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Bennett—Rhamphorhynchus wings


exhibits replacement of actinofibrils by calcite. At such a rate, we might have to collect ~90 more specimens of Archaeopteryx before finding one with feathers preserved by replacement by calcite.


The Zittel wing’s preservation of the retrophalangeal


wedge is similar to that of the Marsh specimen with impressions of sinuous fibers and a distinct posterior margin proximally. The preservation differs from the Marsh specimen in that the posterior margin behind the wing phalanges forms a distinct step down from the plane of the wedge trace to the strips and grooves of the actinofibril impression. This suggests that the matrix formed of sediments trapped within the cancellous spaces of the wedge is for the most part still adhering to the upper slab and did not break off with the counterpart. However, distally where the wedge trace is truncated, the matrix seems to have split off with counterpart. Preservation of blood vessels as positive impres- sions along the trailing edge, whether superimposed on a faint negative impression of the dorsal surface of actinofibrils or a


faint positive impression of the ventral surface of the actino- fibrils, could have resulted if the ventral epidermis and dermis rotted away before the vessels, which were then pressed into the lower sediments to produce a negative impression upon which the upper slab’s positive impression was formed. Previous authors interpreted the raised longitudinal strips


of the Zittel wing as structural fibers in or on the brachiopata- gium though most did not explicitly state whether they interpreted the strips as actual actinofibrils preserved through permineralization or as positive impressions of actinofibrils. The strips cannot be actual structures, permineralized in place, because as noted by Pennycuick (1988) that is contradicted by the fact that gouges into the specimen do not preserve any evidence of internal structure or permineralized soft tissues, but rather show normal matrix. That fact would not preclude the possibility that the strips were positive impressions of structures pressed into the substrate on which the pterosaur carcass came to rest, but the fact that the Marsh specimen, though preserving positive impressions of the dorsal and ventral surfaces of the patagia, does not preserve raised longitudinal strips demon- strates that that is not the case. Padian and Rayner (1993) interpreted the brachiopatagium


as consisting of dorsal and ventral skins of epidermis and dermis surrounding a common hypodermis core (Fig. 3.2). At a mini- mumthe core would have included blood and lymph vessels and nerves, but might also have included other connective tissues, muscle fibers, and other structures. Padian and Rayner (1993) interpreted actinofibrils as keratinous, and Bennett (2000) con- curred because their pattern is inconsistent with tensile struc- tures and the Vienna Pterodactylus (NHMW 1975/1756/0000) shows that they resisted longitudinal compression, which collagen, elastin, and muscle fibers would not. Their relative resistance to decay, which resulted in preservation by replace- ment by calcite in the Zittel wing, also supports the interpreta- tion that they were keratinous. If actinofibrils were keratinous, they must have been epidermal, which argues against Wellnhofer’s (1987; Fig. 3.1) and Frey et al.’s (2007) inter- pretations of fibers in the middle of the brachiopatagium, and because there is no evidence of two layers of actinofibrils in any specimen of Rhamphorhynchus, the actinofibril layer was presumably part of either the dorsal or ventral epidermis.


861


Padian and Rayner (1993) interpreted actinofibrils as on the ventral epidermis and the retrophalangeal wedge as ventral to the actinofibril layer; however, the fact that the Marsh specimen does not preserve evidence of actinofibrils on the skin’s surface and does provide evidence that the retrophalangeal wedge was internal shows that that cannot be the case. In addition, the fact that the blood vessel traces and the retrophalangeal wedge are preserved as positive impressions that appear to be super- imposed on actinofibril traces around the margins of the dactylopatagium of the Zittel wing shows that actinofibrils could not have been part of the ventral epidermis. Blood vessels would not be superficial to the epidermis, but rather would have been within the common hypodermis, and therefore the fact that they were ventral to the actinofibril layer shows that the actinofibril layer was part of the dorsal epidermis. Areconstructed cross-section of the dactylopatagium based


on the above interpretation of the Zittel wing and Marsh speci- mens is shown in Figure 3.3. The structure is similar to the interpretation of Padian and Rayner (1993) in that it has upper and lower skins of epidermis and dermis surrounding a common hypodermis core, but the actinofibril layer was within the dorsal epidermis and not on its surface and the common hypodermis contained the retrophalangeal wedge in addition to the blood and lymph vessels and nerves needed to support the dermis and epidermis (Fig. 9.2). Whereas Wellnhofer (1975) and Padian and Rayner (1993) thought that raised longitudinal strips were actinofibrils that were ~0.05mm in diameter and spaced up to 0.2mmapart, it is now apparent that the grooves between raised longitudinal strips are negative impressions of broad flat actinofibrils up to 0.2mmwide and the raised longitudinal strips represent narrow bands (~0.05mm wide) of tissue between the actinofibrils. Because actinofibrils were not on the skin’s surface, it is probable that they were basal within the dorsal epidermis and were covered by an unspecialized epidermis that was continuous with the epidermis of the narrow intervening bands between the actinofibrils. The reconstruction presented here is compatible with the


negative impressions of actinofibrils preserved between the raised longitudinal strips of the Zittel wing and its calcitic fragment, compatible with the skin impressions of the dorsal and ventral surfaces of the wings of the Marsh specimen, and compatible with the small section of brachiopatagium lying on top of the medial part of the left wing trace of the Marsh spe- cimen that preserves a positive impression of closely spaced broad actinofibrils exposed when the covering layer of unspe- cialized epidermis was lost (Fig. 8). The reconstruction is also compatible with the findings of Frey et al. (2003), who reported that JME SOS 4784 preserves a dorsal actinofibril layer that would have been part of the dorsal epidermis, an intermediate


layer of fibers that was present only where there were actino- fibrils and would have been collagen fibers within the dorsal dermis, and a ventral blood vessel layer that would have been within the common hypodermis. It is also compatible with the fact that Frey et al. (2003) described actinofibrils as 0.2mm wide, the same as the width of the negative impressions of actinofibrils in the Zittel wing and much greater than the 0.05mm width of the raised longitudinal strips. However, the reconstruction presented here is incompatible with reconstruc- tions (Frey et al., 2003; Frey et al., 2007) based on conflations of


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