Wrist supination/pronation in Megalancosaurus?

Megalancosaurus including the palate, the only palate ever figured for a drepanosaur.

Figure 8. Megalancosaurus including the palate, the only palate ever figured for a drepanosaur.

One of the weirdest of the weird
Megalancosaurus has been studied and published previously (see refs below). A recent addition (Castiello et al. 2016) adds fused clavicles, a saddle-shaped glenoid, a tight connection between the radius and ulna that hindered pronation/suppination (but see below) and hypothetical forelimb muscles to our knowledge of this basal lepidosauriform.

Unfortunately 
the authors only go as far as labeling this taxon a drepanosaur and a drepanosauromorph without further identifying the larger and even larger clades these taxa nest within.

News

  1. “unlike those of other drepanosauromorphs [the clavicles] are fused together and possess a small median process caudally directed so that the whole structure looks similar to the furcula of theropod dinosaurs, especially oviraptorids.”
  2. “The scapular blade reaches the modified, expanded neural spines of the third and fourth dorsal vertebra so that the pectoral girdle formed a solid ring, which would have been very rigid.”
  3. “the glenoid fossa has a saddle-shaped structure and lies on the coracoid”
  4. “paired sternal plates are fused to the coracoids forming a craniocaudally elongate coracosternal complex.”
  5. “the coracosternal complex was more vertically oriented than in previous reconstructions” but as figured for Drepanosaurus and Megalancosaurus (Fig. 1) at ReptileEvolution.com.
  6. Rather than a separate olecranon sesamoid (Figs. 1, 2) that Megalancosaurus and all of its sisters share, the authors report on, “the elongate olecranon process of the ulna.”
  7. Rather than recognizing a bone break in the ulna (Fig. 2), the authors report, “a small notch is present on the medial margin of the ulna distal to the articular surface for the humerus. This notch houses the medial corner of the proximal head of the radius, suggesting that in life, the two bones were firmly connected together at their proximal end, preventing pronation and supination of the forearm.” No other sister taxa or tetrapods have such an ulna notch. Note, the notch is not present in figure 2, but the sesamoid is pretty broken up. These bones are hollow, fragile and crushed. Be careful how you interpret them. Earlier we saw another misinterpretation of a drepanosaur forelimb.
  8. When the authors present a hypothetical forelimb myology they do not present a pertinent actual forelimb myology (Fig. 3) for comparison. Such a comparison helps assure the reader that the myology for Megalancosaurus has not been invented and follows actual patterns and sizes.
Megalacosaurus elbow

Figure x. The break and the broken pieces of the Megalancosaurus ulna are reidentified here. The sesamoid is prominent and crescent-shaped as in Drepanosaurus.

Crushed hollow bones
are sometimes difficult to interpret, as we’ve seen before.

Elbow sesamoid in another specimen of Megalancosaurus, MPUM 8437.

Figure 2. Elbow sesamoid in another specimen of Megalancosaurus, MPUM 8437.

The authors provided a hypothetical myology
which they phylogenetically bracketed by lepidosaurs and crocodilians (which means what??) based on prior pterosaur forelimb myology as imagined by Bennett (2003, 2008). Pterosaurs are unrelated to drepanosaurs. The Bennett pterosaur myology had problems because it located extensors and flexors anterior and posterior to the fore arm, rather than dorsal and ventral (palmar) as in Sphenodon (Fig. 3) the closest living taxon to drepanosaurs AND pterosaurs.

Sphenodon hand muscles

Figure 3 Sphenodon hand muscles. Click to enlarge. These were not referenced in the Castiello et al. study.

It would have been appropriate

  1. to show that the fingers of Megalancosaurus had more phalanges (Fig. 4), as seen in sister taxa and as I see them in Megalancosaurus itself.
  2. to show two versions of the manus, with spread metacarpals (as presented) and another with more closely appressed metacarpals, as in sister taxa, Hypuronector, Vallesaurus, and Drepanosaurus (Fig. 4).
  3. to take a closer look at that ulna notch, knowing that such a notch mechanically weakens the cylinder, is produced by broken bone, and is not repeated in other drepanosaurs.
  4. to take a closer look at that olecranon ‘process’ because sister taxa all have a large sesamoid.
  5. to phylogenetically nest drepanosaurs in order to provide the most accurate myology analogy possible.
The sister taxa of Drepanosaurus

Figure 4. Click to enlarge. The sister taxa of Drepanosaurus all had an olecranon sesamoid. Drepanosaurus simply had a larger one.

The above data
has been online for the past six years. Plenty of time to consider it. No need to cite it.

Pronation/supination
Arboreal taxa in general and distant drepanosauromroph sisters (Palaegama and Jesairosaurus) are able to axially rotate the forearm by at least some degree. Like the human forearm, the radius and ulna in these taxa are separated by a long oval space that enables the radius to axially rotate on the ulna.

By contrast 
the radius and ulna of Hypuronector are appressed (Fig. 4), restricting pronation/ supination. Vallesaurus may have been similar, but taphonomic disarticulation makes it difficult to tell. The forearm was relatively shorter than the humerus. Drepanosaurus had a similar short forearm, but also had a giant elbow sesamoid that essentially extended the humerus, separated the proximal radius and ulna, as in birds, but shifted the radius to the sesamoid, deleting the parallelogram effect — AND likely reducing pronation and supination.

Unlike its sisters, but like humans,
the radius and ulna of Megalancosaurus were slender, elongate and separated by an interosseus space. I don’t see any reason to suggest that pronation and supination were restricted to 0º here, but not nearly to the extent found in humans (Homo), about 180º. The radius in Megalancosaurus still appears to articulate with the humerus and if re-inflated from its crushed state, might be a cylinder with a circular proximal articulation, enabling pronation and supination.

References
Bennett SC 2003. Morphological evolution of the pectoral girdle of pterosaurs: myology and function. In: Buffetaut E, Mazin J-M, editors. Evolution and palaeobiology of pterosaurs. Geol Soc Spec Publ. 217. London (UK): Geological Society of London. p. 191–215.
Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. In: Buffetaut E, Hone DWE, editors. Flugsaurier: pterosaur papers in honour of Peter Wellnhofer. München: Zitteliana. B28. p. 127–141.
Calzavara M, Muscio G and Wild R 1980. Megalancosaurus preonensis n. gen. n. sp., a new reptile from the Norian of Friuli. Gortania 2: 59-64.
Castiello M, Renesto S and Bennett SC 2016. The role of the forelimb in prey capture in the Late Triassic reptile Megalancosaurus (Diapsida, Drepanosauromorpha). Historical Biology DOI: 10.1080/08912963.2015.1107552
Feduccia A and Wild R 1993. Birdlike characters in the Triassic archosaur Megalancosaurus. Natur Wissenschaften 80:564–566.
Geist NR and Feduccia A 2000. Gravity-defying Behaviors: Identifying Models for Protoaves. American Zoologist 4):664-675. online pdf
Renesto S 1994. Megalancosaurus, a possibly arboreal archosauromorph (Reptilia) from the Upper Triassic of Northern Italy. Journal of Vertebrate Paleontology 14(1):38-52.
Renesto S 2000. Bird-like head on a chameleon body: new specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of Northern Italy. Rivista Italiana di Paleontologia e Stratigrafia 106: 157–179.

wiki/Megalancosaurus

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