The evolution of the Sordes wing and uropatagia: 1971 to 2011

When insect-specialist, A. Sharov (1971),
first traced the wings of the Sordes holotype (PIN 2585/3) he considered every soft tissue on the specimen some sort of wing membrane with little to no taphonomic displacement (Fig. 1c). Sharov added a skull where there should have been one. He attached the wings to the ankles and considered a V-shaped dark patch the trailing edge of a single uropatagium stretched between the hind limbs (detached from the tail, framed and controlled at least in part by pedal digit 5). Such a structure had never been seen before on any pterosaur.

Sharov also traced large puffs of soft tissue anterior to the right free fingers and lateral to the left ankle (Fig. 1c). These have been ignored ever since.

Unwin and Bakhurina (1994)
echoed Sharov’s observations in an international publication, Nature (Fig. 1d), focusing on the membrane attachment to the ankles and the uropatagium stretching between them and the lateral toes, already extremely bent as in most basal pterosaurs. Unfortunately, less data, rather than more, was presented. And pterosaur workers were left scratching their heads. How does an embryo develop a membrane between the two hind limbs and not attach to the tail (as in bats)? Was the cloaca above or below this membrane? How could a pterosaur walk bound like this? How could it fly bound like this?

Figure 4. Sordes wing drift hypothesis from Peters (2002) which attempted to show that the wings and uropatagia of Sordes were more like those of other pterosaurs than the other way around. The very deep uropatagia are misinterpretations prior to the realization that the left brachiopatagium (main wing membrane) was displaced to the ankle area.

Figure 1. Sordes wing drift hypothesis from Peters (2002) which attempted to show that the wings and uropatagia of Sordes were more like those of other pterosaurs than the other way around. Unfortunately the very deep and hairy uropatagia are misinterpretations prior to the realization that the left brachiopatagium (main wing membrane) was displaced to the ankle area (Figure 3). Click to enlarge.

Peters (1995)
suggested that some elements of Sordes had taphonomically drifted. Moving the elements back to their purported in vivo positions created a morphology more like that of all other pterosaurs. Unfortunately some identification mistakes were made due to inexperience and working with low resolution second hand data, but further work demonstrates the basic idea of taphonomic shifting was sound.

Figure 1. Sordes interpreted by me in Nature 1995. At that time. remember, no one had ever heard of a uropatagium. And the purported "fact" that this flap of skin spanned the hind limbs without connecting to the tail seemed pretty hard to swallow for a majority of paleontologists. The only images available were small and indistinct. Even so, an attempt was made here to understand the taphonomy of the specimen and how it came to sport such a strange autapomorphy that has not been seen since on any pterosaur fossil. Despite the sincerity of this effort, it includes several mistakes rectified now in ReptileEvolution.com/sordes.htm after publication of the specimen in a larger format with higher resolution.

Figure 2. Sordes reinterpreted by Peters (1995). Since no other pterosaur had such a wing and uropatagium plan as Sharov had proposed and Unwin and Bakhurina echoed, I assumed the parts had taphonomically drifted from their in vivo positions. I was largely correct but some mistake were made. The V-shaped patch between the ankles actually is part of the left wing, not the right one, which is undisturbed in the fossil. And what everyone thought was the skull is actually the left hand and m4.1.

Peters (2002)
refigured the Sordes wing (Fig. 1a, b, e), this time attributing certain odd alignments to matrix planes and taphonomic shifting. This was part of a larger study examining a wide range of pterosaur wing plans. Unfortunately, the uropatagia were again incorrectly interpreted, this time as uropatagia that had overlapped medially during taphonomy. The purported skull was ignored.

ReptileEvolution.com (Peters 2011)
offered a new interpretation that identified bones thought to have been the skull roof as left manual 4.1 (Fig. 3). Bones thought had been considered facial bones were reidentified as the free fingers and metacarpus, somewhat busted up. In one of those wonders of taphonomic displacement, in this fossil it appears that manual 4.1 was joined to the distal left humerus,! This left the radius and ulna unaccounted for. And there’s our solution!

The myth of the pterosaur uropatagium

Figure 3. The Sordes uropatagium is actually displaced wing material towed to the ankles by the displaced radius and ulna.

Impressions of the ulna and radius (click here for rollover image) were identified near the left ankle. These formed the straight lines that were thought by previous authors to be the straight trailing edge of an undamaged wing membrane (Sharov 1971, Unwin and Bakhurina 1994). Unfortunately no prior workers raised the question: how could the trailing edge be taut straight when relaxed?

However, it is interesting to note in figure 1 that the tracings of this area made by Sharov (1971) and Unwin and Bakurina (1994) do not match. What were they each looking at?

In the Sordes holotype, along with the displaced ulna and radius came a large chunk of wing membrane, now detached, folded upon itself like a ribbon and spanning the ankles, giving the illusion of a uropatagium trailing edge. Note that no trace of this purported uropatagium exists close in to the femora. Instead faint impressions of the twin uropatagia, identical to those of other pterosaurs and Sharovipteryx, are faintly preserved behind each knee.

Figure 5. Sordes from Elgin, Hone and Frey 2011. While we were all hoping for more detail, we got less.

Figure 4. Sordes from Elgin, Hone and Frey 2011. While we were all hoping for more detail, we got less.

Elgin, Hone and Frey 2011
published a tracing of Sordes, repeating Sharov’s original interpretation, assuming that little to no taphonomic displacement took place (Fig. 4). Based on the left wing membrane trailing edge, this seems to have been traced from Unwin and Bakhurina 1994 (Fig. 1c, more details and links below) rather than Sharov (1971).

Very few pterosaurs are perfect specimens
Prior workers, even after noticing the back-turned cervicals of Sordes, the lack of a skull with identifiable features and the apparent loss of the left free fingers together with m4.1 (originally misidentified as an in-place radius and ulna, Fig. 1c), did not consider the possibility of taphonomic displacement. Nor did they attempt to more accurately trace the skeleton and soft tissue since 1971(!). Nor did they attempt to reconcile the soft tissues of Sordes with those of other pterosaurs, except when Elgin, Hone and Frey (2011) attempted the exact opposite (details and data here and here) with even less detail to their tracings (Fig. 4) than Unwin and Bakhurina (1994)! They explained away all the other wing differences to fit the Sordes model and NOT with greater precision.

Unfortunately
most pterosaur workers (e.g. Unwin 2005; Elgin, Hone and Frey 2011; Witton 2013) continue to reconstruct their pterosaurs with deep chord wing membranes in blind accord with Sharov (1971) rather than changing the Sordes wing to match that of all other pterosaurs (more here on that topic). They argued that the Zittel wing must have been trimmed or must have extended to the ankles. They argued that the Vienna specimen of Pterodactylus (Fig. 4) likewise must have had wings attached to the ankles, when they clearly do not, and explained away the difference with the term, “shrinkage.”

The Vienna Pterodactylus.

Figure 5. The Vienna Pterodactylus. Click to animate. Wing membranes in situ (when folded) then animated to extend them. Contra Elgin, Hone and Frey (2011) there is no shrinkage here or in ANY pterosaur wing membrane. There is only an “explanation” to maintain the Sordes deep chord wing hypothesis when it is the Sordes wing that needs to conform to this very clear data.

If you think this defies all logic,
I agree with you. And yet, it continues…

References
Elgin RA, Hone DWE and Frey E 2011. The extent of the pterosaur flight membrane. Acta Palaeonntologica Polonica 56(1): 99-111.
Peters, D. 1995. Wing shape in pterosaurs. Nature 374, 315-316.
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 from the Mesozoic of Kazakhstan and Kirghizia. – Transactions of the Paleontological Institute, Akademia Nauk, USSR, Moscow, 130: 104–113 [in Russian].
Unwin, D. M. 2005. The Pterosaurs fromDeep Time. Pi Press, New York, 347 pp.
Unwin DM and Bakhurina NN 1994. Sordes pilosus and the nature of the pterosaur flight apparatus. Nature 371: 62-64.
Witton M. 2013. Pterosaurs. Princeton University Press. 291 pages.

wiki/Sordes

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