Peculiar Limb Exaptation – WP#13

In a typical tetrapod limb, the metapodium (that portion of the limb below the elbow and above the wrist, including the ulna, radius, ulnare, radiale, intermedium, and a few other bones in some animals) is fairly basic. Some animals can drastically change this model.

It is usually arranged with a robust ulna attached to the lateral side of the humerus and in line with the outer digits (almost always the fourth, which is again one of the generally first portions of the limb to ossify after the humerus and ulna, in that order); the radius is medial or anterior to this, is rod-like, and is parallel to the ulna, and attached to the medial [internal] side of the humerus and the medial carpals and metacarpals. Those proximal carpals, the ulnare and radiale, are usually small bones that cap the distal ends of the bones they are named for. But in today’s image, you’ll see an animal whose ulnare is larger than its radius, a very peculiar and novel feature.

Drepanosaurus unguicaudatus, forelimb. Based on MCSNB 5287 (holotype); dotted lines refer to missing bones. A-B, humerus (propodium); A, lateral view, B, anterior view. C-D, metapodium and autopodium; C, metapodium in "anterior" or extensor view; D, metapodium in "medial" view. Abbreviations: H, humerus; MCI-V, metacarpals I-V; R, radius; U, ulna; UL, ulnare; g, glenoid; hc, humeral caput.

Pinna, who described the original material [1,2] first, had a little trouble with the element shown above as “U,” the ulna. He excluded the element from the limb, and put the “UL” element, the ulnare, and the ulna. This was corrected since [3], but it’s telling in the expectations and comprehension that go through an anatomist’s head when viewing such a fossil. And this megalancosaur is a doozy.

[1] Pinna, G. 1980. Drepanosaurus unguicaudatus, nuovo genere e nuova specie di Lepidosauro del trias alpino [Drepanosaurus unguicaudatus, a new genus and new species of lepidosaur from the Alpine Triassic]. Atti della Società Italiana di Scienze Naturali 121:181-192.
[2] Pinna, G. 1984. Osteologia di Drepanosaurus unguicaudatus, Lepidosauro triassico del Sottordine Lacertilia [Osteology of Drepanosaurus unguicaudatus, a Triassic lepidosaur of Suborder Lacertilia]. Atti della Società Italiana di Scienze Naturali 24:7-28.
[3] Renesto, S. 1994. A reinterpretation of the shoulder girdle and anterior limb of Drepanosaurus unguicaudatus (Reptilia, Diapsida). Zoological Journal of the Linnean Society 111:247-264.

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