Evolving a Protofeather and Feather Diversity1

SYNOPSIS. It is likely that feathers evolved from a conical shaped tubercle rather than a plate-like structure. Although the morphology of the presumably most prim-itive feather is unknown, minimal conditions for its production include the cellular capacity to synthesize feather proteins (5f-keratin) which provides the molecular phenotype, and a follicular mechanism for production and assembly of molecular and gross structure. Once the minimal structural element, presumably recogniz-able as a barb, existed, a variety of phenotypes followed rapidly. A tubercular growth center of appropriate size could produce a simple barb-like element, with cortex and medulla. This might be recognized externally as a bristle, but need never existed as a separate morphological unit. Rather, if individual placodes gave rise to multiple barb ridges that fused proximally, a structure resembling natal down would have resulted. Subsequent differentiation is controlled by the follicular symmetry, and the feather shape is regulated by barb length. Barb length is di-rectly related to growth period. As feathers appear to grow at roughly similar, size independent rates, shape is determined by individual barb growth periods. The simple fusion of individual proto-barbs would produce a morphology identifiable as natal down. Although this might be the simplest feather structure, others could emerge quickly, perhaps simultaneously, a consequence of the same redundant processing. Once the machinery existed, broad phenotypic plasticity was possible. I constructed a feather phylogram based on these conditions, the fossil record, and ontogeny. I organized the subsequent changes in morphology by perceived com-plexity. The changes are simply individual responses to similar processes that might be time (when in ontogeny) and space (where on body) dependent