Origin of the long basipterygoid process of the basisphenoid in pterosaurs

Today we talk about a throat bone, the basisphenoid.

Figure 1. Elongation of the basipterygoid process of the basisphenoid in pterosaur precursor, Cosesaurus.

Figure 1. Elongation of the basipterygoid process of the basisphenoid in pterosaur precursor, Cosesaurus. Also shown is a precursor, Macrocnemus and two derived taxa, Rhamphorhynchus and Anahanguera. 

The basisphenoid bone connect the palate to the braincase with a pair of basipterygoid (bp) processes. In almost all tetrapods the bp processes are short. They might be small and they might be big but they are never long — except in pterosaurs and their precursors among the fenestrasaurs (Fig. 1) and in sauropod dinosaurs. (If I’m forgetting others, please advise.)

Macrocnemus (Fig. 1) has typically short basipterygoid processes. But they get long and gracile in Cosesaurus (Figs. 1, 2) and more so in Rhamphorhynchus. In Anhanguera (Fig. 1) they are fused, splitting only at their base. These bones are usually the last to be reconstructed, unless they’re obvious, as in the examples above.

Basipterygoid process fusion in pterosaurs
had its origin in Haopterus in ornithocheirids. By convergence this also occurs in the big Pterodactylus longicollum and after Germanodactylus rhamphastinus among the sharp-beak pterosaurs.

That’s the needle-like cultriform process of the basisphenoid between the bp processes. Anhanguera seems to have lost or fused its cultriform process. Evidently this happens as fusion process proceeds.

Figure 2. palatal elements in Cosesaurus. Image from Elleberger 1993. Click to enlarge. 

Figure 2. palatal elements in Cosesaurus. Image from Ellenberger 1993. Click to enlarge. Some parts are easy to identify. Others are more difficult. This is not the way Ellenberger identified the elements. And it is another example of DGS. Pink elements are quadrates.

There has not been much research on pterosaur palates and basipterygoid processes.
For a long time it was thought that the maxillary portions of the palate were the palatines (e.g. Wellnhofer 1978, 1991; Bennnett 1991, 2001`). Peters (2000) solved that problem with comparisons to Macrocnemus (Fig. 1). Osi et al (2010) published their “new interpretation,” but also understood they had only confirmed Peters (2000), who also noted that sometimes the palatine and ectopterygoid fuse, as they do in Rhamphorhynchus (Fig. 1). These elements don’t fuse in Dorygnathus, as Osi et al (2010) showed.

The basisphenoids are rarely exposed in pterosaurs so it will take more digging to see how and when they joined in large derived taxa.

I don’t have a ready answer for why the bp processes elongate in Cosesaurus, other than the fact that it was holding its head more erect while walking around bipedally while flapping its forelimbs. The long bp process kept the palate low and the occipital condyle high on the fenestrasaur skull.

Which throat muscles are attached to the bp and cultriform processes? What function do they serve. I don’t know and there’s not much out there on Google, from what I can tell. Any help would be appreciated.

References
Bennett SC 1991. Morphology of the Late Cretaceous Pterosaur Pteranodon and Systematics of the Pterodactyloidea. [Volumes I & II]. Ph.D. thesis, University of Kansas, University Microfilms International/ProQuest.
Bennett SC 2001. The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I. General description of osteology. Palaeontographica, Abteilung A, 260: 1–112. Part II. Functional morphology. Palaeontographica, Abteilung A, 260: 113–153.
Osi A, Prondvai E, Frey E and Pohl B 2010. New Interpretation of the Palate of Pterosaurs. The Anatomical Record 293: 243-258.
Peters D 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Wellnhoffer P 1991.  The Illustrated Encyclopedia of Pterosaurs. London: Salamander. 192 pp.

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3 thoughts on “Origin of the long basipterygoid process of the basisphenoid in pterosaurs

  1. Out of curiosity — do you think pterosaurs [some or all where palate is known] link upper and lower jaw movements [cranial kinesis] comparable to that in birds and lepidosaurs?

  2. Everything I’ve seen shows no cranial kinesis. Everything in the skull appears to be well-knitted together. The loss of the epipterygoid after Huehuecuetzpalli further attests to this. A reduction in skull bones occurs in anurognathids, but the palate appears to become more robust as this takes place.

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