Fig. 4

Medullary bone observed in the skeleton of Cygnus atratus (CM-S16508) using different techniques. a: isosurface of the tibiotarsi, vertebrae, radii, and ulnae; b: virtual cross-sections of different skeletal elements as shown in A. MB is filling the medullary cavity of the limb bones but is not seen in vertebral elements; c: isosurface of the synsacrum, keeled sternum, coracoids, scapulae, and furcula; d: virtual cross-sections of different skeletal elements as shown in C. MB is observed in all figured elements; e: isosurface of the skull, caudal vertebrae, carpometacarpii, femora, atlas, and axis; f: virtual cross-sections of the skull and mandible as shown in E. MB is visible in the cavities of the pterygoids and surangulars; g: photograph of the skull in ventral view, with higher magnification of the sampled pterygoid; h: Ground section of the pterygoid as shown in G; i: virtual cross-sections of the skull and mandible as shown in E. MB is visible in the cavities of the pterygoids and surangulars; j: photograph of the mandible; k: Ground section of the surangular as shown on higher magnification image. MB is visible in the cavities of the surangular; l: paraffin section of the surangular stained with Alcian Blue. MB is stained in dark blue, contrasting with the lightly stained surrounding trabecular bone, and indicates the distinct chemistries of the two bone types; m: virtual cross-sections of a cervical vertebra, the carpometacarpii, the skull, and a femur as shown in E. MB is visible in all elements; n: Ground section of a caudal vertebrae. The cavities are filled by MB; o: paraffin section of a caudal vertebra stained with Alcian Blue. MB is stained in blue and contrasts with the surrounding periosteal bone tissue that is lightly stained; p: isosurface of the humeri and other elements; q: virtual cross-sections of the humeral epiphyses as shown in P. MB is visible between the trabeculae. r: Ground section of a humeral epiphysis; s: paraffin section of the humeral ephiphysis stained with Alcian Blue. MB is stained in blue and contrasts with the surrounding trabecular bone tissue that is not stained. Abbreviations: CMC, carpometacarpus; Corac, coracoid; C. vert, caudal vertebra; Fem, femur; Furc, furcula; Hum, humerus; Ptery, pterygoid; Rad, radius; Scap, scapula; Sku, skull; Stern, sternum; Sur, surangular; Syn, synsacrum; TT, Tibiotarsus; Uln, ulna; Vert, vertebra