Mammal Teeth: Origin, Evolution, and Diversity

Teeth before the Mammals

6 Teeth before the Mammals Although teeth rarely excite the attention that their importance warrants, their evolution among the early vertebrates without doubt played an unrivaled role in the successful adaptation of these animals and their achievement of rapid and effective dominance in the organic world. — weller, 1968 WORLD DOMINATION! There can be little doubt that improved efficiencies of food acquisition and processing make teeth among the most important innovations in the evolution of life. While these structures reached a new level of complexity and variation with the radiation of mammals, the story of teeth began well before the earliest members of our class. If we are to understand the origin and early evolution of these structures in all their glory, we must venture back in time more than half a billion years, to the earliest vertebrates. This chapter surveys briefly our current understanding of the origin(s) of teeth in vertebrates and briefly sketches some early innovations in dental form in fishes, amphibians , and reptiles. Before we begin, however, a very brief sketch of some relevant aspects of vertebrate classification is necessary to put the discussion of dental evolution into context. Readers interested in more detail can see the many recent classifications available in the literature (e.g., Hedges and Kumar 2009). One common scheme for extant vertebrates is as follows. First, the living vertebrates are divided into Cyclostomata or Agnatha (jawless fish) and Gnathostomata (jawed vertebrates). Teeth likely first evolved in stem gnathostomes. Gnathostomes include Chondrichthyes (cartilaginous fishes) and Osteichthyes (bony jawed vertebrates). Osteichthyes comprises Actinopterygii (ray-finned fishes) and Sarcopterygii, the latter of which divides into Actinistia (coelacanths ), Dipnoi (lungfishes), and Tetrapoda. Tetrapods include Lissamphibia (amphibians ) and Amniota. Amniotes are the first fully terrestrial vertebrates, thanks in no small measure to eggs in which embryos develop inside amniotic membranes so that they can survive on dry land. These tetrapods are sometimes contrasted with the anaminotes, a paraphyletic group that includes fishes and amphibians. Finally, the amniotes are split into Sauropsida (reptiles and birds) and Mammalia (Fig. 6.1). THE ORIGIN(S) OF TEETH The past few years have witnessed a remarkable flurry of research on the origin or origins of vertebrate teeth. While this work is progressing, the details of when, where, why, and how teeth first appeared still elude consensus. Indeed, there is not even agreement on the fundamentals, such as how we define a tooth. From a functional perspective, we might think of teeth as hardened structures in the mouth that function in feeding (Fig. 6.2). According to this definition, a very 74 t h e e v o l u t i o n o f m a m m a l t e e t h broad range of vertebrates and invertebrates have teeth (Peyer 1968; Weller 1968). Gastropod radulae, for example, are chitinous ribbons in the mouth with longitudinal rows of recurved “teeth” used for scraping algae from rock surfaces or soft parts from the shells of other mollusks (Hickman 1980). Sea urchins also have “teeth” in a mouth structure that has been called Aristotle’s lantern. Each “jaw” has five calcium carbonate elements that are used to scrape food (R. Z. Wang, Addadi, and Weiner 1997). Aristotle described this structure in book 4 of Historia animalium as resembling “a horn lantern with the panes of horn left out” (Aristotle, trans. 1910:531a). Other examples abound, such as the sharp, hardened “teeth” lining spider chelicerae above the fangs to grip and crush prey. Jawless vertebrates (agnathans) also have “teeth” in this sense. Hagfish and lampreys have keratinous cones attached to cartilaginous plates that surround the mouth (Peyer 1968). These are formed from cornified epidermal cells and are used to rasp flesh to create wounds or to bore into carrion. While these may all be considered “teeth” from a functional perspective, none are homologous with or play any role in the origin or evolution of teeth in the sense used in this book. The Etiology of Gnathostome Teeth When most vertebrate morphologists speak of teeth, they are referring to those of gnathostomes. In fact, conventional wisdom has long held that “real” teeth are inextricably linked with the appearance of the jaw, though some researchers are now beginning to question this association (see below). The etiology of teeth has been the subject of debate and discussion for decades. In the mid-twentieth century, Ørvig (e.g., 1951) and Stensiö (e.g., 1961, 1962) developed a model to explain teeth (and scales) as differing...