MADERSON, P.F.A.*; ALIBARDI, L.: Origin and evolution of mammalian integument

Maderson’s (1972) model proposed that hairs derived from mechanoreceptive epidermal appendages located between theropsid scales. It argued an initial role in thermo-regulatory behavior because such “protohairs” lacked insulatory properties (IP). Later quantitative changes in developmental fields underlying their form and distribution produced a “protopelage” whose IP were the subject of later selection. The model did not consider the skin’s role as a barrier to cutaneous water loss (CWL) and could not readily explain the origin or initial selective advantage of the postulated quantitative modifications of morphogenesis. Paleoecological analyses suggest that “true” terrestrial ecosystems only appeared in the L.Trias, a time of major radiations of advanced therapsids. Morphological diversity of CWL barriers in living amniotes suggests independent evolution in different clades. Residing in delicate epidermal tissues, such barriers must be protected from abrasive environmental contact either by tough scales (extant reptiles), or by appendages (hairs or feathers). The demonstration that a simple molecular trigger causes hair follicle multiplication suggests an explanation for pelage origin and primary role. Unscaled, early therapsids living in “wet” habitats employed behavioral thermo-regulation as do many anuran amphibians. While some epidermal toughening may have been achieved by incorporation of histidine-rich proteins into ALPHA-keratinizing cells, invasion of “dry,” potentially desiccating, abrasive environments necessitated an appropriately protected CWL barrier. A mutation causing multiplication of protohairs would have provided such protection and would have spread rapidly permitting speciation in the new “terrestrial” environment. The IP of this pilose skin form were a protoadaptation (Gans, 1979) later favored as endothermy emerged.