Since muscles rarely fossilize and when they do only the major muscles are discernible, reconstructing pterosaur myology (muscle, tendons and ligaments) depends on analogy with living taxa and the correct identification of muscle scars. Papers on musculature in living taxa are also rare. Virginia Abdala, an expert in this niche, sent my papers from 1939 and 1946, along with her own work.
According to the large reptile tree, among living reptiles pterosaurs are most closely related to Sphenodon, a basal lepidosaur, and Varanus (Fig. 1) a living lizard (squamate), so we make comparisons to them. All traditional paleontologists, just so you know, think birds and crocs are the closest living relatives, but then they are not referencing the only study – ever- that gives lizards a fair shake.
While most muscle anchors on these two lepidosaurs are the same, some shifts have taken place on the wrist anchors. In Sphenodon most of the extensor digitorum brevis anchors are on the intermedium, while in Varanus all have shifted to the ulnare. The intermedium, the former anchor, is a vestige in Varanus. Establishing those shifts sets us up for the possibility of shifting wrist extensor anchors in the phylogenetic ancestors of pterosaurs (Fig. 2). This is key, especially when you get to taxa in which the carpals are poorly ossified.
A strange thing happens in Huehuecuetzpalli. Only the ulnare ossifies. The rest of the carpal elements, it appears from the evidence of phylogenetic bracketing (Fig. 2) are undergoing a transformation/migration. Basal tritosaurs. When the carpal elements re-ossify in Cosesaurus the two centralia are missing and a pteroid and preaxial carpal are present on the medial margin of the wrist (Fig. 3).
The shift in the two centralia corresponds to a shift in morphology elsewhere in the anatomy of Cosesaurus, including the development of a pterosaur-like pectoral girdle that enabled flapping. Huehuecuetzpalli does not demonstrate such changes, but it does have proportions similar to those of living lizards capable of bipedal locomotion, such as Chlamydosaurus, the frilled lizard.
No large extensors were anchored to the two centralia in Sphenodon or Varanus. In Sphenodon a small extensor was anchored to the medial centralia, but it attached to digit 1. Any large extensors that might have anchored on the preaxial carpal would have had to migrate there.
More evidence of migration
In many basal tritosaur lepidosaurs more derived than Huehuecuetzpalli many of the carpals are poorly ossified. For instance, Tanystropheus ossifies the radiale, ulnare and distal tarsal 4. However in Dinocephalosaurus, a phylogenetic descendant of Macrocnemus you find a well ossified carpus that demonstrates the migration of the centralia to the medial wrist. In this case (Fig. 6), only one centralia is ossified.
On the other hand, in drepanosaurs (arboreal tritosaurs) basal members have poorly ossified carpals, but Drepanosaurus and Megalancosaurus ossify all the carpals with the exception of the radiale. The intermedium and ulnare become elongated. The centralia of derived drepanosaurs remain in the middle of the wrist.
Tomorrow we’ll see more pterosaur arms and muscles.
Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. Pp. 127–141 in E Buffetaut and DWE Hone eds., Flugsaurier: pterosaur papers in honour of Peter Wellnhofer. Zitteliana, B28.
Haines RW 1939. A revision of the extensor muscles of the forearm in tetrapods. Journal of Anatomy 73:211-233.
Haines RW 1946. A revision of the movements of the forearm in tetrapods. Journal of Anatomy 80: 1-11.
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