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.
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 taxidermy.net. 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.