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862


Journal of Paleontology 89(5):845–869


Frey et al.‘s (2003) evidence with the interpretation of Martill and Unwin (1989). The layer of collagen fibers described by Frey et al. (2003) seems to be arranged as Bennett (2000) sug- gested collagen fibers would be to redirect spanwise tension in the more medial parts of the brachiopatagium anteriorly to the


basale would have been high enough to produce a surface epidermis that could have been shed or lost through abrasion. In Figure 3.3, actinofibrils are shown as having cross-


proximal wing phalanges. Thus, the actinofibril and collagen fiber layers were within the dorsal epidermis and dermis, respectively, and together they formed a dorsal skin that was the primary functional and structural part of the dactylopatagium, whereas the ventral epidermis and dermis were presumably unspecialized.


Actinofibrils.—Actinofibrils probably were modified epidermal scales that formed in place as part of the dorsal epidermis of the dactylopatagium. In order for this to occur, the stratum basale of the dorsal epidermis of the dactylopatagium would have been divided into broad germinative strips of fibril keratinocytes separated by narrow intervening strips of unspecialized keratinocytes, with the fibril keratinocytes synthesizing and accumulating hard keratin such as is found in claw sheathes and the unspecialized keratinocytes synthesizing and accumulating the softer keratins of unspecialized epidermis. An initial embryonic layer of fibril keratinocytes probably formed during the late embryonic development of the individual, and as the individual grew the fibril germinative strip increased in length and width. The added area of fibril keratinocytes might have


been symmetrically distributed around the previous layer of fibril keratinocytes such that the thickest part was in the center of the actinofibril. However, it is also possible that the added keratinocyte area was not symmetrically distributed in much the same way that the development of the dermal shields on the carapace of box turtles (Terrapene; Zangerl, 1969) and the plastron of mud turtles (Kinosternon; Mosimann, 1956) is asymmetrical such that the embryonic shield is toward one edge of the dermal shield. If the added area was biased toward the posterolateral end of the germinative strip, then the thickest part of the actinofibril would be toward the anteromedial end of the germinative strip, which would be consistent with the fact that in the Zittel wing the raised longitudinal strips are tall and sharply defined in the more anterior and medial parts of the dactylopa- tagium and indistinct to absent near the trailing edge and wing tip, and the fact that thicker anteromedial ends would be better suited to the function of resisting compressive loads and redirecting tensile loads to the proximal wing phalanges. Actinofibrils formed deep in the epidermis would not have


been shed and replaced during the lifetime of the individual, so the rate of cell division in the stratum basale of the fibril germinative strip need only have been high enough to keep up with the growth of the individual and produce a sufficiently stiff actinofibril. The layer of unspecialized surface epidermis that covered over the actinofibrils might have been formed by having the stratum basale of the intervening strips spread laterally over the actinofibrils and merge with that of adjacent intervening strips to cover the actinofibrils and form a continuous sheet of unspecialized stratum basale that produced the surface epidermis or by having the developing keratinocytes from the intervening strips spread laterally and merge to cover the actinofibrils and form a continuous sheet of surface epidermis. The rate of cell division in the unspecialized stratum


sections equivalent to a plano-convex lens. Such a cross-section would result if the actinofibrils were built up through the addition of multiple planar layers of keratinocytes produced by a planar germinative strip as the animal grew from embryo to adult. The cross-section could also have approximated a biconvex lens or a plano-convex lens with a flat external surface if the germinative strip was concave rather than planar, or could conceivably have taken other shapes if the rate of addition of keratinocytes was not uniform across the width of the fibril germinative strip. Note that the gabled roof appearance of the calcitic fragment suggests that the germinative strip was V-shaped. The widths and lengths of actinofibrils are variable across


the dactylopatagium. In the central area posterior to WP2 and 3, actinofibrils were ~0.2mm wide, whereas anteriorly, close behind the wing phalanges they were narrower as they also seem to be in the more medial parts of the dactylopatagium posterior to WP1. In the medialmost part of the dactylopatagium and in the adjacent lateral plagiopatagium the raised longitudinal strips are short, closely spaced, and less regular than elsewhere, which suggests that the actinofibrils were shorter and narrower than elsewhere. However, it is possible that that appearance is partly a result of the manner of preservation of the patagium, such that the actinofibrils may have been longer and more regularly arranged than the appearance of the raised strips suggests. Note that there may have been a limit to the permissible width of actinofibrils if the unspecialized stratum basale or the develop- ing keratinocytes of the adjacent intervening strips were to merge and form a continuous sheet of surface epidermis covering the actinofibrils. In the middle of the dactylopatagium behind WP2 and 3 actinofibrils seem to be quite long and straight. A special case of varying width seems to be found in the actinofibrils that are adjacent to the prominent fold lines in the dactylopatagium. In the case of Fold Line C, the actinofibrils immediately on either side of the fold line are narrow and their raised longitudinal strips less prominent than most strips whereas the actinofibrils a short distance on either side of the fold line are of normal breadth and prominence. While mapping the raised longitudinal strips of the Zittel


wing I did not see any evidence that the grooves between them, and thus the actinofibrils that had occupied the grooves, were not continuous longitudinally. The only possible evidence of discontinuity was in places where a raised strip seemed to jog to one side; however, that might merely represent a local change in the width of the germinative strip rather than the posterolateral end of an anteromedial actinofibril abutting the anteromedial end of a more posterolateral actinofibril. Thus it is probable that individual germinative strips and actinofibrils extended unbro- ken from the wing spar to the trailing edge, which in the Zittel wing could be up to 125mm long. It seems that there would be no difficulty and no disadvantage in having such long actinofibrils, though there probably also would be no disadvan- tage to having shorter ones if their ends were staggered. As noted by Padian and Rayner (1993) there are many


places on the Zittel wing where a raised longitudinal strip was intercalated between two adjacent strips. These instances might


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