Flipped wingtips – Rio Ptero Symposium

Continuing our look at the Rio Ptero Symposium abstracts…

Hone et al. (2013) proposed a reconstruction of the tiny Rhamphorhynchus specimen, Bellubrunnus with wing phalanx four (m4.4) concave anteriorly — in other words, different from all other pterosaurs (in which m4.4 is either straight or convex anteriorly). Hone et al. proposed that odd reconstruction because that is exactly how the wingtip appears in the fossil (Fig. 1).

In an attempt at explaining away the autapomorphic concave anteriorly curvature, which would have played havoc with the wing membrane, Hone et al. (2013) promoted the idea that the wingtip might straighten out under load, pulled and straightened posteriorly by a stretching wing membrane under tension, the way an archer’s bow bends when the archer pulls back the bow string.

There’s some confusion here,
so let’s take the problem apart with DGS.

Figure 1. Bellubrunnus (center) in situ. The Zittel wing (left) and the WDC specimen (right) showing how the interphalangeal joints are typically pointed posteriorly, toward the membrane. The Bellubrunnus specimen appears to break that model, but note the wings have been torsioned so the dorsal side is shown in green and the ventral side appears in red. Confusing, yes!

Figure 1. Click to enlarge. Bellubrunnus (center) in situ. The Zittel wing (left) and the WDC specimen (right) showing how the interphalangeal joints are typically pointed posteriorly, toward the membrane. The Bellubrunnus specimen appears to break that model (see insets at bottom), but note the right wing has been flipped (ventral surface in red) and the distal half of the left wing has been flipped (ventral surface in red) based on the shape of the wing joints. This makes the wing tips typically convex anteriorly when flipped back to their in vivo orientation. 

Clue #1. The wing tips are not exposed in dorsal view
While the fossil is largely preserved in dorsal view, the left distal elements, m4.3 and m4.4 (in red) have been twisted axially 180 degrees, exposing their ventral surfaces. The right wing is exposed ventrally (in red) in its entirety. We know this because of the shapes of the joints. Examples (Fig. 1) from the Zittel wing and the WDC specimen illustrate what the unrotated wing joints look like. The largest, most pointed parts of the wing joints generally point toward the wing membrane. That’s basic pterosaur morphology. That’s how you can tell dorsal from ventral… most of the time.

The confusion arises
from the proximal process of the left m4.4 which is larger and more pointed — the opposite of most pterosaur wing joints. This is what Hone et al. keyed on. The right wing doesn’t show any expansion of the joint in either direction. So it is oddly uninformative. Basically Hone et al. chose that joint expansion clue on the left m4.4 to decide that joint was on the wing phalanx posterior rim, giving it a concave anterior bow. There’s a certain logic in that that I can appreciate. But then, we have to deal with the extreme oddity of concave wing tips!

Let’s look at some other related wing tips.

Figure 2. M4.4 from another Rhamphorhynchus, showing the joint can expand both anteriorly and posteriorly.

Figure 2. M4.4 from another Rhamphorhynchus, showing the joint can expand both anteriorly and posteriorly.

Other Rhamphorhynchus specimens (Fig. 2) demonstrate that the proximal joint of m4.4 can expand in both directions. This may solve the problem. A reconstruction (Fig. 3) also helps.

Figure 1. A reconstruction of the new pterosaur, Bellubrunnus.

Figure 3. A reconstruction of the new pterosaur, Bellubrunnus with wingtips (m4.4) convex anteriorly as in most other pterosaurs. Otherwise we’ll have problems with the trailing wing membrane.

A sister taxon – Qinglongopterus
The large pterosaur tree nests the small Qinglongopterus as a sister to Bellubrunnus and it also has a double-expanded proximal joint on m4.4 (Fig. 4). So, now, if you’re a paleontologist, you have to decide:

  1. did the preparator knock off the joint expansion on the concave side of the Bellubrunnus left wing during prep? … and also both joint expansions on the right wing?
  2. are the wing tips flipped ventral side up? (See Fig. 1).
  3. did Bellubrunnus have an odd sort of convex anterior wing with the joint expansion anteriorly and only on one wing?
  4. or did Bellubrunnus have a concave anteriorly phalanx, different than all other pterosaurs?

Hone et al. chose #4. Then they had to explain it away but employing the “under tension” hypothesis. Occam’s razor might have helped here. It’s like trying to explain away a folded wing membrane as “membrane shrinkage” during fossilization, which  Elgin, Hone and Frey (2011) did earlier.

Figure 4. Qinglongopterus, the sister to Bellubrunnus, has a  proximal m4.4 that is expanded in both directions. So, it happens to close relatives.

Autapomorphies
As I learned with regard to that funny little “anterior process” on the ilium of Cosesaurus that turned out to be the stem of the prepubis, autapomorphies can turn out to be mistaken interpretations. If you find an autapomorphy, a trait that no other sisters have, like a concave wingtip, it’s best to think through the possibility that you may have made a mistake. Unfortunately, Hone et al. did not explore all the possibilities, nor did they find the same trait on the other wing or other sister taxa. There is something odd about that one joint, but nothing odd about the convex anterior phalanx.

It’s always good to discover things, but potentially embarrassing when someone points out “kind sir, you have it upside down.”

Once pterosaur wings evolved, very little changed about them other than the relative sizes of wing bone elements. An anteriorly concave wing would have been quite a difference — and one that was reversed back again in descendant taxa, as no later Rhamphorhynchus specimens share this trait.

This also hearkens back to Bennett 2008
who claimed the pterosaur wing finger and membrane began as part of a supinated limb (in other words, not facing the substrate) — but lost the forward-pointing claw from the wing finger because it would have pointed forward hooking on nearby tree trunks — and the naturally concave palmar bow of the the terminal phalanx would have had to reverse to convex forward — along with turning the natural flexion of the metacarpophalangeal wing joint to hyper-hyper extension. All this completely imaginary hoo-hah is detailed here.

References
Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. Pp. 127–141 in E Buffetaut and DWE Hone eds., Flugsaurier: pterosaur papers in honour of Peter Wellnhofer. Zitteliana, B28.
Elgin RA, Hone DWE and Frey E 2011. 
The extent of the pterosaur flight membrane. Acta Palaeontologica Polonica 56 (1), 2011: 99-111. doi: 10.4202/app.2009.0145
Hone DWE, Habib MB and Van Rooijen M 2013. Wingtips in Pterosaurs: functional and ecological implications. Rio Ptero Symposium 77-78.

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