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864


Journal of Paleontology 89(5):845–869


consistent with both fan folding of a stiff dactylopatagium and contractive furling of an elastic dactylopatagium; however, the right wing of the Marsh specimen presents an appearance that is consistent with fan folding but not consistent with contractive furling (Fig. 6.1). The outer part of the dactylopatagium is folded along Fold Line E such that the edge of the fold rather than the trailing edge of the dactylopatagium forms what looks like the trailing edge of the wing behind the middle third of WP4. I cannot conceive of a manner in which contractive furling could produce such an appearance. It is possible that the idea of genetically controlled fold


lines was arrived at independently by others: Monninger et al. (2010, p. 52) commented in an abstract that actinofibrils were “the guiding structure for formation of ... folds.” However, it is not clear to me whether they meant merely that folds were parallel to the long axes of actinofibrils, which had been noted before (Padian and Rayner, 1993; Bennett, 2000) or if they meant that specific variation in the size and pattern of actinofibrils controlled the location of the folds.


Retrophalangeal wedge.—Previous authors noted the retro- phalangeal wedge immediately behind the wing spar, but interpreted it as only behind the wingfinger (Padian and Rayner, 1993; Tischlinger and Frey, 2010; Monninger et al., 2012) whereas the Zittel wing shows that it extended proximally almost to the elbow, and the wedge’s arcing posteromedial margin on the Marsh specimen also suggests it extended behind the antebrachium. Various functions have been proposed for the retrophalangeal wedge including: (1) streamlining the transition between the thick wing spar and thin brachiopatagium (Padian and Rayner, 1993; Tischlinger and Frey, 2010; Monninger et al., 2012), (2) containing generative tissue that produced the actinofibrils (Padian and Rayner, 1993), (3) anchoring the actinofibrils to the wing spar and transferring lift forces from the actinofibrils to the wing spar (Padian and Rayner, 1993; Tischlinger and Frey, 2010; Monninger et al., 2012), and (4) reinforcing the IP joints and preventing their flexion (Monninger et al., 2012). Faced with the problem of explaining the development


of cylindrical keratinous fibers on the ventral surface of the epidermis, Padian and Rayner (1993) suggested that the retrophalangeal wedge contained the generative tissues from which the fibers grew like fingernails, posterolaterally in a radiating pattern to be abraded away at the trailing edge. This suggestion is not supported because as discussed above actinofibrils developed in place within the epidermis, and so no concentrated mass of generative tissue was necessary. Padian and Rayner (1993) viewed the brachiopatagium as


essentially cantilevered behind the wing spar with the actinofibrils transferring lift forces anteromedially to the wing spar. Such a structure would require that the actinofibrils were anchored to the wing spar in such a way that the lift forces could be transferred to the spar, and Padian and Rayner (1993, fig. 12) suggested that local tension in the wedge was important in the force transference. The suggestion that the wedge was involved in anchoring the actinofibrils and transferring aerodynamic forces to the wing spar is not supported because actinofibrils formed as part of the epidermis would be securely attached to the underlying dermis, which in turn could have been securely


attached to the periosteum of the bones of the wing spar, so no broad wedge of tissue would be necessary to transmit forces. However, a planar rather than interwoven array of actinofibrils alone or as part of composite of actinofibrils and an elastic membrane would not have been stiff enough to transmit lift forces. McGowan (1991) noted that cylindrical actinofibrils 0.05mm in diameter would be too slender to resist bending forces, and though the actual actinofibrils were broader than previously thought, they were probably little thicker and little better suited to resist bending out of the plane of the patagium. This is clearly shown by the presence of the Zittel wing’s undulations and fold lines; a patagium flexible enough to undulate and fold compactly would not be stiff enough to transfer lift forces as Padian and Rayner (1993) proposed. The dactylopatagium must have been tensioned between the plagiopatagium medial to it and the wingfinger anterior to it, and lift forces must have been transferred medially and anteriorly by tension in the collagen fiber layer. Note that other authors who have viewed the dactylopatagium as self- supporting and self-cambering with actinofibrils transmitting lift forces (Schaller, 1985; Frey et al., 2007; Tischlinger and Frey, 2010; Monninger et al., 2012) have not provided evidence or argumentation that actinofibrils were stiff enough to resist bending and transfer lift forces to the wing spar or that the layer of collagen fibers did not transfer such loads to the wing spar and body by tension. Frey et al. (2007) suggested that intrinsic muscle tissue ventral to the actinofibrils contracted to bend the actinofibrils to camber the dactylopatagium, but did not provide any evidence that there were muscle fibers associated with the actinofibrils or collagen fibers. The suggestion that the retrophalangeal wedge consisted of


dense fibrous connective tissue that reinforced the IP joints and prevented them from flexing is not supported because the wedge extended proximally behind the MCP joint of digit IV, because the wedge was as well developed behind the middle of the wing phalanges as behind the IP joints, and because such reinforce- ment was unnecessary. Monninger et al. (2012) stated that there is no evidence of strong IP ligaments, which may be true in the case of Rhamphorhynchus because of the small size and immaturity of most specimens, but Bennett (2000) described prominent IP ligament attachment scars in mature individuals of Pteranodon so there is evidence that pterosaurs had strong IP ligaments in their wingfingers. Even without that evidence, the extant phylogenetic bracket (Witmer, 1995) informs us that pterosaurs had IP ligaments. Pterosaurs, like their extant relatives (e.g., lepidosaurs and birds), exhibit the osteological correlates of synovial joints in their wingfingers (e.g., expanded articular ends with complementary surfaces, different bone texture on the articular ends indicating that they were covered by articular cartilages), and so it can be concluded that the synovial joints of pterosaurs included a fibrous joint capsule with parts thickened into ligaments to hold the bones together and control their movements. Moreover, IP ligament attachment scars on most pterosaur wing phalanges would be relatively smaller and less prominent than those of their non-volant relatives because of the large radius of the joint surfaces and the extremely limited movement allowed by the joint. In the case of a typical ginglymoid IP joint, the joint surfaces have a small radius and allow considerable flexion and extension, and so the IP


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