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.
Figure 1. Italian Drepanosaurus at left, compared to the New Mexico drepanosaur at right. Original and corrected identifications are shown at 5 second intervals.
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.
Figure 2. Hayden Quarry drepanosaur as originally presented by Pritchard et al. 2016. No other tetrapod ever had such an arrangement of bones. Compare to figure 3 where the ulna is in its usual position and shape. Also, note the lack of detail presented here despite firsthand access. Finally, note that most scale bars do not scale with one another, nor with the reconstruction. These problems are repaired in figure 3.
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.
Figure 3. The Hayden Quarry drepanosaur as interpreted here with scale bars all to the same scale and the ulna in its usual place with an olecranon sesamoid at the elbow. Note DGS was able to tease out the former metacarpals and carpals here.
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.”
Figure 4. Carpus of Megalancosaurus in situ, in vivo and compared to a relatively close outgroup taxon that preserves carpal elements, Daedalosaurus, a kuehneosaur. Note that bones Pritchard et al. identified as intermedium and ulnare are actually the intermediaum and pisiform. Here the megalancosaur ulnare continue to be located proximal to distal carpals 4 and 5. The medial carpal enlarges to replace the dislocated and elongated intermedium, parallel to an equally elongated pisiform. Finally, note that the artist of Daedalosaurus misidentifies distal carpals 1 and 5. Carpal 1, proximal to the thumb, should always be on the radius side.
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).
Figure 5. The elbow of Megalancosaurus. The perfect alignment of the olecranon sesamoid with the ulna masked the separation of these two bones, which are often fused in taxa with an olecranon process, like the kuehneosaur, Daedalosaurus. Note the ulna no longer articulates with the humerus as in Drepanosaurus.
Figure 6. Vallesaurus forelimb as drawn by Renesto and Binellit 2006.
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).
Figure 7. Megalancosaurus showing the elongation of the astragalus and calcaneum in the pes.
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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