Origin of the archosauromorph temporal fenestrae

Yesterday we looked at the evolution of the temporal fenestrae in lepidosauromorph reptiles. Today we’ll look at the same sequence in archosauromorph reptiles. Note that both develop their temporal fenestrae by convergence from “anapsid”-type precursors.

Figure 1. Development of the diapsid architecture in archosauromorph reptiles. This was done by convergence with lepidosauromorph reptiles. This sequence includes basal synapsids (development of the lateral temporal fenestra first) and from basal synapsids demonstrates the variation in basal diapsid skull architecture. Here, Petrolacosaurus is not the basalmost diapsid in this sequence which breaks a long-standing tradition. This image was updated September 2, 2013.

Figure 1. Development of the diapsid architecture in archosauromorph reptiles. This was done by convergence with lepidosauromorph reptiles. This sequence includes basal synapsids (development of the lateral temporal fenestra first) and from basal synapsids demonstrates the variation in basal diapsid skull architecture. Here, Petrolacosaurus is not the basalmost diapsid in this sequence which breaks a long-standing tradition. This image was updated September 2, 2013.

Figure 2. Subset of the large reptile tree highlighting taxa in figure 1 in the lineage of basal archosauromorph diapsids.

Figure 2. Subset of the large reptile tree highlighting taxa in figure 1 in the lineage of basal archosauromorph diapsids.

Starting with the basal “anapsid” reptile Protorothyris, the lateral temporal fenestra developed large in Elliotsmithia and small in Arachaeothyris, both considered basal synapsids. Heleosaurus is also a basal synapsid, but phylogenetically it nests among the protodiapsids alongside Millerosaurus (not related to Milleretta). Early diapsids include EudibamusSpinoaequalis and Petrolacosaurus.

Contra tradition, Petrolacosaurus is not the basalmost diapsid.

Thereafter this lineage of diapsids splits into a major marine clade, the Enaliosauria and a more terrestrial clade, the Thadeosauria, which gave rise to crocs and dinos along with several other terrestrial reptiles.

Preceding and within the Enaliosauria the lateral temporal fenestra fills in with Araeoscelis and both upper and lateral fenestrae fill in with Mesosaurus. The lower temporal bar is lost or modified in several marine taxa. The upper temporal fenestra is squeezed shut in thalattosaurs, but for the most part the temple and cheek architecture remain conservative in the archosauriformes.

Bordering bones
At the first appearance of the lateral temporal fenestra, the postorbital, squamosal, jugal and quadratojugal all border the rim in Aerosaurus. The quadratojugal is excluded in most other synapsids due to a long jugal process. Heleosaurus follows the Aerosaurus pattern. Millerosaurus has an odd and autapomophic squamosal process that divides the lateral temporal fenestra into anterior and posterior holes. The contribution of the quadtratojugal in two of the diapsids is less clear, but Petrolacosaurus demonstrates contact with the ltf.

The upper temporal fenestra appears in Eudibamus bordered by the postorbital, parietal and squamosal, a pattern retained by the other diapsids listed.

Reconstruction of the in situ skull bones demonstrates the autapomorphic lateral placement of the upper temporal fenestra in Petrolacosaurus, different from the dorsal placement in prior reconstructions.

A reordering of the reptile family tree is required
Due to these various appearances and disappearances of temporal fenestrae, reptiles must be nested and classified according to their overall morphologies, not strictly according to their cheek and temple architecture.

Moreover, the diapsid skull architecture did not appear all at once, but evolved from a simpler architecture;

Archosauromorph cheek and temple architectural patterns are distinct from those in the new Lepidosauromorpha, as we saw yesterday. These two clades share no common ancestor with a diapsid skull architecture, which is a heretical departure from the traditional paradigm.

4 thoughts on “Origin of the archosauromorph temporal fenestrae

  1. These two posts still don’t get to the nub of my criticism earlier: the circularity of the method. Its a problem I tried to point out when talking about pterosaur embryos. In both cases you are effectively using the method to show that you can use the method.

    You can’t assume these fenestrae are homologous unless you have used an independant test; that is code them separately (add a couple of extra character states to the character) and see if the clades still come out together. If they do, then you can start to assume homology

    • No I don’t mean lower and upper, I mean if you have the fenestra in the same position but with different morphologies (bordered by different bones), code them seprately i.e. split up your character state for lower temporal fenestera to absent (0), boardered by Jugal and quadratojugal (1), boardered by postorbital, jugal and quadratojugal (2), not just present/absent. That way you are not starting your analysis by assuming the two morphologies are homologous.

      That is what I meant by circular: You start your analysis by biasing it towards saying the two different morphologies of LTF are homologous, and they use the analysis to say they are homologous.

      As for where Diapsids come from, I’d say the Araeoscleidae (Petrolacosaurus and kin) probably provide the best idea of the early Diapsid morphology. Brouffia and Paleothyris of the anapsid ancestor from which they came

      PS you often critisise the use of Anapsid as an outdated term in your blog. I think you’ll be hard pressed to find a palaeontologist who uses it as anything other than a morphological discription of the primitve skull condition. I don’t know of anyone who still thinks its a monophyletic group. Its just a quick and easy was of describing a particular skull morpphology

  2. Quick and easy works for me too. Remember, though, the big Kahuna for my results is the diphyletic reptile tree going back nearly to the very first amniote. Also remember, the purported ‘bias’ as it stands was able to nest mesosaurs within Diapsida and Lanthanosuchids, Caseasauria and various millerettids outside of any synapsid-type clade. So, there’s that.

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