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Bennett—Rhamphorhynchus wings


p. 308) noted that the surface of the wing impression was different from that of the surrounding matrix and suggested it was covered by a layer of encrusting calcite. Based on my examinations, the Vienna Pterodactylus exhibits closely spaced broad actinofibrils similar to those of the Zittel wing preserved by way of calcification with dendritic deposits of manganese and iron oxides in the intervening strips (Bennett, 2013b); however, a detailed description of the wings of the Vienna Pterodactylus is beyond the scope of this paper and will be undertaken elsewhere. The wings of Sordes pilosus described and illustrated by Unwin and Bakhurina (1994) included long, straight, closely


packed fibers in the dactylopatagium that are clearly actino- fibrils. Their preservation is consistent with the interpretation of actinofibrils as closely spaced broad keratinous structures in a somewhat stiff and inextensible dactylopatagium. Unwin and Bakhurina (1994, fig. 2c) did not provide measurements, but based on their figure the actinofibrils, though somewhat variable in width, were ~0.11mm wide and spaced 6–7 per mm. Sordes had an estimated wingspan of 65cm (Unwin and Bakhurina, 1994), but if scaled up to the size of the Zittel wing the actinofibrils would be ~0.17mm wide and spaced 3.8–4.5 per mm, quite close to the measurements of the Zittel wing’s actinofibrils. Note that advocates of widely spaced cylindrical actinofibrils on or in an extensible elastic patagium might interpret the close spacing of the actinofibrils as evidence of the contraction of the elastic membrane in order to furl the wing; however, it is also consistent with the interpretation presented here, and the actinofibrils are much wider (>2 times absolute width and >3 times relative width) than the Zittel wing’s raised longitudinal strips that some may still wish to interpret as


actinofibrils. Unwin and Bakhurina (1994) did not describe fibers of any sort in the plagiopatagium, but described shorter fibers from the uropatagium, which they interpreted as having


unraveled in some cases. They did not make it clear whether they interpreted the short fibers as homologous with the actinofibrils preserved in the lateral dactylopatagium of Sordes. Bennett (2000) argued that the short fibers were not homologous with the actinofibrils of the dactylopatagium and suggested that they were elastic fibers, but I now wonder if they might not be the branching integumentary structures that Kellner et al. (2010) termed pycnofibers. Kellner et al. (2010) identified two types of fibers in the


patagia of Jeholopterus, and suggested that one and possibly both were actinofibrils. They suggested that actinofibrils could shorten or expand to provide flexibility and that such actinofibrils would not be incompatible with Padian and Rayner’s (1993) load- bearing element interpretation or my spreading element inter- pretation (Bennett, 2000); however, extensible actinofibrils are incompatible with the latter two interpretations. Unfortunately,


Kellner et al. (2010) did not provide illustrations or diagrams of fiber orientation and distribution, and they made no attempt to determine whether one or both of the two fiber types was homologous with the actinofibrils of the Zittel wing, JME SOS 4784 (Frey et al., 2003), or the Vienna Pterodactylus,sotheir description adds little to our knowledge of pterosaur wings. However, based on their description it is probable that the short Type A fibers that were interpreted as actinofibrils are not actinofibrils but rather are collagenous tensile fibers like the


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longitudinal strings Frey et al. (2003) noted in JME SOS 4784, whereas the Type B fibers that were interpreted as possibly also being actinofibrils are keratinous actinofibrils.Note, however, that because anurognathids are the likely sister group to all other pterosaurs and possesswingfinger IP joints capable of flexion and extension unlike all other pterosaurs (Bennett, 2007), it is possible that the structure of the patagium of Jeholopterus and other anurognathids was distinctly different from that of other pterosaurs.


Bennett’s (2000) spreading elements interpretation.—Bennett (2000) accepted that actinofibrils were widely spaced cylind- rical structures on an elastic patagium and argued that they resisted longitudinal compression to prevent chordwise narrowing and redirected spanwise tension anteriorly to reduce loads on the distal phalanges. The present interpretation that the dactylopatagium did not include an extensible elastic membrane that would tend to narrow chordwise when under spanwise tension appears to negate the importance of preventing chord- wise narrowing, but it is the presence of the closely spaced actinofibrils that would have made the dactylopatagium inextensible and determine its chord and area. The proposed function of redirecting spanwise tension from the plagiopata- gium into more chordwise tension on the proximal wing phalanges would still be important for reducing the loads applied to the distal phalanges and allowing slender distal phalanges to bear the loads placed on them. One reviewer simultaneously expressed doubt that kerati-


nous actinofibrils would function to resist compressive loads and acceptance that they would function to redirect tensile loads anteriorly to proximal wing phalanges. Those views are incompatible because actinofibrils could not have redirected tension without resisting longitudinal compression. Bennett (2000) discussed and illustrated this with what were termed strut and cable systems including struts representing actinofibrils. If actinofibrils could not resist any compression, they would when loaded in compression shorten until the lines of tension ran straight between their attachments to the body wall and hindlimb medially and the wing phalanges laterally, and so would not redirect tension at all. If actinofibrils could resist compression, they would not shorten significantly when loaded in compression and the lines of tension between their attachments to the body wall and hindlimb medially and the wing phalanges laterally would be bent by the actinofibrils, and the tension would be redirected to proximal wing phalanges rather than borne on distal ones. Perhaps the reviewer took my statements that actinofibrils would resist compression to suggest that they were subjected to and could resist compressive forces other than or exceeding those that tension within a tensed dactylopatagium would subject them to. However, I have never suggested that the role of keratinous actinofibrils was in resisting the sort of compressive loads that would be placed on them if one were to snatch a Rhamphorhynchus out of the air, put one’sthumb on themidshaft ofWP1 and one’s index andmiddle fingers on the trailing edge behind WP1, and squeeze. In such a situation, the longitudinal compressive forces would exceed those which could be resisted by actinofibril and collagen fiber composite, the actinofibrils would bend, and the plane of the dactylopatagium would wrinkle and fold. However, Bennett (2000) argued that in


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