Fig. 1

Morphological diversity and rachis structure. RDFs exhibit at least three general outlines: a with barbs that appear to extend all the way down rachis, one of the two feathers in DIP-V-16186; b with naked rachis basally, DIP-V-16208 overview; c with more pointed apices and narrower vanes, one of the two feathers in DIP-V-16186. The general structure of all RDFs shares some common features: d diagram of RDF indicating rachidial ridge (dark red), and fused and unfused barb ridges making up rachis (blue), with barbule distribution indicated only on right side of feather apex (pale red), and with cross-sections showing branching pattern of barbs within vaned section of RDF (upper section), and tissue generation at follicle (lower level) required to produce barbs at posterior margin, or along lateral margin of rachis (right side vs. left side of each section, respectively). Rachidial ridge variants include: e prominent ridge throughout length, DIP-V-17138; f faint, DIP-SY-06231; g or even sporadic, MCAC-0322. Arrowheads in f and g indicate barbs attached to posterior margin of rachis, while arrows indicate those attached to lateral surface. Abbreviations: ant – anterior, lam – lamina, lb. – lateral barb, pmb – posterior margin barb, post – posterior, rr – rachidial ridge. Scale bars = 2 mm in (a, b, c, e and f); 0.5 mm in (g)