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Journal of Paleontology 89(5):845–869


aiming for stability. In addition, I used tenopatagium and actinopatagium not as Schaller (1985) had, for types of patagia, but rather for the medial and lateral parts of the brachiopatagium. I accepted Padian and Rayner’s (1993) interpretation that pterosaur patagia consisted of dorsal and ventral skins of epidermis and dermis surrounding a common hypodermis core, and also accepted their interpretation that actinofibrils were on the undersurface of the patagia. I noted that superficial actinofibrils would be epidermal and keratinous, and because there is no evi- dence that they were elastic or that they stretched and contracted I interpreted actinofibrils as formed of a hard keratin that would better resist longitudinal compression. I argued against the previous interpretations of the function of actinofibrils, which I referred to as the stiffening element (Wellnhofer, 1975) and load- bearing element (Padian and Rayner, 1993) interpretations on the grounds that elastic actinofibrils would not stiffen or stabilize the brachiopatagium and keratinous fibers alone would not be stiff enough to resist dorsoventral bending to camber the brachiopata- gium and transfer aerodynamic loads to the wing spar. I presented a new interpretation of their function: actinofibrils prevented narrowing of the patagium under tension and redirected spanwise tension to the proximal wing phalanges, reducing loads on the


distal wing phalanges. I argued that the combination of actino- fibrils and an elastic membrane including collagenous and elastic fibers in the dermis would produce a composite fabric that


transmitted lift forces to the wing spar and body by tension, spread the brachiopatagium chordwise, maintained the chord even as tension increased from aerodynamic loading, and redirected spanwise tension in the brachiopatagium into chordwise loads on the wingfinger. This interpretation, which I called the spreading


element interpretation, explained the pattern of radiating actino- fibrils in the lateral part of the brachiopatagium and their absence from the medial part. Czerkas and Ji (2002) described the rhamphorhynchoid


Pterorhynchus wellnhoferi Czerkas and Ji (2002) from the Haifanggou Formation of Inner Mongolia, which preserves a soft tissue cranial crest and extensive traces of a hairy body covering on the neck and trunk. They claimed that it also pre- served part of the brachiopatagium with actinofibrils; however, the supposed patagium is a lanceolate leaf with parallel venation coincidentally preserved near the skeleton, so the specimen provides no evidence as to pterosaur patagium structure. Frey et al. (2003) described the internal structure of the bra-


chiopatagium based on visible and ultraviolet (UV) light exam- inations of a specimen of Rhamphorhynchusmuensteri, JMESOS 4784, from the Solnhofen Limestone. They identified three dif- ferent layerswithin the patagium: a layer of actinofibrils averaging 0.2mm in diameter, ranging from 30 to 80mm in length, and bifurcating near the trailing edge; a layer consisting of bifurcating andmerging longitudinal strings that lay ventral to the actinofibril layer and crossed the actinofibrils at angles of 30°–90°; and a vascular layer with one large vessel subparallel to the wing pha- langes giving rise to smaller branches and loops. They noted that the large vessel and branches are also preserved on the Vienna specimen of Rhamphorhynchus (NHMW 1998z0077/0001) and that traces of them are visible on the Zittel wing under UV illu- mination. Frey et al. (2003) rejected Kellner’s (1996) criticisms of Martill and Unwin’s (1989) interpretation of the structure of the patagium in the supposed Santana wing specimen, and conflated


Martill andUnwin’s (1989) interpretationwith their own evidence from JME SOS 4784 in order to conclude that pterosaur brachiopatagium consisted of at least five layers: a thin dorsal epidermis, a spongy subdermis, the actinofibril layer, a layer of muscle fibers that presumably included the longitudinal strings, and a ventral vascular layer; although note that neither dorsal dermis nor ventral epidermis and dermis were mentioned. Frey et al. (2007) expanded the reconstruction in a popular article, suggesting that the spongy subdermis was pneumatic and that contraction of the muscle fibers resisted by actinofibrils would have enabled active control of camber. Tischlinger and Frey (2010) described the band of soft


tissue that Padian and Rayner (1993) had noted just behind the wing phalanges of the Zittel wing on two new specimens of Rhamphorhynchus muensteri. They characterized the band of tissue as massive and interpreted it as forming a streamlined transition between the thick wing phalanges and thin patagium, and as representing the:


“insertion tissue for the aktinofibrills [sic], which probably interwove with the connective tissue of the wedge. Thus, the wedge itself effected force transduction between aktinofibrills and the wing skeleton.” (Tischlinger and Frey, 2010, p. 64)


In an extended abstract, Monninger et al. (2012) interpreted the band as dense fibrous connective tissue that reinforced the interphalangeal joints of the wingfinger to keep them fully extended in addition to streamlining the transition between the wing phalanges and patagium. They suggested this was necessary because there was no evidence of strong inter- phalangeal ligaments. They also argued that there was a trailing edge structure that they termed a trailing edge ligament. Kellner et al. (2010) described the patagia of the anur-


ognathid Jeholopterus ningchengensis Kellner et al. (2010) (IVPP V12705) from the Daohugou Beds of Inner Mongolia. They used the term plagiopatagium for the entire membrane between the fore- and hindlimbs, but followed Bennett (2000) in using tenopatagium and actinopatagium for its medial and lat- eral parts. They described three types of fibers: small, closely spaced, parallelly arranged fibers up to 0.1mm thick and 4–8mm long, which they interpreted as actinofibrils; less abundant, larger, and much longer fibers that they suggested might also be actinofibrils; and short branching hair-like struc- tures they termed pycnofibers. They described two layers of actinofibrils close to the wing phalanges with the more dorsal layer subparallel to the wing phalanges and smaller, thinner, and more closely spaced than the other more ventral layer that was oblique to the wing phalanges. They also stated that in places the


specimen had three or more layers of actinofibrils with different fiber orientations, yet seemed to reject the possibility that the appearance of multiple layers resulted from overlapping layers of a folded patagium. Kellner et al. (2010, p. 327) rejected the interpretation that actinofibrils were external and keratinous, instead interpreting them as internal structures that “could


potentially shorten or expand and therefore provide more flexibility to the wing membrane,” and yet suggested that their model was not incompatible with Padian and Rayner’s (1993) load-bearing element interpretation or my spreading element interpretation (Bennett, 2000).


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