Mammal Teeth: Origin, Evolution, and Diversity

Monotremata and Marsupialia

10 Monotremata and Marsupialia You should feel your pulse quicken and your brow flush at the mere mention of the title of this chapter: the most fascinating mammal radiation. —m. archer, 1984 WHEN MOST OF US THINK about mammals, we think about lions and tigers and bears, or elephants and giraffes, or cows and sheep, but not platypuses and koalas. To be sure, living monotremes have little to offer dental researchers, but the marsupials present an incredible variety of teeth. They provide us with an impressive example of the diversity of forms that evolution can produce, all independent of the placental radiation. It is enough to give one what M. Archer (1984) referred to as “pouch envy.” This chapter surveys the radiation of tooth form in recent monotremes and marsupials , family by family. The classificatory scheme, detailed in chapter 5, combines the third edition of D. E. Wilson and Reeder’s Mammal Species of the World (2005) with current understandings of morphological and molecular systematics. This review and those in the chapters that follow are not meant to be comprehensive descriptions of tooth traits for taxonomic identification or phylogenetic analysis. These chapters serve merely as an introduction to the diversity of dental form in the radiation . More detail can be found in the references cited for individual taxa. PROTHERIA The mammalian subclass Protheria comprises a single living order, Monotremata. Monotremata There are two recent families of monotremes, Ornithorhychidae (the platypus) and Tachyglossidae (echidnas). It takes little effort to survey dental variation in these mammals because echidnas and the adult platypus are edentulous. Nevertheless, we cannot ignore the egg-laying mammals, because the platypus has teeth as a juvenile and all monotremes have specialized structures in the mouth that function in food breakdown. ornithorhynchidae (fig. 10.1). There is only one living ornithorhynchid , Orithorhynchus anatinus, the platypus. This monotreme is semiaquatic and lives along freshwater streams, lakes, and lagoons on the Australian mainland and Tasmania . Adults average between about 500 g and about 2 kg and are excellent swimmers and burrowers. Ornithorhynchus forages for invertebrates and small vertebrates mainly by probing the mud and gravel along the bottoms of freshwater streams using its bill, a specialized structure with both electrosensory and mechanosensory receptors (Grant 1989, 1995; Pettigrew, Manger, and Fine 1998; Nowak 1999). 130 t h e t e e t h o f r e c e n t m a m m a l s Platypus occlusal morphology was described in detail by early-twentieth-century researchers (see, e.g., G. G. Simpson 1929b; and H. L. H. H. Green 1937) and may be of relevance to debates concerning a possible dual origin for the tribosphenic molar (see chapter 8). The main cusps of the larger teeth are positioned buccally on the upper crowns and lingually on the lowers. Crests or lophs run transversely across the occlusal surface from each of these cusps toward a series of small cuspules that line the opposite edge of the crown. Each of these teeth is divided roughly into anterior and posterior halves by a deep, transverse basin, resulting in what M. O. Woodburne (2003:207) called a “bilobate, essentially bilophodont, molar morphology.” Platypuses lose their dentitions at about three to four months of age; the teeth are replaced by keratinous pads that function in crushing and grinding food (Grant 1989). Specialized epithelial ridges on each jaw anterior to those pads are used for grasping and holding prey (A. M. Musser and Archer 1998). Food is stored in cheek pouches behind the bill and then moved to the keratinous pads for crushing and grinding. The tongue also assists in food breakdown through the action of two small projections near the base that work against the palate (Grant 1995). tachyglossidae (fig. 10.1). There are, according to Groves (2005c), four species of echidnas. These monotremes are endemic to a broad range of environments, from forests to more open settings, in Australia, New Guinea, and on nearby islands. Echidnas are larger than platypuses, adults weighing between about 2.5 kg and about 16.5 kg. They are excellent burrowers and consume mostly termites and ants, though they do eat other invertebrates, such as earthworms and scarabs, on occasion (Griffiths 1989; Nowak 1999; Nicol and Andersen 2007). Echidnas do not have teeth as adults or as juveniles. Like the platypus, echidnas crush and grind food with the help of keratinous structures and tongue action (Griffiths 1989; Rismiller 1999). They ingest mostly termites and ants with the...