Shifting extensor anchors in lepidosaurs and pterosaurs

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.

Extensors color coded from Haines 1939 and 1946 in Sphenodon (left) and Varanus (right). Note the shifts in muscle anchors.

Figure 1. Click to enlarge. Extensors color coded from Haines 1939 and 1946 in Sphenodon (left) and Varanus (right). Note the shifts in muscle anchors from the intermedium in Sphenodon to the Ulnare in Varanus, in which the intermedium is greatly reduced. This sets up the hypothesis that muscles and tendons do shift from bone to bone. The long extensor from the humerus (gray) connects only to the proximal metacarpals.

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.

Metacarpal extensors in tritosaurs, fenestrasaurs and pterosaurs.

Figure 2. Click to enlarge. Metacarpal extensors in tritosaurs, fenestrasaurs and pterosaurs. Huehuecuetzpalli ossifies only the ulnare. Cosesaurus ossify the wrist elements and the two centralia migrate to the medial margin where they become known as the pteroid and preaxial carpal. Longisquama and pterosaurs co-ossify the distal carpals. For extinct taxa the metacarpal extensors are hypothetical.

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 origin of the pterosaur pteroid and preaxial carpal from lepidsoaur centralia.

Figure 3. Click to enlarge. The origin of the pterosaur pteroid and preaxial carpal from lepidsoaur centralia.

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.

Digit extensors

Figure 5. Digit extensors anchored between the metacarpals had to migrate or disappear when the metacarpals became appressed. Here they have hypothetically migrate to the dorsal surfaces of the metacarpals.

The carpus of Dinocephalosaurus, a basal tritosaur close to Macrocnemus. Here only one centralia is ossified and it occurs on the medial wrist.

Figure 6. The right carpus dorsal view  of Dinocephalosaurus, a basal tritosaur close to Macrocnemus. Here only one centralia is ossified and it occurs on the medial wrist as a vestige.

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.

References
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.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods
Ichnos, 7: 11-41.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29:1327-1330.
Wild R 1978. Die Flugsaurier (Reptilia, Pterosauria) aus der Oberen Trias von Cene bei Bergamo, Italien. Bolletino della Societa Paleontologica Italiana 17(2): 176–256.
Witton M. 2013. Pterosaurs. Princeton University Press. 291 pages.

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