First of all
it’s always good to see fresh, exciting material coming out of the Triassic. In this case a 3D preservation of a new type of Drepanosaurus forelimb (Figs. 1-3). but this time retaining only two fingers and digit 2 is quite robust. This specimen GR 737 deserves a new genus, but it was recognized by Pritchard et al. as conspecific with the holotype MCSNB 5728.
Previously (Renesto 1994)
identified the large bone at the elbow of Drepanosaurus as a displaced and flattened ulna while the ulnare and intermedium evidently fused and took the place and shape of the cylindrical, but proximally concave ulna. That interpretation was dismissed with evidence five years ago here by showing that the purported elbow bone was in reality an olecranon sesamoid, similar to that found in sisters Megalancosaurus (Fig. 4 in yellow) and Vallesaurus (Fig. 5). The new specimen does nothing to change that and has a long list of reconstruction and identification problems here solved by DGS (digital graphic segregation) using a computer monitor as a microscope having not seen the actual specimen firsthand.
Pritchard et al. 2016 followed the earlier Renesto mistake. Occam’s razor is once again ignored in favor of bizarre morphologies never before seen. The ulna always has been a cylindrical bone parallel to the radius. The elbow sesamoid is a flattened plate. Moreover, Pritchard et al. were unable to see the fused sutures between the metacarpals and phalanges (Fig. 3) in their specimen. They did not realize that when cylindrical bones, like the ulna, are crushed and scattered they need to be put back together before you add them to a reconstruction (Fig. 3). Massive bones, like digit 2, are never backed up by gracile and perforated never-before-seen structures as Pritchard et al. reconstructed. Finally, bones Pritchard et al. identified as the intermedium and ulnare in Megalancosaurus are actually the intermediaum and pisiform (Fig. 4) based on taxa close to megalancosaurs preserving a complete carpus.
From the Pritchard et al. abstract:
“Along with the crushed type specimen from Italy, these specimens have a flattened, crescent-shaped ulna with a long axis perpendicular to that of the radius and hyperelongate, shaft-like carpal bones contacting the ulna that are proximodistally longer than the radius.”
the keywords ‘sesamoid‘ and ‘olecranon‘ do not appear in this paper. So the authors did not test alternate identities that were presented online 5 years ago. Drepanosaurus is a highly derived drepanosaur and the elbow bone in question is quite large. It is reasonable to look for smaller versions of this olecranon sesamoid in more primitive taxa. And when you look for them, you find them in Megalancosaurus (Fig. 5) and Vallesaurus (Fig. 6).
Pritchard et al.
rely on the interpretation of the apparent elongation of the intermedium and ulnare (a misidentified pisiform) in Megalancosaurus as their transitional stage enabling the three-part forelimb in the Hayden Quarry drepanosaur. The more primitive Hypuronector has an unossified carpus. The more derived Vallesaurus has a poorly ossified carpus and a small elbow sesamoid. The more derived Megalancosaurus has a larger olecranon sesamoid and a more completely ossified carpus (Fig. 4). Not mentioned by the authors, the pes (foot) of Megalancosaurus has an identical elongation of the calcaneum and astragalus (Fig. 7).
Unfortunately, Pritchard et al. have no clue as to what drepanosaurs are.
They report, “Drepanosaurus is a member of Drepanosauromorpha, a group of Triassic reptiles with lizard-like body proportions and elongate, slender digits likely adapted for specialized grasping.” By contrast the large reptile tree nests them as basal Lepidosauriformes, sisters to Jesairosaurus and this clade is a sister to the gliding kuehneosaurs and their immediate arboreal ancestors.
And finally, the link to supplementary material in the paper: http://dx.doi.org/10.1016/j.cub.2016.07.084 is broken and does not connect with it. There are several high-impact, new generation PhDs authoring this paper. Let’s hope next time they test alternate identities, conduct a proper phylogenetic analysis and produce more precise reconstructions.
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