Prismless enamel in amniotes: terminology, function, and evolution

Abstract

Introduction Amniota, the group consisting of reptiles, birds, and mammals, is characterized by monotypic enamel, a hypermineralized tissue secreted exclusively by ectodermal cells, the ameloblasts. While the enamel of Tertiary, Quaternary, and Recent mammals has received much study (Koenigswald and Sander, 1997 a ; Chapter 8), that of reptiles, toothed birds, and mammal-like reptiles is much less well known. One reason is that most mammals have prismatic enamel, which is easily studied in thin sections with polarized light, whereas the enamel of most non-mammalian amniotes (summarily referred to as reptiles hereafter) lacks prisms. This prismless enamel can only adequately be studied with the scanning electron microscope (SEM) because the light microscope cannot resolve individual crystallites. Enamel types differing in crystallite arrangement thus can look the same in light micrographs. Published SEM surveys of reptile enamel do not predate the last decade (Carlson, 1990 a , b ; Dauphin, 1987 a , b , 1988; Dauphin et al ., 1988; Sahni, 1987). There are two additional reasons for the low level of interest in reptilian enamel structure. First, the main impetus for studying reptile enamel in the past was not interest in the matter per se but in the origin of mammalian prismatic enamel. Second, paleontologists and systematists working with mammals traditionally focus on the teeth because mammalian teeth are rich in characters compared with the postcranial skeleton. The reverse is true for non-mammalian amniotes. The purpose of this chapter is to provide an overview of the structural complexity and diversity of reptilian enamel and to explore the evolutionary constraints on its structure, be they functional or phylogenetic.