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Bennett—Rhamphorhynchus wings


represent the posterolateral end of an anteromedial actinofibril abutting the anteromedial ends of two more posterolateral actinofibrils so as to accommodate the radiating pattern of actinofibrils on the dactylopatagium while limiting the maxi- mum width of individual actinofibrils. However, there is no evidence that such actinofibrils ended adjacent to one another and the instances could also represent the branching of one germinative strip into two. Frey et al. (2003) stated that in JME SOS 4784 actinofibrils bifurcated near the trailing edge but did not provide photographs or diagrams so it is not clear if this is what they were describing. Note that Padian and Rayner (1993) also stated that some structural fibers appeared to bifurcate near the trailing edge of the dactylopatagium, but this was because


they misinterpreted branching blood vessel traces as structural fibers.


Skin surface.—The Marsh specimen preserves positive impressions of the dorsal and ventral surfaces of the dactylo-


patagium that show that the epidermis was smooth except for fine striae that are best observed under low angle illumination. The striae form a posterolaterally radiating pattern that loosely reflects the posterolaterally radiating pattern of the underlying actinofibrils. The sections of epidermis bounded by striae do not appear to have been heavily keratinized. This is shown by the distortions of their pattern associated with the oval depressions in the Marsh specimen’s left wing (Fig. 7.2), and so the surface epidermis seems to have formed a soft, compliant covering of the dorsal and ventral surfaces of the wings. Interestingly, the Marsh specimen seems to have been largely ignored by pterosaur workers because its preservation of traces of a smooth skin with only fine striae differed markedly from the raised longitudinal strips of the Zittel wing and so was deemed less informative.


Undulations and fold lines.—The undulations in the Zittel wing indicate that the dactylopatagium was lax, which is to be expected because the wing was no longer attached to the trunk. The series of undulations behind WP2 and 3 probably resulted because the wingfinger was unloaded and flattened into the plane of the substrate, whereas when spread as in flight the curvature of the wingfinger and tension within the brachiopa- tagium would have taken up the slack, eliminating the undula- tions. Similarly, the undulations along the trailing edge behind WP1 in the Zittel wing and the left wing of the Marsh specimen would have flattened out when the brachiopatagium was under tension. The sharp edged creases of the Zittel wing, which are also evident in BSP 1907 I 37, indicate that the membrane was somewhat stiff and suggest that it had some natural camber, at least in the distal part of the dactylopatagium. The stiffness of the dactylopatagium is also evident in the deep furrows along the fold lines on the right wing of the Marsh specimen, which because the impression is a positive impression of the ventral surface of the patagium represent places where two sections of the dactylopatagium lying on the substrate resisted compression perpendicular to the long axes of the actinofibrils within the plane of the patagium and were pushed up into ridges along fold lines. It is not clear how extensible Wellnhofer (1975, 1987) thought the brachiopatagium was, but Padian and Rayner (1993,


863


fig. 15A) thought it was significantly extensible perpendicular to the long axes of actinofibrils and Bennett (2000) thought it was as extensible as bat patagium perpendicular to the long axes of actinofibrils within the limits of a load-bearing collagen fiber network. However, the stiffness and resistance to compression perpendicular to the actinofibrils noted above plus the remark- able uniformity of the widths (~0.05mm) of the raised longitudinal strips representing the strips of unspecialized epidermis between the actinofibrils indicate that the dactylopa- tagium was essentially inextensible. A significantly extensible membrane would not exhibit any stiffness, and it is probable that an extensible membrane, even a lax one, would exhibit some variation in the width of the intervening strips. The gentle undulations seem to represent the only significant shortening that could occur in the dactylopatagium. Thus, the dactylopa- tagium was somewhat stiff and inextensible. Because the dactylopatagium was somewhat stiff and


inextensible, it was necessary to fold it in order to store it compactly. The dactylopatagium would have folded up some- what like a traditional folding hand fan consisting of a sector of paper supported by slender slats pivoting around a single point. The Zittel wing, Marsh specimen, and other specimens such as BSP 1907 I 37 exhibit a consistent pattern of prominent fold lines, of which five have been identified by letters. In both specimens described here Fold Line B extends from roughly one-third of the distance along WP1 from its proximal end to the trailing edge behind the mid-point of WP2, Fold Line C extends from roughly the mid-point of WP1 to the trailing edge behind the second IP joint and in part cuts across the regular pattern of raised longitudinal strips in the Zittel wing and the pattern of striae that reflects the pattern of underlying actinofibrils in the Marsh specimen, Fold Line D extends from a point one-fifth along WP2 to the trailing edge behind the mid-point of WP3. and Fold Line E extends from a point three-fifths along WP3 to the trailing edge behind the distal third of WP4. The similarity of the positions and shapes of the folds and the deviations from the pattern of actinofibrils suggest that the folds were not merely the result of incidental folding of a uniform patagium but rather were genetically controlled lines of increased flexibility that enabled the otherwise somewhat stiff dactylopatagium to fold consistently and compactly. It is not clear how the dactylopa- tagium folded up along each of the fold lines, but the folding along Fold Line E of the right wing of the Marsh specimen indicates that the patagium posteromedial to the fold line folded up and over the patagium anterolateral to the line. Presumably there was another fold close behind Fold Line E that brought the patagium down and backward. One reviewer seemed to think that a wing with a somewhat


stiff and inextensible dactylopatagium would present a different appearance when folded as proposed here than a wing with an elasticmembrane bearing widely spaced cylindrical fibers furled by contraction of the membrane, and furthermore suggested that no pterosaur specimen preserves evidence of the proposed fan folding. Bennett (2000; fig. 3) discussed and illustrated the sort of folding proposed here, with the dactylopatagium folded into a narrow band alongside the wingfinger just as is seen in


specimens such as BSP 1938 I 503 (Wellnhofer, 1975, pl. 4, fig. 1). I find that the majority of specimens with soft tissue preservation of wing membranes present an appearance that is


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