Another lizard with an antorbital fenestra!

Earlier we looked at the nesting of pterosaurs within a third clade of lepidosaurs (lizards), the Tritosauria, outside of the Squamata (Iguania + Scleroglossa). Pterosaurs, as everyone knows, have an antorbital fenestra. That’s the principal reason why most pterosaur workers try to force fit them into the Archosauria.

The frilled lizard, Chlamydosaurus kingii, is famous for many things: bipedal walking, frilled neck skin and that cantankerous attitude. Now let’s add: antorbital fenestra (Fig.1). This is the only living lizard that I know of (there may be more!) that has an antorbital fenestra. That makes six non-homologous appearances in the Reptilia. Here and here are the other five.

Figure 1. Frilled lizard skeleton. Note the small skull opening between the naris and orbit. That's an antorbital fenestra.

Figure 1. Frilled lizard skeleton. Click to enlarge. Note the small skull opening between the naris and orbit. That’s an antorbital fenestra. And this one raises the total of distinct non-homologous antorbital fenestra to six and the second among lepidosaurs. Fenestrasaurs were the first. Drepanosaurs may be homologous with Jesairosaurus at the base.

With its bowed hind limbs
The frilled lizard presents a good analog for how bowlegged pterosaurs (chiefly derived forms) would have run bipedally, perhaps prior to flight. This is the first time I’ve seen a skeleton of Chlamydosaurus, having featured this lizard in an early paper (Peters 2000) as an example of a reptile capable of bipedal locomotion, convergent with fenestrasaurs. I am pleased to note the ilium of Chlamydosaurus has a small anterior process (a hallmark of bipedal reptiles, exaggerated in fenestrasaurs, including pterosaurs). The tail, with those deep chevrons and wide transverse processes, would have been more robust than in any fenestrasaur. The closely apprssed tibia and fibula are also cursor traits. The asymmetric foot is no impediment to bipedal locomotion, contra the opinion of many pterosaur workers.

That antorbital fenestra has an unknown (to me) function. If anyone has that data, please let me know.

Added note: Darren Naish was kind enough to refer me to other lizards with this sort of antorbital fenestra, Pogona vitticeps, the bearded lizard is one, and here is a Digimorph link to it. A quick Googling revealed that the Harderian gland is located at the medial corner of the orbit. The lacrimal gland is smaller and appears at the posterior eyelids. According to Wikipedia, “The Harderian gland is a gland found within the eye’s orbit which occurs in tetrapods (reptiles, amphibians, birds and mammals) that possess a nictitating membrane and the fluid it secretes (mucous, serous or lipid) varies between different groups of animals.”

And how about that retroarticular process~! Perhaps related to the neck frill. I understand not all of the bones of the frill are included here.

The frilled lizard can be seen in action here on YouTube.

More on bipedal pterosaur tracks here.

This image (Fig. 1) comes from There are several more images of Chlamydosaurus from other angles there.

Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.

8 thoughts on “Another lizard with an antorbital fenestra!

  1. Firstly, let’s say that this _is_ an antorbital fenestra homolog — how would it be relevant to your hypothesised (and, I’m sure, erroneous) nesting of Pterosauria within Squamata? Wouldn’t it be an irrelevant example of convergence? I mean, you’re not nesting pterosaurs within Agamidae, are you? Secondly, an opening in this position is seen in quite a few lizards and is especially prominent in other agamids (there’s a really big one in Pogona as well): I’m pretty sure it’s the opening for the lacrimal gland. Therefore, nothing to do with the antorbital fenestra, since animals with an antorbital fenestra also have a lacrimal gland.

    • Indeed it is an irrelevant example of convergence with pterosaurs. Thank you for the reference to other lizards, like the Bearded Dragon. For other readers, here it is:

      The only relevance might be that not only archosaurs have that antorbital fenestra, but that convergence spreads that trait fairly widely among the Reptilia. And that gives us even more incentive to consider pterosaurs as lepidosaurs.

      • “The only relevance might be that not only archosaurs have that antorbital fenestra, but that convergence spreads that trait fairly widely among the Reptilia. And that gives us even more incentive to consider pterosaurs as lepidosaurs.”

        I think you are missing the point. The point is not that Pterosaurs have an opening in the skull that is roughly in the same general location as the antorbital fenestra of archosaurs, and therefore could have evolved the feature convergently. The point is that Pterosaurs have an opening, formed by the same skull bones, in the same patterns, and is logically presumed to have the same function (cranial sinuses) as archosaurs. Thats multiple levels of convergences, in just one feature (out of the hundreds of other characters) that you have to explain away to pull Pterosaurs out of archosauria.

  2. To Chris: Pterosaurs: no fossa. Fenestrasaurs, no fossa. Chroniosuchus, no fossa. Parasuchians, fossa. Jaxtasuchus, no fossa. Same skull bones in each case. Explain away hundreds of features? Is this opposite day? Haven’t you been reading this blog? Let me remind you that pterosaurs nest with turtles and sauropterygians before they nest with archosaurs, given the opportunity to do so.

    To Darren: The other comment may still be on its way. It has not come through yet.

  3. I wrote a long-ish comment on the skull opening you’re referring to in iguanians. I’ll say again: you’re looking at an enlarged lacrimal foramen here, a character sometimes mooted as a synapomorphy of Acrodonta.

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