Mammal Teeth: Origin, Evolution, and Diversity

Xenarthra and Afrotheria

11 Xenarthra and Afrotheria There’s nothing that an evolving animal cares about more than how its teeth are getting along. —lewis, 1960 MOLECULAR SYSTEMATISTS RECOGNIZE four basic clades of placental mammals, two groups associated with the Southern Hemisphere , Xenarthra and Afrotheria, and two from northern continents, Laurasiatheria and Euarchontoglires (see chapter 5 for details). While the northern, or Laurasian, clades are usually joined together as Boreoeutheria, there is less agreement on how Xenarthra and Afrotheria are related to each other or to the other placental mammals. Some have rooted the placental tree with Xenarthra (see, e.g., Shoshani and McKenna 1998; Kriegs et al. 2006; and Svartman, Stone, and Stanyon 2006), following McKenna and Bell (1997). Others have suggested that Afrotheria is at the root (see, e.g., Madsen et al. 2001; W. J. Murphy, Eizirik, Johnson, et al. 2001; W. J. Murphy, Eizirik, O’Brien, et al. 2001; Waddell, Kishino, and Ota 2001; Delsuc et al. 2002; Springer et al. 2005; and Nikolaev et al. 2007). Yet others have joined Xenarthra and Afrotheria into a single root clade called Atlantogenata (Waddell et al. 1999; Waddell and Shelley 2003; P. D. Waters et al. 2007). This chapter presents a brief survey of tooth form for the two southern, or Gondwanan, supraordinal placental clades, Xenarthra and Afrotheria. These groups are considered together both for convenience and to give the reader some appreciation for the extent of the Southern Hemisphere radiations. Unless otherwise noted, the classificatory scheme used here follows D. E. Wilson and Reeder (2005) and references therein and is described in detail in chapter 5. XENARTHRA Where better to begin our survey of placental mammal teeth than with Xenarthra, core of the now defunct “Edentata,” the “toothless” mammals? The xenarthrans include the orders Cingulata (armadillos) and Pilosa (sloths and anteaters). Despite their former designation, only the anteaters lack teeth entirely. Most xenarthrans do not have differentiated anterior teeth, but they do have single-rooted, ever-growing cheek teeth, which in most cases are homodont, enamel-less pegs. Cingulata There is only one extant family in the order Cingulata, Dasypodidae. dasypodidae (fig. 11.1). Gardner (2005b) recognizes 21 species of armadillos . These terrestrial and fossorial mammals have a widespread distribution in forests and more open areas throughout the Americas, from the Straits of Magellan x e n a r t h r a a n d a f r o t h e r i a 145 Pilosa Pilosa is divided into two suborders, Folivora (sloths) and Vermilingua (anteaters). Each suborder is divided into two families. The sloths comprise Megalonychidae (two-toed) and Bradypodidae (three-toed); the anteaters include Cyclopedidae (the silky anteater) and Myrmecophagidae (the other anteaters) (Gardner 2005a). Folivora megalonychidae (fig. 11.1). There are two species of two-toed sloths, both in the genus Choloepus. These are found in the neotropical forests of Central and South America. Sloths are slow-moving arboreal mammals with specialized adaptations for life upside down in the canopy. Choloepus adults weigh between about 4 kg and about 8.5 kg. They are predominantly folivorous, though they also take some fruits and twigs (P. J. Adam 1999; Nowak 1999). Barlow (1984) reports the sloth dental formula to be P+M 5/4–5, but there has been debate concerning whether the anteriormost teeth might actually be canines (Grassé 1954; Goffart 1971). These teeth are caniniform in Choloepus and are separated from the remainder of the dentition by a diastema , at least in adults. The upper caniniforms occlude on the posterior face of the opposing lowers, forming selfsharpening , elongated triangular structures. The permanent cheek teeth usually erupt in utero as simple conical structures . These comprise a soft, internal core of amorphous dentin surrounded by an outer layer of harder, more vascularized orthodentin. This configuration causes differential wear, with deep basins and sharp edges between the dentin types. Sloths may also have a thick layer of cementum, but they lack enamel (Naples 1982; Ferigolo 1985; Gaudin 2004). bradypodidae (fig. 11.1). Gardner (2005a) recognizes four species of three-toed sloths, all in the genus Bradypus. These sloths are also found in neotropical forests of Central and South America. They resemble Choloepus in many ways. They overlap in size with two-toed sloths, with adults weighing between about 2.25 kg and about 6 kg. Bradypus is predominantly folivorous but consumes twigs on occasion . The diets of this genus are said to be more limited than those of Choloepus (Chiarello 1998; Nowak 1999; B. Urbani and...