Digital rendering, photography, and line-drawing of the enantiornithine fossil specimen (Shanyang sp., IVPP V26899) (a), (c), (d) - dorsal; (b)- ventral view showing the major skeletal elements.

The feather extracted from the crown are labeled as feather sample (c). Abbreviations: al, alular metacarpal; co, coracoid; cv, cervical vertebra; fe, femur; fi, fibula; fr frontal; fu, furcula; hac, hallucal ungual; hc, head crest; hu, humerus; hy, hyoid; il, ilium; is, ischium; lt, lateral trabecula; mcf, molted crest feather; mc II, major metacarpal; mp, midline process; ph II-1, first digit of phalanx II; phIII-1, first digit of phalanx III; pa, parietal; pu, pubis; py, pygostyle; r, radius; rec, rectrices; rem, remiges; rs, rostrum; rw, right wing; sc, scapula; sk, skull; st, sternum; tb, tibiotarsus; and tmt, tarsometatarsus.

Photograph (a) and simplified line-diagram (b) of the sampled isolated long feather with a focused area from SEM imaging (c-g).

As shown in the SEM images, the long elliptical or oval-shaped melanosomes are tightly aligned with their long axis parallel to the elongated barbs (e, g). The position of enlarged images (d, e, f, and g) is indicated as square in (c, d); black arrows in (e) and (g) indicate the melanosome clusters.

Photograph of the crest feather (a) and prepared blocks of the feather barb (b) for thin sectioning.

The sampled spots for the sliced feather are indicated as long red-colored cubes with their respective positions in (b). (c) to (f) are (STEM) images obtained from individual slices as indicated in (b). The shape and arrangement of the melanosome packing (ACHP, asymmetric compact hexagonal packing) is clearly revealed in (d); (c) and (d) are derived from the cross-section; (e) and (f) are derived from the nearly longitudinal section. As indicated by the red arrows in (d) and (f), the dorsal and ventral hooks are dominant in the cross-sectional images. On the other hand, hooks connecting the proximal and distal arrays of melanosomes are better visualized in the longitudinal section, as labeled with blue arrows in (f). Orientation of the slices were labeled (left, right, proximal, distal) with their respective petitioning in the fossil before and during sectioning.

FDTD modeling results.

(a) Representative multilayer melanosome model; (b) Schematic drawing of the reflectance calculation setup of the feather barb. X-Y incident plane (blue) is perpendicular, and the Y-Z incident plane (orange) is parallel to the special melanosome longitudinal axis. θ1 and θ2 are incident angles in X-Y and Y-Z incident planes, respectively. The blue dashed box (R) indicates the area where the reflection spectra are simulated and calculated. (c) Angle-dependent reflectance spectra calculated for p- and s-polarized light by FDTD modeling with X-Y incident plane. (d) Angle-dependent reflectance spectra calculated for p- and s-polarized light by FDTD modeling with Y-Z incident plane. Keratin cortex layer was considered in the modeling.