Evolving a protofeather and feather diversity

It is likely that feathers evolved from a conical shaped tubercle rather th an a plate-like structure. Although the morphology of the presumably most p rimitive feather is unknown, minimal conditions for its production include the cellular capacity to synthesize feather proteins (=phi -keratin) which provides the molecular phenotype, and a follicular mechanism for production and assembly of molecular and gross structure, Once the mininal structural element, presumably recognizable as a barb, existed, a variety of phenotyp es followed rapidly. A tubercular growth center of appropriate size could p roduce a simple barb-like element, with cortex and medulla, This might be r ecognized externally as a bristle, but need never existed as a separate mor phological 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 symmet ry, and the feather shape is regulated by barb length. Barb length is direc tly related to growth period. As feathers appear to grow at roughly similar , size independent rates, shape is determined by individual barb growth per iods. The simple fusion of individual proto-barbs would produce a morpholog y identifiable as natal down. Although this might be the simplest feather s tructure, others could emerge quickly, perhaps simultaneously, a consequenc e of the same redundant processing. Once the machinery existed, broad pheno typic plasticity was possible. I constructed a feather phylogram based on t hese conditions, the fossil record, and ontogeny. I organized the subsequen t changes in morphology by perceived complexity. The changes are simply ind ividual responses to similar processes that might be time (when in ontogeny ) and space (where on body) dependent.