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re: More on the Jeholopterus pycnofibre study

A recent paper

The soft tissue of Jeholopterus (Pterosauria,
Anurognathidae, Batrachognathinae)
and the structure of the pterosaur
wing membrane

Alexander W. A. Kellner1,*, Xiaolin Wang2, Helmut Tischlinger3,
Diogenes de Almeida Campos4, David W. E. Hone2 and Xi Meng2

Proc. R. Soc. B (2009) 00, 1–9

came to light. As this is one of my favorite pterosaurs a few comments are offered.

First and foremost, this is an important contribution with lots of great details offered. Now on to the observational and hypothetical nitpiks.

Kellner et al. (2009) report in their description of the plagiopatagium (trailing wing portion from body to wing tip): "Although no distinctive trailing edge is discernible, the wing membrane extends along the body and is connected to the hind limbs, reaching the ankle (Wang et al. 2002)."

Curious comment for several reasons. No distinct trailing edge? Yes and no. Wang et al. 2002 and Peters 2002 both found a trailing edge, as did Kellner et al. (see Figure 1), but only distally. Kellner et al. no doubt refers to the proximal trailing edge as being indistinct. Peters 2002 found more trailing edge because I superimposed one slab tracing on another to connect the elements missing from the opposite slab - demonstrating the value of photographs! Ironically, Kellner et al. Fig. 4b do show the trailing edge as it nears the elbow. Note the hairpin turn in membrane midway between the labels ra/ul and inc. That's the wing trailing edge approaching, but not reaching, the elbow, dividing the actinopatagium (distal wing) from the tenopatagium (proximal fillet). This is key to any discussion of wing shape.

Importantly NONE of these authors illustrate the wing membrane reaching the ankle or in any proximity to the ankle. Why not? Because it cannot be shown. This is a key character widely discussed and an opportunity was missed to show it, if present. The "sketch" by Kellner et al. 2009 shows only a lot of ptero-fur. Peters 2002 found a distinct connection to the mid thigh, as in Pterodactylus and Rhamphorhynchus (and several other taxa). The first line in the conclusion of Kellner et al. discusses this character, so it was important to them. Unfortunately, no presented evidence leaves us all wondering.

Kellner et al. report in their description of the uropatagium ("horizontal stabilizer" membrane trailing from hind limbs): "The shape of the uropatagium cannot be determined owing to the lack of a distinct posterior edge and the medially displaced feet. Two sets of fibres are observed: one running parallel to the longitudinal axis of the body and the second running perpendicular to the tibiae."

Peters 2002 showed for all pterosaurs, the trailing edges of the uropatagia in Jeholopterus (yes there are two, as in Sharovipteryx) nearly meet in a parasagittal midline extending from the ankle (not the tip of digit V) toward the tip of the ischium, but diverge proximally. As Kellner et al. noted, this is more clearly seen in the lower slab. The same situation holds true for Sordes. Two uropatagia, slight overlap. The fibres in Jeholopterus, as in Sharovipteryx, extend at right angles to both hind limb elements, femur and tibia, as Kellner et al. report. They are perpendicular to one another in Jeholopterus because the hind limb elements are perpendicular to one another. In flight the tibia would have extended far beyond 90 degrees from the femur forming the leading edge of the pterosaur horizontal stabilizer, as shown in many of John Conway's renderings. Then the fibres straighten out and run parallel to one another during extension in flight, as in Sharovipteryx. Here again, a reconstruction would have shed light on the situation.

One of the biggest puzzles presented by Jeholopterus is the distal extent of the epidermis/dermis, especially so since the wings and skeletal elements are not disturbed - at all. Kellner et al. give a name for ptero-hair "pycnofibres", meaning dense, bushy. Kellner et al. finish their description of these hairs with the following puzzling statements after stating the pycnofibres are not homologous with mammalian hair.

"The pycnofibres are further formed by smaller fibrils of unknown nature. Possibly, they were mostly composed of keratin-like scales, feathers and the mammalian hair."

I have a feeling that last sentence was intended to be deleted because it doesn't make sense in that just earlier the authors had disconnected pterosaurs from mammals and birds.

I wish Kellner et al would have described and illustrated these pycno- areas further. They needed to be mapped out in a full-page figure. The visible and UV light images indicate that long structures radiating from the torso extend about a torso's width away(!) in several directions. A reconstruction might have produced something akin to a flying fur ball - unless there was a pattern to these and they were restricted to one plane or another, which makes more sense. Areas that were still connected to the torso need to be noted along with any areas apparently sloughed off. What was the actual pattern of these pycno areas? Missed opportunity.

Kellner et al. wrote in their discussion: "Right after the description of J. ningchengensis by Wang et al. (2002), Peters (2002) argued that the wing membrane in the Chinese taxon did not reach the ankle but extended only to the elbow. However, despite the fact that no trailing edge of the posterior portion of the plagiopatagium is clearly discernible, an extensive portion of soft tissue that is attributable to the wing membrane is closely associated with the hind limbs, particularly with the tibiae (figure 3a). Apparently, Peters (2002), who based his studies on photographs, has only identified the limits of the actinopatagium that indeed terminate at the articulation of the humerus with radius and ulna, but the tenopatagium extends up to the ankle."

I'm confused by this. On one hand Kellner et al. is saying the actinopatagium terminates at the elbow, which is in accord with Peters 2002, then reports the tenopatagium extends to the ankle, without showing this. Here a reconstruction would have helped. Or even a dotted line on the tracing. If one extends the wing and hind limb, as in flight, then a taut trailing edge from wingtip to ankle does not go anywhere near the elbow. If instead the taut trailing edge ran from wingtip to a short distance posterior to the elbow, as shown in Peters 2002 and described by Kellner et al. 2009, then there's no possibility of a pre-hindlimb membrane from elbow to ankle that can be pulled taut by extension. And no such pre-hindlimb membrane is indicated. If the standard pterosaur wing, stretched between wingtip and ankle was in place, one would expect to see longer aktinofibres closer to the body. This is not the case as shown by every pterosaur wing. Rather the wing fibres do not deepen proximally - at all.

In bats the wing shape is dynamic and ever-changing, tightening and stretching between the various elements (several fingers and the hind limbs). In birds the wing shape is more constrained by the orientation of the stiff feathers of unyielding length and the bending of the elbow and wrist. The hindlimbs are not involved. Which of these two are pterosaurs more similar to? Certainly the forelimb membrane has the flexibility to extend as a wing and contract to almost nothingness during folding, as in bats, but pterosaurs have only one wing finger and the akinofibres restrict the shape of the wing. If the Zittel wing is correct, as Peters 2002 showed when attached to the rest of a Rhamphorhynchus, AND the various well-preserved Pterodactylus wings are reconstructed correctly, the trailing edge is stretched taut between the wing tip and whatever lies just posterior to the elbow, then the proximal portion extends to the anterior thigh forming a fuselage fillet. There is no way a tenopatagium could extend to the ankle. Let's say it did in a thought experiment. If the pterosaur wing does extend to the ankle and the ankle extends beyond the elbow in flight, then the tenopatagium (proximal patagium) stretches MOST when walking, not flying, because the distance between these elements becomes far greater. Thus the tenopatagium would have been less than taut in flight than in walking. That doesn't make sense.

The solution to all this confusion may lie in a better illustration of the various fibers and membranes and their respective orientations in Jeholopterus. Plus, when an opposing hypothesis, like Peters 2002, is on the table, every aspect of it has to be disassembled and falsified. Experiments and reconstructions have to be replicated with different results. Simple statements without backup evidence leave everyone wondering. I think an opportunity for argument, evidence and clarity was lost here. Sure it's nice to see ungual sheaths on claws. But the real issues were sidestepped. Show the evidence!

Best regards,

David Peters
St. Louis