When? why? and how? Some speculations on the evolution of the vertebrate integument

Abstract

SYNOPSIS The basic structure of the vertebrate integument is briefly reviewed The system is either scaled non scaled, or a mixture of the two Scales are not appendages of the integument, but are patterned folds in which the dermal and/or epidermal components may be elaborited An appendage is the product of specialized patterns of cell differentiation localized within the dermis and/or ep dermis Scales, and appendages (whether borne within scaled or non scaled integuments) can only be correctly defined with reference to the chemical or molecular nature of the end products of dermal and/or epidermal cell differentiation Truly homologous integumentary structures probably do not exist above the class level in modern vertebrates. Anatomical, developmental neuiological and paleontological data are presented in support of a model for the origin of mammalian hair It is suggested that hairs arose from highly specialized sensory appendages of mechanoreceptor function which facih tated thermoregulatory behavioral activity in early synapsids Specialization of cellular differentiation within these units led to the appearance of dermal papillae A chance mutation led to subsequent multiplication of the originally sparsely, but spatially arranged papillae, causing the induction of a sufficient density of "sensory hairs to constitute an lnsulatory body covering". The lnsulatory properties of this "prolopelage were the subject of subsequent selection but the sensory function of mammalian hairs remains important". © 1972 by the American Society of Zoologists.