In 1971 Alexander Sharov described and illustrated the holotype of Sordes pilosus, a Jurassic pterosaur with extensive soft tissue preservation (Fig. 1). Sharov (1971) illustrated a uropatagium spanning the hind limbs and not attached to the tail which was angled off to the left.
Unwin and Bakhurina 1994
Twenty-three years later, Drs. David Unwin and Natasha Bakurina (1994) reported on the fibers embedded in the uropatagium of Sordes, which they described as attached to and controlled by the medially-directed fifth toe. Unfortunately since then another pterosaur with the same sort of uropatagium has not been found. The cartoonish drawing presented by Unwin and Bakhurina (1994) was a disappointment in view of the Sagan Standard, “Extraordinary claims require extraordinary evidence.”
Peters (2002) noticed stratification beyond and in line with the proposed trailing edge membranes of earlier workers, and also noted that the right wing preserved a narrow-aft-of the elbow configuration, contra earlier reports. Noting the medial overlapping of membranes between the ankles, Peters (2000) suggested that twin uropatagia may have slightly overlapped one another. This turned out to be a mistake (see below).
For several decades paleontologists have been perplexed by the concept of a membrane spanning and binding the hind limbs, wondering if the cloaca opened above it or below it, wondering how such a morphology could have evolved, wondering how the hind limbs could have extended laterally with the tension of the membrane constantly pulling them medially. They wondered if the the tail had become detached form the midline of the uropatagium, which had an apparent V-shaped notch midway between the ankles.
Despite these questions and misgivings, the concept of the “single uropatagium” has remained firmly in place and widely accepted to the present day. This has become the traditional hypothesis. You can see recent illustrations of just such a uropatagium here, here, here and a description by Dr. David Hone, “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 could only really sprawl and not walk upright.”
The big problem is, no other pterosaur has a uropatagium. All other specimens that preserve anything like it preserve paired uropatagia, shallower membranes extending behind each hind limb to the tail, with little to no membrane aft of the ankle and no direct connection to pedal digit 5. The pterosaur precursor, Sharovipteryx also had distinct paired uropatagia.
So what’s going on back there in Sordes?
Earlier we looked at the myth of the Sordes uropatagium. Here more detail is offered. The apparent membrane lateral to the left tibia, (directed a little above the ankle actually) is bordered by the displaced and otherwise missing ulna. The ulna is attached to the similarly displaced proximal portion of the left wing membrane, complete with aktinofibrils. Note: the membrane is quite narrow. The wing membrane extends beneath the left ankle, folds between the ankles and the torn tip continues toward the right fifth toe. A small portion of the torn wing membrane appears above the right foot. You can see the in situ specimen in closeup here. Earlier workers assumed that only the margin of the uropatagium was preserved because nothing is found more proximal to the body. Here, what you see is what you get.
Note the continuity of the membrane beneath the left tibia and tail. Note the doubling of the membrane, like a folded, crushed ribbon, creating the illusion of a medial angle between the ankles. Note the presence of aktinofibrils, which otherwise are found only on the wing. Note the pale borders of the actual paired uropatagia behind each knee — as in other pterosaurs and basal fenestrasaurs. Note pedal digit 5 was beneath the metatarsus.
Jumbles of hair lateral to the left ankle have no “core” and no certain origin, but may have come from the left anterior femur along with the drifting of the left ulna and radius and other hairs lateral to the left knee (which has a patella).
A Taphonomic Coincidence
By coincidence the ulna was oriented toward the ankle. By coincidence the membrane folded like a ribbon between the ankles.
The Value of DGS (Digital Graphic Segregation) in Testing Observations
It has often been said that the DGS (aka: Photoshop) method is intrinsically inferior to seeing the specimen first-hand. Well, apparently not in Sordes, nor in Jeholopterus, the IVPP embryo and several other pterosaurs and other reptiles, like Vancleavea in which either more detail was uncovered without seeing the actual fossil and subsequent testing failed to confirm original findings.
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.
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 DM and Bakhurina NN 1994. Sordes pilosus and the nature of the pterosaur flight apparatus. Nature 371: 62-64.