Skulls at the Protodiapsid/Synapsid Split

Updated February 23, 2015 with a new image of Mycterosaurus.

Earlier we looked at taxa at the Protodiapsid/Synapsid split. Here’s an update with skulls alone after the addition of two Protorothyris skulls (Fig. 1).

Figure 1. Reptile skulls at the protodiapsid/ synapsid split to scale with Protorothyris as the proximal outgroup. Note the elongation of the rostrum and the appearance of the the lateral temporal fenestra. The two clades were originally quite similar. Two versions of Mycterosaurus are shown. Click to enlarge.

Descending from a sister
to Coelostegus, Protorothyris (Fig. 1) nests as the proximal outgroup to the split between the Synapsids and Protodiapsids (which gave rise to the Diapsida). The narrower skull of the more derived MCZ 2149 specimen of Protorothyris continues in both clades. The relatively short rostrum of the MCZ 2149 specimen of Protorothyris was an autapomorphy.

Both protodiapsid and synapsid clades display 

1. a new lateral temporal fenestra (with concurrent temporal bone shape changes)
2. a longer rostrum
3. longer ascending process of the premaxilla
4. the maxilla was at least as deep as the lacrimal
5. the pineal opening was larger
6. the frontal was not so narrow
7. the supratemporal contacted the postorbital
8. the cervical ribs were not robust and parallel to the centra
9. metatarsal 1 was less than half of metatarsal 3
10. pedal 5.1 did not extend beyond metatarsal 4

In basal protodiapsids

1. the naris was ventrally bordered by the maxilla chiefly.
2. the maxilla expanded dorsally, blocking contact between the lacrimal and naris, but only in the Heleosaurus/Mycterosaurus/Mesenosaurus clade. Other protodiapsids and basal diapsids did not have his trait.
3. the naris was elongated
4. the nasals and frontals were subequal
5. the basipterygoid processes were prominent
6. retroarticular process descended
7. cervical neural spines were not taller than the centra
8. medially the clavicles were not broad
9. the coracoid, even when fused, was larger than the scapula
10. the radiale and intermedium were elongated
11. the radius and ulna were together longer than 3x their width
12. the tibia is not less than twice the ilium length

In basal synapsids

1. the naris is ventrally bordered by the premaxilla ventrally.
2. most of this clade had nasals longer than frontals
3. most taxa had a convex ventral maxilla (but so did Helosaurus).
4. the occipital was anterior to the jaw joint
5. broader supraoccipital
6. mandible tip rises
7. retroarticular process rises
8. transverse processes appear on the vertebrae
9. the manus and pes were subequal
10. the ilium is longer than tall
11. the pelvic elements are fused
12. pedal 4.1 is shorter than three times its width
13. overall length longer than 30cm

This phylogenetic split has not been recognized in academic literature as generic diapsids are rarely to never tested with basal synapsids.

References
Clark J and Carroll RL 1973. Romeriid Reptiles from the Lower Permian. Bulletin of the Museum of Comparative Zoology 144 (5): 353-408 .
Price LI 1937. Two new Cotylosaurs from the Permian of Texas: Proceedings of the New England Zoological Club, v. 16, p. 97-102.
wiki/Protorothyris