Basal Diapsida and Proto-Diapsida

Updated February 23, 2015 with the deletion of Echinerpeton and the inclusion of a new skull for Mycterosaurus. 

Yesterday we looked at basal synapsida with a peek at basal proto-diapsida. Here a bit more on the latter.

Figure 1. Basal diasids and proto-diapsids. Largely ignored these putative synapsids actually split from other synapsids while retaining the temporal fenestra trait that serves as the basis for the addition of upper temporal fenestra in diapsids. Included here are Protorothyris, Archaeovenator, Mycterosaurus, Heleosaurus, Mesenosaurus, Broomia, Milleropsis, Eudibamus, Petrolacosaurus, Spinoaequalis, and Tangasaurus.

Figure 1. Basal diasids and proto-diapsids. Largely ignored these putative synapsids actually split from other synapsids while retaining the temporal fenestra trait that serves as the basis for the addition of upper temporal fenestra in diapsids. Included here are Protorothyris, Archaeovenator, Mycterosaurus, Heleosaurus, Mesenosaurus, Broomia, Milleropsis, Eudibamus, Petrolacosaurus, Spinoaequalis, and Tangasaurus.

Oddly,
the most basal member here, Protorothyris, has a later appearance in the Early Permian. Other members are often from the Late Carboniferous. Still others appear throughout the Permian. So, these are the reptiles that preceded and skirted between the large pelycosaurs that typically grab the headlines. With Spinoaequalis and Petrolacosaurus (Fig. 1) appearing in the Late Carboniferous, all these taxa likely had sisters back then, with many late survivors in the Permian.

  1. Protorothyris – Early Permian
  2. Archaeovenator –  Late Carboniferous
  3. Mycterosaurus – Middle Permian
  4. Broomia – Middle Permian
  5. Heleosaurus – Middle Permian
  6. Mesenosaurus – Late Carboniferous
  7. Broomia – Middle Permian
  8. Milleropsis – Early Permian
  9. Eudibamus – Early Permian
  10. Petrolacosaurus – Late Carboniferous
  11. Spinoaequalis – Late Carboniferous
  12. Tangasaurus – Late Permian

There are very few reptiles known from earlier strata, although reptiles must have been present in great numbers throughout the Late Carboniferous. Paleothyris and Hylonomus are known from around 310 mya. Casineria and Westlothiana appear 335 mya in the Visean, 30 million years before their putative predecessors, the gephyrostegids, although Silvanerpeton is equally old. Evolution proceeded at a slow pace in this era.

Not a bolosaurid
Eudibamus
has bolorsaurid-like teeth, and so has been considered a bolosaurid (known chiefly from cranial material), but note that Mycterosaurus (not directly related) also had blunt teeth. So it can happen! BTW, bolosaurid sisters are often known from skulls only, but the few that do have post-crania generally have very large, bulky, post-crania, nothing close to the bipedal possibility in Eudibamus.

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