re: More on the Jeholopterus pycnofibre study

[David Peters <davidrpeters@charter.net>]

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
doi:10.1098/rspb.2009.0846

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