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.

The Myth of the Bat Wing Pterosaur

Toy Pteranodon, ca. 1962, from the Marx Company.

Toy Pteranodon, ca. 1962, from the Marx Company.

To paraphrase Dr. Robert T. Bakker in The Dinosaur Heresies: I remember the first time the thought struck me! There’s something very wrong with that toy Pteranodon. It was a Marx toy (Fig. 1) with big, floppy wings that could NOT have been folded away. The wings extended, like bat wings, to the hind limbs, and so had a very deep chord, a shape still in favor by most pterosaur workers. Even as a kid I knew birds and bats could fold their wings close to their bodies, as compact as possible. The Marx Pteranodon looked to be incapable of that. The pterosaurs in the Jurassic Park series suffered from the same deep-chord malady (Fig. 2).

Saggy blanket Pteranodon wing membrane from Jurassic Park 2

Figure 2. The deep chord wing model produces a "sagging blanket" wing membrane whenever the wing is folded. That is NOT reflected in known fossils.

Comparing pterosaurs, bats and birds.

Figure 3. Comparing pterosaurs, bats and birds.

Wings made pterosaurs the first vertebrates to achieve powered flight, but the shape of those wings has been argued about for decades. Most experts (Unwin and Bakhurina 1994; Elgin, Hone and Frey 2011, for instance) follow the traditional bat-wing model (Figure 3), with a deep trailing edge curving back to attach at the ankle. Such a wing is incapable of being folded away without sagging.

Another wing model emerged when Bennett (1987) reported on bicondylar and fused tail bones in Pteranodon that could move only up and down. He proposed a wing model in which trailing edge attached to the tail tip and so could be lifted and depressed like an airplane elevator (Figure 4).

At about the same time, Padian (1987) and Padian and Rayner (1993) argued against the bat-wing model in favor of a more bird-style model by attaching the trailing edge to the middle or posterior of the pelvis (Figure 4) with hind legs tucked in. Ironically, while arguing against the bat-wing paradigm, Padian and Rayner (1993) fell sway to its influence and invented more wing tissue aft of the elbow in their reconstruction than they showed in various fossils (see below).

Padian and Bennett wing models

Figure 4. Various "narrow chord" wing membrane proposals, but note that all, except Peters (2002) include a large amount of material aft of the elbows, as in the bat-wing model.

Here’s the evidence for the traditional bat-wing model.
Elgin, Hone and Frey (2011) documented the latest evidence for pterosaur wing shape. Unfortunately, rather than firmly documenting their conclusions, they could only report, “A review of relevant pterosaur specimens … strongly suggests that the trailing edge of the wing extended down to the lower leg or ankle in all specimens where the brachiopatagium is completely preserved.” Strongly suggests???!!!  I was hoping for something a little more fim. Here’s their evidence:

Darkwing Rhamphorhynchus

Figure 5. Darkwing Rhamphorhynchus traced by Elgin, Hone and Frey (2011). The displaced humerus is highlighted in yellow. Note the trailing edge of the wing is directed toward the elbow.

1. The “darkwing” Rhamphorhynchus was traced “as is” with a continuous tone deemed to represent a wing membrane that attached at mid thigh (Figure 5). Note that the left humerus was disarticulated and displaced posteriorly. Even so, the wing membrane trailing edge was directed precisely at the elbow, as in the Peters (2002) model. The strip of soft tissue extending down the thigh was on a level below the wing membrane, not continuous with it. The Zittel wing specimen of Rhamphorhynchus was not used by Elgin, Hone and Frey (2011) despite preserving a exquisite wing membrane that was extremely short at the elbow, supporting the Peters (2002) model.

Jeholopterus wing and soft tissue traced

Figure 6. This Elgin, Hone and Frey (2011) tracing did not distinguish between hairs and wing membranes. It also did not include data from the counterplate.

2. The holotype Jeholopterus specimen (Figure 6) was traced only from the plate with no distinction made between the long pycnofibers (hairs) and wing membrane. An alternative reconstruction is offered here in which the counterplate is also employed and a more precise tracing of the soft tissues are documented separating the narrow wings from the long pycnofibers.

Vienna specimen of Pterodactylus.

Figure 7. Elgin, Hone and Frey excused this narrow-at-the-elbow preservation as the result of "membrane shrinkage."

3. The Vienna specimen of Pterodactylus (Figure 7) was traced accurately, without much membrane posterior to the elbow. Rather than taking this example at face value (and it compares well with the Zittel wing, among others), Elgin, Hone and Frey (2011) invented the taphonomic malady “membrane shrinkage” to explain away the absence of wing membrane needed to fulfill their bat-wing paradigm. Nevermind that there was no “membrane shrinkage” elsewhere on the specimen. The Vienna specimen is animated below. It shows that if one adopts the “what you see is what you get” approach to evidence, the wing membrane extends and folds away very nicely indeed– as is!

Figure 8. Bennett (2007) Anurognathus? with yellow added to show extent of wing membrane when only three wing phalanges are employed.

4. The private specimen of Anurognathus (Bennett 2007a, Figure 8 ) was employed by Elgin, Hone and Frey (2011), but they did not use gray tones to delineate what they considered wing membranes. Rather three lines indicated it. They noted that no membrane attached to the ankle, but failed to note that the trailing edge they drew extended straight to the elbow, as in the Peters (2002) model. The DGS method revealed fourth wing phalanges largely buried in the matrix beneath the left foot and right tibia, revealed here.

Eosipteruswing

Figure 9. Elgin, Hone and Frey (2011) traced the wing membranes of Eosipterus, noting that no membranes were present lateral to the tibiae. Here a yellow tone completes the membrane patches and produces a complete shape that follows the Zittel wing/Peters (2002) model.

5. Elgin, Hone and Frey (2011) employed Eosipterus (Figure 9) in which they traced patches of wing membrane not realizing that when one completes the puzzle, the results support the Peters (2002) wing shape model. Again, no material was found lateral to the ankles or shins.

6. The best example of the bat-wing model has always been Sordes pilosus (Sharov 1971, Figure 10). This is the only specimen in which wing material was preserved lateral to the ankles–but there’s more to this story! Elgin, Hone and Frey (2011) traced few details and overlooked the displaced radius (or ulna) that formed the trailing edge of their interpretation of the left wing (details here). It was indeed the left wing, but torn, displaced and preserved in such a way that the membrane continued between the ankles where it created the illusion of a uropatagium. Elgin, Hone and Frey also failed to note the continuation of the right wing trailing edge as it headed toward the metacarpophalangeal joint before turning back to the elbow and anterior femur, again as in the Peters (2002) model.

Sordes as traced by Elgin, Hone and Frey 2011

Figure 10. Click to enlarge. Sordes as traced by Elgin, Hone and Frey 2011 on the left. On the right wing they failed to note the continuing trailing edge of the wing membrane that curved toward the elbow. On the left wing they failed to note the displaced radius (or ulna) that produced the illusion of a trailing edge membrane that extended to the ankle and created a false uropatagium between the ankles. These errors are highlighted in yellow, orange and pink on the right hand image.

Elgin, Hone and Frey (2011) made several more mistakes when they created their reconstruction of the pterosaur wing, detailed in Figure 11. These workers, like many others before them, ignored and made excuses for clear “what you see is what you get” evidence supporting the Peters (2002) model in favor of the unsupportable, centuries-old bat-wing paradigm.

Figure 8. Click to enlarge. Problems with the Elgin, Hone and Frey (2011) pterosaur wing model with corrections proposed by Peters (2002).

Figure 11. Click to enlarge. Problems with the Elgin, Hone and Frey (2011) pterosaur wing model with corrections proposed by Peters (2002).

Elgin, Hone and Frey (2011) say these specimens suffer from "membrane shrinkage."

Figure 12. Click to enlarge. Elgin, Hone and Frey (2011) say these specimens suffered from "membrane shrinkage." Actually, as you can see, the wing fingers are twisted on the Rhamphorhynchus such that the membrane was preserved in front of the bones. Overlooked were the proximal membranes which are visible even in this black and white photo. The Jeholopterus? specimen has torn wing membranes. They also did not "shrink."

Now for the heretical view.
Peters (2002) argued for a “what you see is what you get”model and reconstructed a narrow trailing edge essentially stretched between the elbow and wing tip with a short fuselage fillet filling the gap between the elbow and mid thigh (Figures 13, 14). This wing shape is duplicated in every pterosaur in which wing material is known. Note how well it folds up to prevent damage. A deep-chord wing (at the elbow) cannot do that. Note that phalanx four either curves or is bent posteriorly, matching the curvature of the trailing edge of the tip caused by aktinofibrils separating like strips on a bamboo fan, stretching membrane between them. Note that the wing is aerodynamically taut only at full extension and that tension runs between the elbow and wing tip. The hind limb is independent.

Pterosaurs originated as flapping leapers, not as gliders. Those wings developed distally first, not as gliding membranes close to the body (contra Bennett 2008).

Figure 13. Click to animate. The Vienna specimen of Pterodactylus (wings folded). Animation opens the wings and legs to reveal the true shape of pterosaur wings, stretched between the elbow and wingtip with a short fuselage fillet extending from elbow to mid femur.

Arthurdactylus

Figure 14. Click to enlarge. The left wing of the pterosaur Arthurdactylus in dorsal view based on the Peters (2002) bird-wing model.

The bird-wing model for pterosaur wings reaches its acme in ornithocheirids like Arthurdactylus (Figure 14.) Not only does this wing shape create a nice airfoil like that of a sailplane, but it folds away to virtual invisibility!

There never has been evidence for any membrane lateral to the tibia. Please send any, if found. All available evidence supports the bird-wing model.

(There were also earlier rumblings about the correct orientation of the pteroid: forward and variable (Frey) vs. medial and fixed (Bennett 2007b, Peters 2009). Medial and fixed is correct, but we’ll take a look at that issue later.)

As always, I encourage readers to see the 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:
Bennett SC 1987. New evidence on the tail of the pterosaur Pteranodon (Archosauria: Pterosauria). Pp. 18-23 Currie, PJ and Koster EH, eds. Fourth Symposium on Mesozoic Terrestrial Ecosystems, Short Papers. Occasional Papers of the Tyrrell Museum of Paleontology, #3.
Bennett SC 2007a. A second specimen of the pterosaur Anurognathus ammoni. Paläontologische Zeitschrift 81(4):376-398.
Bennett SC 2007b. Articulation and Function of the Pteroid Bone of Pterosaurs. Journal of Vertebrate Paleontology 27(4):881–891.
Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. Pp. 127–141 in E. Buffetaut & D.W.E. 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 online pdf
Padian K 1987. The case of the bat-winged pterosaur. In: Czerkas, S.J. and Olson, E.C., eds, Dinosaurs Past and Present (Natural History Museum of Los Angeles County/ University of Washington Press, California/Seattle) Vol. II, pp. 65–81.
Padian K and RaynerJV 1993. The wings of pterosaurs. American Journal of Science 239-A, 91–166.
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.