalis

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