The Myth of the Pterosaur Uropatagium

Two things helped pterosaurs fly: bones and the various soft tissues the bones supported. Unfortunately, soft tissues were not so well preserved. While pterosaur wings have warranted the lion’s share of attention, relatively little has been said about the membranes pterosaurs carried behind their knees. See Figure 1. And what little that has been reported, unfortunately, is, well, um… based on a single misidentification that has wormed its way into most pterosaur books, reports and artwork. Most pterosaur experts have accepted this misidentification as fact, and have drawn erroneous inferences from this misstep.

Take, for instance, Dr. David Hone’s contribution to Pterosaur.net, “On the ground the ‘rhamphorhynchoids’ were probably pretty poor. Their large rear membrane would have shackled their hindlegs together making walking difficult, and the shape of their hips and upper legs meant that [they] could only really sprawl and not walk upright.”

The “large rear membrane” Hone referred to is the hypothetical “uropatagium,” purportedly a single sheet of skin, etc., spanning the hind legs of basal pterosaurs from the groin to the tips of the bent back lateral toes, but not involving the tail. See Figure 2.

Click to animate. This is the Vienna specimen of Pterodactylus, which preserves twin uropatagia behind the knees.

Figure 1. Click to animate. This is the Vienna specimen of Pterodactylus, which preserves twin uropatagia behind the knees.

Never mind that Sharovipteryx clearly had paired uropatagia (one membrane trailing each hind limb). Nevermind that pterodactyloid-grade pterosaurs (Figure 1) also had paired uropatagia. Nevermind that no other animal ever had such a membrane. Nevermind the evolutionary and embryological implications and consequences. Don’t even ask if the cloaca (common egg and waste exit) was located above or below the apex of this membrane. According to most pterosaur experts, one basal pterosaur “clearly” had a uropatagium… so they all did.

Bats have analogous structures, left and right uropatagia (plural of uropatagium), but their membranes stretch between the ankle and the tail.

In pterosaurs Unwin and Bakhurina (1994) reported the “uropatagium” stretched from lateral toe to lateral toe and these “controlled” the trailing edge of this aerodynamic surface. Not sure how exactly. Not sure what pivoted or extended. That was never detailed. Look down at your own feet and imagine a membrane spanning the gap between your legs, but not from knee to knee — from outer toe to outer toe. That’s what we’re dealing with. The long stiff pterosaur tail had to be free of such a membrane. Otherwise, if embedded, it would have pointed directly into the ground between the ankles.

Uropatagium of Sordes according to Sharov 1971 and Unwin/Bakhurina 1994.

Figure 2. Uropatagium of Sordes according to Sharov 1971 and Unwin/Bakhurina 1994.

The short history of the uropatagium.
The “uropatagium problem” goes back to 1971 when Aleksandr G. Sharov described a well-preserved, small Jurassic pterosaur, which he named Sordes pilosus. It was complete, articulated and only slightly damaged (only the skull appeared to be much the worse for wear). Best of all, the holotype of Sordes preserved soft tissue: wing membranes, hair and an additional, completely unexpected membrane between the hind legs.  See figures 2 and 3.

Fast forward to 1994 and we find Dr. David Unwin and Dr. Natasha Bakhurina reporting on the same specimen in Nature, agreeing with Sharov (1971) that the soft tissue between the legs was a genuine uropatagium and reporting “well preserved wing membranes show that the hind limbs of pterosaurs were intimately involved in the flight apparatus; connected externally to the main wing membrane and internally by a uropatagium, controlled by the fifth toe.”

This false paradigm continues unchecked today.

Currently on Pterosaur.net, contributor Dr. Dave Hone writes: “The key issue here is the uropatagium – with the hindlimbs shackled together in rhamphorhyncoids, they are left with all of their limbs effectively joined together, a wing membrane from finger to ankle, the uropatagium linking the two legs, and then the other wing on the other side.”

On closer examination (using the DGS method), the “uropatagium” turns out to be something else entirely. Turns out Sordes was no different than Sharovipteryx and dozens of other pterosaurs in having two small uropatagia, each one filling the angle behind each thigh, knee and shin. See Figure 3. The other, darker material between the ankles drifted there during taphonomy. Its the proximal part of the left wing.

The myth of the pterosaur uropatagium

Figure 3. Click to enlarge. The Sordes uropatagium is actually displaced wing material carried between the ankles by the displaced radius and ulna.

The famous Sordes holotype specimen is preserved crushed upon its belly. The right wing was virtually pristine, naturally articulated and nicely preserving a narrow wing membrane. The well-defined trailing edge extended from the wing tip up to an acute curve behind the metacarpals then back toward the body, paralleling the ulna to a point aft of the elbow and finally inserting near the middle of the front of the femur (Peters 2002 and Fig. 3).

The proximal left wing of Sordes did not fare so well. The three distal phalanges of the left wing were in their correct place, but the first (proximal) wing phalanx was bent in toward the body, just a short distance from the distal humerus. Sharov (1971) tentatively identified this phalanx as the radius + ulna, probably because it was in the mirror-image position of the right radius + ulna. The left humerus was parallel to the right one, but less well preserved. A complete hand with articulated short fingers was in the area of the supposed skull. Nevermind the skull.  The left radius and ulna were missing! No one ever noticed this before.

I found the radius and ulna. They had drifted back between the left ankle and left wing, along with a certain amount of proximal wing tissue. Some of this folded, spindled and mutilated wing membrane came to rest between the ankles, creating the illusion of the rear margin of a symmetrical “uropatagium. It is easy to see there is no preservation of membrane material in the center of the gap between the legs. Also note that the actual uropatagia have arcing shapes matching those of sister pterosaurs. So, the purported uropatagium, the soft tissue around and between the ankles, turns out to be nothing more than displaced material from the left wing..The sharp lines preserved between the left wing and left ankle drawn by prior workers turn out to be either the radius or the ulna (they are very much alike) with the other forearm bone intersecting it at a slight angle.

I have never seen the fossil itself, only an image on my monitor. Those who viewed the fossil first hand overlooked this data. I also overlooked this data in Peters (2002) when I noted that lines marking “trailing edges” were continued beyond the outline of the specimen, suggesting that they represented geological faults created during the splitting of the plate and counterplate. That still holds true for the right wing. I didn’t realize the left radius and ulna were missing until I undertook a detailed tracing of the area.

So, just like all other pterosaurs and their fenestrasaur sisters, Sordes had twin uropatagia behind its knees. No membrane stretched from ankle to ankle and toe to toe. Rhamphorhynchoids did not have shackled hindlimbs. Nor did the hind leg affect or restrict the movements of the wing because there was no attachment of the wing to the ankle (Peters 2002). Hope this clears things up.

Read more about the evolution of fenestrasaur/pterosaur uropatagia here.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Peters D 2002.
A New Model for the Evolution of the Pterosaur Wing—with a twist. Historical Biology 15:277-301.
Sharov AG 1971. New flying reptiles fro the Mesozoic of Kazakhstan and Kirghizia. – Trudy of the Paleontological Institute, Akademia Nauk, USSR, Moscow, 130: 104–113 [in Russian].
Unwin DM and Bakhurina NN 1994. Sordes pilosus and the nature of the pterosaur flight apparatus. Nature 371: 62-64.

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