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383 A2 Appendix 2. Skeletal Anatomy of Xenarthrans The skeletal elements of xenarthrans reflect the clade’s distinct character among mammals, provide support for uniting xenarthrans and appreciating their particular adaptations. McDonald (2003) provided a broad review of several of the distinctive skeletal features of xenarthrans. Given the importance of such features for our discussions on fossil (as well as living) forms, we discuss their presence or absence or degree of similarity to features present in other mammals. Three major skull shapes may be recognized among living Xenarthra, reflecting its three main groups: armadillo-like, slothlike, and anteater-like. These three are easily recognizable among fossil xenarthrans, despite the vastly increased taxonomic diversity over extant forms. Clearly, xenarthran skulls share a close resemblance in bone number and pattern with the skulls of other mammals, except that of glyptodonts (Appendix Fig. 2.1), in which the skull has undergone a telescoping process to accommodate its center of mass near the neck joint (Fariña, 1985, 1988; Fariña and Vizcaíno, 2001). Among the elements to consider is the septomaxilla (or os nariale). Although a small bone, its absence has been viewed as support for a more derived status among mammals. The septomaxilla is a small bone associated with the nasal region, particularly in basal tetrapods, and there has thus been much discussion over the presence in xenarthrans of a septomaxilla (which is present in monotremes but absent in other mammals). Based on an examination of fetal specimens of Tamandua and Choloepus, Zeller et al. (1993) concluded that the xenarthran septomaxilla was homologous with the central part of the septomaxilla of monotremes and several Mesozoic (that is, truly ancient) mammals, although they noted that in armadillos it has two additional components. On the other hand, Wible and Gaudin (2004) noted that it is probably not homologous with that element. Clearly, additional work is needed to fully evaluate the significance of this structure. Among the features generally considered characteristics of xenarthrans is the reduction and loss of the zygomatic arch, which is regarded as part of a functional complex usually related to feeding upon social insects that also evolved in several other, though not closely related, mammalian groups. Indeed, absence of the arch in vermilinguas and its reduction in pangolins was among the evidence used to support a relationship between these xenarthrans and pangolins. However, among xenarthrans an incomplete or reduced zygomatic arch only occurs in pilosans (i.e., anteaters and many sloths), while in all of the cingulates (armadillos, pampatheres, and glyptodonts) the zygomatic arch is not only complete, but often strongly developed. In glyptodonts (Appendix Fig. 2.1), for instance, the zygomatic (bone in this case; Appendix 1) bears a strong ventral extension or flange Cranial skeleton Appendix 2 384 that projects below level of the tooth row (similar projections occur in sloths, despite incompleteness of the arch). As the earliest xenarthrans were armadillo-like, and thus probably insectivorous, the argument that loss or reduction of the arch necessarily reflects such dietary habits is not at all firmly established. Further, an incomplete arch is present among the earliest known sloths (and is thus probably the ancestral state) and some authors have considered this evidence that the sloth ancestor was insectivorous, even though the earliest sloths have been interpreted as herbivores. A secondarily complete arch (i.e., in which the zygomatic bone and zygomatic process of the temporal bone come into contact) has evolved independently in some fossil sloths. In the ear region, the ectotympanic is usually incomplete or semicircular (horseshoelike), a feature that has also been considered a primitive mammalian character (Van der Klaauw, 1931), although in some taxa this bone forms a circular ring. The floor of the middle ear is not usually covered by an ossified tympanic bulla. Hyoid apparataus In the hyoid apparatus, several of the elements that are usually independent in other mammals (Appendix 1) may be fused in xenarthrans. One of the most characteristic and widespread of these is the fusion of the thyrohyoid elements to the basihyoid to form a single “V-bone” (Pérez et al., 2010). Besides this fusion, many other xenarthrans have the typical form mentioned in Appendix 1, as noted by several authors (e.g., Pérez et al., 2010; De Iuliis et al., 2011), but in some cases fusion is even more extensive, as in Megatherium americanum and glyptodonts. In the latter, several of the elements (stylohyoid, epihyoid, and probably ceratohyoid) fuse to produce a rodlike element termed the...

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