MID-CRETACEOUS FEATHERS IN AMBER REVEAL UNUSUAL MORPHOGENESIS OF RACHIS-DOMINATED FEATHERS

We describe three-dimensionally preserved feathers in mid-Cretaceous Burmese amber that share macro-morphological similarities (e.g., proportionally wide rachis with a “medial stripe”) with lithic, two-dimensionally preserved rachis-dominated feathers. These feathers in amber reveal a unique ventrally concave and dorsoventrally thin rachis, and a dorsal groove (sometimes pigmented) that we identify as the “medial stripe” visible in many rachis-dominated rectrices of Mesozoic birds. Differentiated proximal and distal barbules, the latter with hooklets clearly observable, produce interlocking barbs of these pennaceous vanes. The three-dimensional preservation of these Burmese feathers confirms previous claims, based on two-dimensional fossils, that they correspond to an extinct morphotype. The highly differentiated barbules combined with the lack of obvious histodifferentiation of the barb rami or rachis suggests that these feathers could have been formed without the full suite and developmental interplay of intermediate filament alpha keratins and corneous beta-proteins that is employed in the cornification process of modern feathers. This study highlights how the development of these feathers might have differed from that of their modern counterparts, namely in the morphogenesis of the ventral components of the rachis and barb rami. We suggest that the concave ventral surface of the rachis of these extinct feathers is not homologous with the ventral groove of modern rachises. Consequently, our results caution about the common practice of extrapolating developmental aspects (and mechanical attributes) of modern feathers to those of stem birds (and their dinosaurian outgroups) because the latter need not to have developed through identical pathways.