When Drepanosaurus (Pinna 1980, Figs. 1-3) was first discovered and described, this oddity, metaphorically from the land of Dr. Seuss, presented several never-before-seen morphologies including a hooked tail, humped shoulder, a giant olecranon sesamoid (earlier misidentified as a displaced ulna) and an odd hand with a super-claw on finger two — all on one headless body.
After fielding a question,
I told a reader that I would take another look at the Drepanosaurus hand. I’m glad I did. The prior tracing was not based on DGS techniques or high resolution images. This one (Figs. 1-3) is. Earlier I mistakenly reconstructed ungual 2 extending beyond the others. Now I find that the middle three unguals terminated at about the same line (Fig. 3).
several of the phalanges are apparently fused together. Nevertheless their former joints are still visible and are traced here. The penultimate phalanges are very short, the opposite of most arboreal lizards. The cervicals are also quite short, the opposite of other drepanosaurs.
Why did the phalanges fuse?
Perhaps because that big claw prevented the typical flexion function among phalanx sets. Ungual 2 is so big that several former PILs (now fused phalanges) ran through it.
Like almost all digits, the acted together for grasping. The large size of ungual 2 simply made up for the relative brevity of metacarpal 2 and the proximal phalanges, traits that are plesiomorphic for reptiles. That the extensor surface of the ungual is much larger than the flexor surface suggests that the claws were often held retracted, like cat claws. So these were more like paws, the tendril like arboreal lizard toes. Others have considered drepanosaurs slow movers. I agree.
Unlike earlier chameleon-like hypotheses
for Megalancosaurus, the manual digits of Drepanosaurus appear to have swung through parallel arcs, as in most tetrapods.
the tall narrow claws lie on their sides, as is typical of ungual preservation in crushed fossils. In figure 3, I imagined them in dorsal view, which is the typical presentation of a manus for other tetrapods. Atypically the ungual extends proximally over the penultimate phalanges in dorsal view. So the transparent colors help to visualize this. One can only imagine the size of the extensor tendons on those hands. The flexors were strong too. Don’t let one of these climb on your arm or hand. You might not ever be able to shake it off.
Wikipedia reports that Drepanosaurus nests within the Protorosauria, a terrestrial clade or small to large archosauromorphs. In counterpoint, and with actual phylogenetic testing (not tradition), the large reptile tree nests Drepanosaurus and the drepanosaurs with Jesairosaurus and the Keuhneosaurs at the base of the Lepidosauriformes. This clade was arboreal.
Pinna G 1980. Drepanosaurus unguicaudatus, nuovo genere e nuova specie di Lepidosauro del trias alpino. atti Soc. It. Sc.Nat. 121:181-192.
Pinna G 1986. On Drepanosaurus unguicaudatus, an upper Triassic lepidosaurian from the Italian Alps. Journal of Paleontology 50(5):1127-1132.
Renesto S 1994. The shoulder girdle and anterior limb of Drepanosaurus unguicaudatus (Reptilia, Neodiapsida) from the upper Triassic (Norian of Northern Italy. Zoological Journal of the Linnean Society 111(3):247-264