Soft-tissue and dermal arrangement in the wing of an Early Cretaceous bird: Implications for the evolution of avian flight

Despite a wealth of fossils of Mesozoic birds revealing evidence of plumage and other soft-tissue structures, the epidermal and dermal anatomy of their wing’s patagia remain largely unknown. We describe a distal forelimb of an enantiornithine bird from the Lower Cretaceous limestones of Las Hoyas, Spain, which reveals the overall morphology of the integument of the wing and other connective structures associated with the insertion of flight feathers. The integumentary anatomy, and myological and arthrological organization of the new fossil is remarkably similar to that of modern birds, in which a system of small muscles, tendons and ligaments attaches to the follicles of the remigial feathers and maintains the functional integrity of the wing during flight. The new fossil documents the oldest known occurrence of connective tissues in association with the flight feathers of birds. Furthermore, the presence of an essentially modern connective arrangement in the wing of enantiornithines supports the interpretation of these primitive birds as competent fliers.

Scientific RepoRts | 5:14864 | DOi: 10.1038/srep14864 digits, are all surrounded by abundant remains of epidermal and dermal connective tissues in close association with plumage. Eight to nine large and strongly asymmetrical primaries, 10-12 secondaries, and the remains of plumulaceous coverts are preserved as carbonized keratinous structures. A halo of brownish-to-yellowish epidermic and dermic tissue outlines the skeletal elements of the wing except for the ungual phalanges of digits I and II and their keratinous sheaths. The connective tissue associated with the plumage shows the same post-mortem folding of the skeletal elements, indicating that these soft tissues are preserved in anatomical position.
Three epidermal structures (i.e., patagia 10 ) are noticeable in MCCMLH31444: the propatagium, the alular patagium, and the postpatagium (Fig. 1). The morphology of the propatagium is remarkably similar to that of modern birds 10,17 , other enantiornithines 7 , and the Chinese Early Cretaceous Confuciusornis sanctus 8 , in which this large epidermal fold connects the wrist to the shoulder (Fig. 1). The propatagium shows a scattered pattern of feather follicles distributed across its entire surface (Suplementary Fig. S1, online). The alular patagium extends from the cranial surface of the midshaft of the metacarpal II to almost the caudodistal end of the first phalanx of digit I (Fig. 1), thus showing a similar distribution to that of most modern birds 10,17 . Particularly remarkable is the presence of an almost completely preserved postpatagium, extending from the caudoproximal base of the ungual phalanx of digit II to the preserved proximal-most caudal portion of the ulna and in clear association with the wing's plumage (Fig. 1). The preserved calami of flight feathers (i.e., primaries and secondaries) are embedded in the postpatagium, forming the same sinuous caudal outline visible in the naked (i.e., unfeathered) wing of modern birds 10,17 (Fig. 1). Also, the reduced digit III appears to be completely lodged in the postpatagium, as in modern birds 10,17 (Fig. 1), thus confirming previous claims that in Enantiornithes this reduced digit was attached to the digit II by soft-tissue structures 18 .
The connective tissue surrounding the proximal-most portion of the calami of the remiges is characterized by a serial striation pattern (Fig. 2). The striations are arranged in discrete and truncated cone-shaped to ribbon-shaped bundles that attach diagonally to the medial portion of the calamus of each feather (Fig. 2). In some of the best-preserved regions, it is possible to identify several striated bundles of fibers that attach to the calamus at different angles and one above the other (Figs 2 and 3A). In the proximal-most region of each bundle-opposite to their feather attachment-the striation lines are more tightly joined together, slightly curving with respect to the main orientation of the bundle (Fig. 2).
SEM inspection reveals that the majority of these striated bundles are composed of plait-like fibers arranged in parallel and alternating with smooth regions of subequal width (here label as Type 1 striated bundles, Fig. 3A,C). The plait-like fibers are made of smaller fibers rolled in a helical pattern (Fig. 3C);  Fig. 2A. Abbreviations: alp, alular patagium; cl, calamus; ks, keratinous sheath; mc-II; metacarpal II; mc-III, metacarpal III; pf, plumulaceous feathers; pr, primary remex; prg, propatagium; psg, postpatagium; r, radius; sr, secondary remex; ul, ulna; I-pI, first phalanx of digit I; I-u, ungual phalanx of digit I; II-pI, first phalanx of digit II; II-pII, second phalanx of digit II; II-u, ungual phalanx of digit II; III-pI, first phalanx of digit III; III-pII, second phalanx of digit III.
Scientific RepoRts | 5:14864 | DOi: 10.1038/srep14864 the size range and arrangement of these smaller structures is congruent with their interpretation as fibrous collagenous fibers 19 . Additionally, we label as Type 2 some of the striated bundles in the postpatagium containing strap-like fibers. These are flat and wider than the plait-like ones, and are patterned with diagonal striations with respect to the main axis of the fibers (Fig. 3B). These strap-like fibers are only visible in two of the best-preserved striated bundles and they are associated diagonally with the calami at a higher angle than the plait-like fibers that compose the Type 1 striated bundles (Figs 2 and 3).  Element analyses (EDAX) indicate that the connective tissues of MCCMLH31444 are mainly preserved as calcium phosphate while the plumage has likely undergone a carbonization process (Fig. 4). Phosphatization of soft tissues is generally associated with changes in pH levels and conditions that allow steady mineral precipitation 20 . These decay conditions fit the combination of taphonomic factorsobruption, stagnation, and bacterial sealing-proposed as responsible for the exceptional preservation of the Konservat Lagerstätte of Las Hoyas 11 .

Discussion
The parallel pattern of plait-like fibers alternating with an unorganized matrix in MCCMLH31444 is characteristic of modern tendons and ligaments that are subject to multidirectional mechanical stresses 19,21 . Even though the structural composition of modern tendons and ligaments is highly variable depending on their size and mechanical requirements 19,21 , the strap-like fibers also preserved in the new fossil also show an overall morphology similar to that of structures interpreted as fossilized muscle tissues in theropods 22,23 . The general morphology and arrangement of the system of ligaments, elastic tendons, and smooth muscles associated with the follicles of flight feathers in the postpatagium of MCCMLH31444 is strikingly similar to that present in living birds 9,10 , strongly suggesting that a comparably elaborate dermal system is preserved in the new fossil.
In modern birds, networks of smooth muscles attached by elastic tendons to the outer walls of feather follicles control the coordinated movement of groups of feathers 9,10,24,25 . In the wing, a well-developed array of such tendons and feather muscles-erectors, depressors, retractors and rotators 9 -together with other ligamental structures (i.e., Ligg. phalangoremigalia distalia, digitationes remigiales 10 ) bestows structural strength and allows the range of movement crucial for controlling lift and maneuverability during Feathers are composed by higher concentrations of K, O, S, C and Fe as well as traces of Ti; connective tissues show a remarkably different composition from both the matrix and the fossil feathers, with a much more flat profile and a higher peak of P and Ca, evidencing the underlying calcium phosphatization process. One sample was collected from each region. flight 9,25 . For instance, the contraction of rotators results in a counter-clockwise movement of the flight feathers 9,25 , which counteracts the passive rotation of the feathers caused by aerodynamic stresses and impacting the wing's ability to generate lift 9,25 . The comparable arrangement and microstructure of the plait-like muscle fibers observed in MCCMLH31444 suggest that they were capable of providing structural support and buffer against multidirectional forces, such as the ones exerted on the remigial feathers during flight in modern birds. This overall similarity suggests that the wing of MCCMLH31444, and most likely that of other flighted enantiornithines, formed an airfoil capable of enduring important aerodynamic stresses.
The evolution of the modern network of smooth muscles, elastic tendons and ligaments involved in the function of the wing's flight feathers was a paramount event in the fine-tuning of aerodynamic capabilities in birds. By allowing these feathers to be repositioned in tandem, this sophisticated dermal system helped the wing to maneuver as a single functional unit and to cope with the strenuous aerodynamic stresses of flapping flight. The 125-million-year-old MCCMLH31444 provides the oldest reported evidence of this intricate connective network and its earliest phylogenetic occurrence. The remarkably modern anatomy and arrangement of the connective tissues preserved in the wing of MCCMLH31444 implies that Early Cretaceous enantiornithines had already developed forelimbs morphologically well adapted for flight, losing most of the primitive grasping functions attributed to the dinosaurian forerunners of birds 5 .
Although still showing a suite of primitive skeletal traits 4,14 , even the earliest enantiornithines (i.e., Protopteryx fengningensis 26 ) had already developed forelimb elements of modern proportions, a carinate sternum, and an advanced pectoral girdle including a triosseal canal for the passage of flight muscles 4,14 , all of which suggest active flapping flight and a wing stroke similar to that of present-day birds 4,14 .
Integumentary similarities with modern birds, such as wings with identical feather arrangement and a well-developed alula (i.e., bastard wing) 18,27 , also point to the same functional conclusions 4,18,27 . The preservation of three important patagia-propatagium, alular patagium, and postpatagium-together with the above-mentioned dermal system in the wing of MCCMLH31444, an Early Cretaceous enantiornithine, lends strong support to the notion that these primitive avians had achieved aerodynamic competence comparable to those of many modern birds.