860


Journal of Paleontology 89(5):845–869


Solnhofen Limestone.” Therefore I offer the following spec- ulations as to how some specific features of the Zittel wing and Marsh specimen might have been preserved. The wing impres- sions of the Zittel wing and Marsh specimen were both covered by a thin layer of matrix that formed a counterpart that was chipped away to fully expose the impressions on the upper slabs. The counterpart of the Marsh specimen seems to have come off in many small pieces as shown by the large number and rather close spacing of the gouges (Fig. 2). Whereas the main impressions of both wings of the Marsh specimen preserve positive impressions of smooth epidermis and wrinkles that were impressed into the sediments that formed lower (i.e., counterpart) slab, the two areas on the left wing that differ from the smooth epidermis require more complicated explanations. In the case of the impression interpreted as the retrophalangeal wedge, it presents the appearance of loose sinuous fibers like those of the retrophalangeal wedge trace of the Zittel wing and is separated from the adjacent epidermis impression by a distinct groove. The lateral part of the wedge trace consists entirely of the sinuous fiber impressions whereas in the medial part the sinuous fiber impressions are superimposed on a pattern of posterolaterally oriented striae. The superimposition might be interpreted as evidence that the retrophalangeal wedge and its sinuous fibers were a distinct structure that in life lay on top of the epidermis behind the wing spar and so was pressed into the bottom sediments that took the negative impression of the wing; however, that seems unlikely. An alternative interpretation relies on the probability that the anterior part of the dactylopa- tagium that included the retrophalangeal wedge was more substantial than the posterior part and so took longer to decay, and the fact that the wedge is interpreted below as a cancellous pneumatic structure that could have trapped and molded sediments as it decayed. The trapped and molded sediments would have lain on top of the negative impression of the epi- dermis in the bottom sediments, and as covering sediments formed the upper slab and took the positive impression of the epidermis they would also have taken an impression of the sediments trapped and molded by retrophalangeal wedge. As such, the impressions of the sinuous fibers of the retro- phalangeal wedge on the upper slab would seem to be negative impressions of the sediments trapped within and molded by the decaying retrophalangeal wedge tissues but not necessarily positive impressions of the wedge’s internal structure. The lateral end of the wedge trace ends abruptly behind the proximal quarter of WP2, perhaps because sediments did not get into the wedge distal to that point. Note that the narrow fillet of matrix with a fractured surface behind WP2 and the proximal third of WP3 of the right wing may also consist of sediments that were trapped within the retrophalangeal wedge. The small section of soft tissue trace that appears to be


lying on top of the main impression near the medial edge of the left dactylopatagium indicates that at least that part of the soft tissue impression consisted of more than one layer. This could have occurred if the medial part of the patagium was folded, perhaps along Fold Line A, such that the lateral plagiopatagium overlapped the medial dactylopatagium lateral to Fold Line A and sediments got in between the two layers of patagium before preservation so as to form a thin layer of sediment lying on top of the main impression and preserving an impression of the


folded layer. When the counterpart was split off the part slab, the small section of sediment bearing the impression of the folded patagium adhered to the main slab rather than splitting off with the counterpart. The small section appears to preserve a positive impression of closely spaced broad slightly convex structures separated by narrow grooves in its lateral part and smoother surface epidermis in its medial part. The positive impression of broad convex structures compares well with, and is complementary to, the negative impression of closely spaced broad actinofibrils in the Zittel wing, and so it is interpreted as a positive impression of actinofibrils. In the context of the


reconstruction of the wing presented below, in which actino- fibrils were not on the wing surface but rather were covered by a thin layer of surface epidermis, it seems that the part of the surface epidermis that covered the actinofibrils was ripped off or fell off or rotted away so as to expose the actinofibrils before the small section’s impression formed. In the case of the Zittel wing, the counterpart seems to have


come off in larger pieces, perhaps as a result of greater preparation skill on the part of the finder. According to Zittel (1882; quoted above), the finder removed all of the counterpart to fully expose the impression, producing gouges and tool marks and making the specimen more attractive but less scien- tifically informative. Removal of the counterpart could have exposed the layer of actinofibrils preserved by replacement by calcite in the central area of the dactylopatagium. It is probable that chipping off the counterpart would have damaged the dull, pale calcite layer in places and exposed the remarkable pattern of raised longitudinal strips on the underlying matrix, in which case the finder would have prepared away the rest of the damaged calcite layer in order to expose the entire pattern of strips, thereby increasing the specimen’s visual appeal and monetary value. Alternatively, much of that layer might have adhered to the counterpart and come away with it as the counterpart was chipped off, in which case the finder would have removed whatever of the calcite layer remained on the slab so as to increase the specimen’s visual appeal and monetary value. Whichever happened, it is remarkable that the tiny calcitic fragment was left to demonstrate that soft tissues pre- served by replacement by calcite had once covered much of the impression; however, it was probably overlooked because it was very short and in a relatively narrow groove. Preservation of resistant structures by replacement by


calcite is rare in the Solnhofen Limestone except in the case of cartilage in joint capsules, and so one might ask why keratinous actinofibrils were preserved in the Zittel wing when no kerati- nous feathers have been so preserved in any specimen of Archaeopteryx. I expect that the exact set of circumstances and conditions that permitted such replacement by calcite was rare. Wanderer (1908) described the Dresden specimen (SNSD-MMG BaJ 2210) as agreeing with the Zittel wing in all details, but based on my examinations although the specimen does preserve traces of actinofibrils from the medialmost dactylopatagium, the preservation is quite different in manner and quality from the preservation of the both Zittel wing and Marsh specimen (description of the Dresden specimen may be undertaken elsewhere). Thus the Zittel wing is the only 1(<1%) out of the ~108 Rhamphorhynchus specimens in museum collections monographed by Wellnhofer (1975) that


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