When? Why? and How? Some Speculations on the Evolution of the Vertebrate Integument

Abstract

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 elaborated. An appendage is the product of specialized patterns of cell differentiation localized within the dermis and/or epidermis. 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.