Bennett—Rhamphorhynchus wings
ligaments must attach to a small area around the center of joint rotation on the medial and lateral sides of the proximal phalanx, resulting in prominent attachment scars. In all pterosaurs except anurognathids (Bennett, 2001, 2007), the joint surfaces have a large radius and allowed almost no flexion and extension, and so the ligaments need not have attached to a small area but rather could attach to broad areas resulting in less prominent scars. Lastly, dense fibrous connective tissue though strong in tension
is not suited to resist compression, and so a band of dense fibrous connective tissue (or even cartilage) behind the IP joints would not be suited to preventing flexion of the joints. Having rejected other proposed functions of the retro-
phalangeal wedge, only a streamlining function remains. The retrophalangeal wedge would have streamlined the airflow over the wing, reducing drag (Palmer, 2010). Note that the wedge is widest behind the carpus and MCP joint where the wing spar is thickest, and tapers out proximally where streamlining of the elbow might be more difficult because the wedge would be on the extensor side of the joint and perhaps because it would be well behind the leading edge of the wing where streamlining might be less important. It is possible that there was also a streamlining structure medial to the elbow, but if so the Zittel wing does not preserve any trace of it. Various tissues might have formed the retrophalangeal
wedge; however, most pterosaurs, including Rhamphorhynchus (Bonde and Christensen, 2003), exhibit extensive skeletal pneumaticity, which was presumably evolved to displace marrow and lighten the skeleton. Therefore, it is unlikely that the wedge would have been formed of dense connective tissues that would add significantly to the mass of the wing if a lighter alternative were available. It is probable that the wedge was an extra-skeletal pneumatic feature. If, as reconstructed here, the dorsal and ventral surfaces of the wedge were in contact with the dorsal and ventral dermis (Fig. 9.2), then the wedge could be a pneumatic diverticulum bounded by thin epithelia and connected to the intra-skeletal pneumatic spaces. It could deflate and collapse when the wing was folded and inflate when tension in the dactylopa- tagium pulled the dorsal and ventral skins taut. The facts that the Zittel wing preserves a separate fold line in the retrophalangeal wedge and that there are places where raised longitudinal strips can be seen in gaps in the wedge are consistent with the wedge being a pneumatic diverticulum bounded by thin epithelia. It is possible that the unusual wing phalanx cross-sections
seen in Rhamphorhynchus (Fig. 9.2) and Nesodactylus with streamlined anterodorsal and ventral surfaces but markedly concave posterior surfaces reflect the presence of the retro- phalangeal wedge. It is conceivable that other rhamphorhynchoid pterosaurs that did not have such wing phalanx cross-sections did not have pneumatized retrophalangeal wedges behind their wing phalanges. However, it is unlikely that Rhamphorhynchus and Nesodactylus were the only taxa with intrapatagial pneumatized structures to streamline the wing cross-section.
Vascular supply to the brachiopatagium.—Frey et al. (2003) described and illustrated vasculature within the dactylopatagium of Rhamphorhynchus as consisting of one large vessel subparallel to the wing phalanges that gave rise to smaller branches and loops that in turn sent off small branches, and noted that the Zittel wing preserved some vessel traces visible
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under UV illumination. The pattern is similar to that supplying the small intestine in humans, in which a large superior mesenteric artery gives rise to multiple smaller intestinal arteries interconnected by looping arcades that in turn send off still smaller vasa recta, and the positive impressions of the small vessels along the trailing edge correspond to the vasa recta. Note that if the Zittel wing originally preserved physical traces of the large vessel and loops in addition to those visible under UV illumination, they would have been ventral to the actinofibril layer and lost when the counterpart was chipped off. There is no evidence that the dactylopatagium contained
any muscle tissues and no reason to think that it did. Rather the dactylopatagium seems to have consisted of little more than two layers of skin and only enough hypodermis containing vasculature and nerves needed to support those two layers. Therefore, the dactylopatagium probably was rather inactive metabolically, such that it is unlikely to have required high levels of perfusion to supply its tissues with oxygen and nutrients. The presence of large vessels suggests that the patagium was important in thermoregulation (Frey et al., 2007), in which case it could have been used to lose heat by radiation and convection or absorb heat for warming after a night’s cooling. Note that it is not clear whether the vessel traces of JME SOS 4784, NHMW 1998z0077/0001, and the Zittel wing represent arteries, veins, or artery-vein pairs, and so there is no evidence of separation of the arterial and venous supplies, which would be necessary to prevent countercurrent heat exchange if the dactylopatagium was to be used for thermoregulation.
Plagiopatagium.—The Zittel wing presents a plagiopatagium that may not be complete, but certainly is contracted considerably from its extent in flight. That contraction is indirect evidence that the plagiopatagium was extensible such that it would have behaved much as bat patagium does, extending spanwise and chordwise when appropriately tensed and con- tracting so as to be stored compactly when relaxed. The Zittel wing provides no direct evidence as to the structure of the plagiopatagium except for the fine lineations that are parallel to the trailing edge. The fine lineations are smaller than the raised longitudinal strips associated with actinofibrils and their orien- tation is inconsistent with keratinous elements that would resist longitudinal compression. Rather their orientation is consistent with collagen and/or elastin fibers that would bear tensile loads within the patagium. The Marsh specimen provides no information about the plagiopatagium except for the indirect evidence that there are essentially no traces of the plagiopata- gium despite well-preserved traces of the brachiopatagia and the tail vane, which suggests that the dactylopatagium was a soft tissue that was not more resistant to decay than most of the body’s soft tissues.
Trailing edge tendon?—Bramwell and Whitfield (1974, p. 543–544) modeled pterosaur wings with and without a collagenous load-bearing trailing tendon and preferred the model without, whereas Pennycuick (1988) suggested that a trailing edge tendon was present. Padian and Rayner (1993) argued against a trailing edge tendon on the grounds that there is no evidence for one, that the tip of the dactylopatagium was rounded whereas a load-bearing tendon would follow a straight
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