MADERSON, PFA; HILLENIUS, WJ; Brooklyn College CUNY; College of Charleston: Exploding the myth of tubular feathers: a major evolutionary innovation requires only one novel mechanism

The Prum/Brush model for feather origin and diversification (a) posits multiple developmental novelties appearing in an inherently untestable evolutionary sequence, (b) ignores aspects of form that reflect essential developmental and organismic roles, (c) is unsubstantiated by data from living sauropsids, and (d) lacks paleontological support. Expressly attributed to Davies (1889), their criticism of alternative elongate scale models as �creating a developmental and topological conundrum� ignores presently known morphogenetic mechanisms and misrepresents recent formulations of such models. Examination of barbules, barbs, and rachis (in toto = spathe) reveals the one feature that distinguishes feathers from other epidermal appendages, e.g., hairs and nails: localized populations of cells separate from one another during terminal maturation. Mutant and experimentally perturbed feathers show that disturbance of developmental patterning signals produces fusion of some or all parts of the spathe. This implies that an �Ur-feather� could have evolved via a single, unique developmental novelty: new signals, acting post-mitotically, produced a pre-pattern later manifested in differential cell adhesion within a continuous sheet of maturing β-keratogenic cells that grew beyond the apex of a scale�s dermal core. The observable simultaneous emergence and subsequent differentiation of presumptive inter-barb spaces, outer and inner vane surfaces, and deployment zone opposite the rachidial ridge, bely the putative �fundamentally tubular nature of a feather� (only sheath and pulp caps are tubular) and reveal the fallacy of the topological conundrum. From a primordial, contour-like appendage, all known feather morphs are derivable by invoking sequential heterochronies, and perhaps genes affecting symmetry.