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355 A1 Appendix 1. A Primer on Skeletal Anatomy The skeleton of vertebrates forms a bony internal framework that serves several functions, among which are that it supports the body, offers protection for vital organs, participates in ventilation of the lungs, provides attachment sites for musculature that together with the bones produce movement, and serves as a store for several minerals. In some vertebrates, such as turtles and some xenarthrans (Appendix 2) and dinosaurs (mainly ankylosaurs), bony structures, such as osteoderms, may be widely distributed and associated with the integumentary system (the skin) to provide external defensive protection. The bony osteoderms are covered by keratinized (i.e., made from essential the same material as nails and hooves) scutes, which often closely mimic the form of the bony element. The skeleton includes numerous bones (Appendix Fig. 1.1), and while the number and types of bones remain reasonably constant among mammals , their shapes and proportions change, largely in response to functional differences. This primer (adapted mainly from De Iuliis and Pulerà, 2010, with contributions from Getty, 1975) provides a basic overview of the mammalian skeleton, allowing the reader a familiarity with it parts, their main functions, and the functional implications of some of the differences among mammals. It is based primarily on the skeleton of the cat, a mammal with which we are all reasonably familiar, but we include some skeletal elements of other mammals, especially the sheep, beaver, horse, and cow, also all reasonably familiar, to provide a wider scope. This allows a means for assessing and appreciating both the similarities and differences, as well as for evaluation of the functional adaptations reflected by their form (that is, the relationship between form and function). The skeleton is subdivided primarily into the axial skeleton and appendicular skeleton. The axial is the central part and includes the cranial skeleton (that of the head), the vertebral column (or spine), the ribs, and the sternum (or breastbone). The appendicular skeleton includes the bones of extremities (forelimbs and hind limbs or, more simply, the arms and legs), as well as the elements that connect them to the axial skeleton: the pectoral girdle (shoulder blade) in the case of the arm and the pelvic girdle (hips) in the case of the legs. Often, as for example in paleontology, the skeleton is subdivided for descriptive purposes into the cranial (that of the head) and postcranial (the rest of the body) skeleton, as we do here. The cranial skeleton includes the skull and the lower jaw, or mandible, as well as the hyoid apparatus, the bony structure that mainly supports the tongue. The skull of mammals, as with other vertebrates, protects and supports the brain and sense organs and is used in food gathering and processing. It may be conveniently divided into a facial or rostral region, Cranial skeleton Appendix 1 356 Appendix 1.2. Skull of the cat in (a) left lateral view, (b) ventral view, (c) dorsal view. Images © Dino Pulerà reproduced by kind permission of Elsevier Inc. including the nose, orbits, and upper jaws, and a cranial region, including the braincase and ear. In adults the skull is a single structure formed from various centers of ossification (regions of bone development). These centers expand and their edges generally fused together so as to obscure their boundaries. However, it is still possible to note the position and form of the bones. The great diversity of mammals is reflected in the form of their skulls. Among the more extreme modifications are the elongated, tubular, and edentulous skulls of anteaters, the enormous and highly pneumaticized skulls of elephants, and the massive skulls of baleen whales, in which the skull seems little more than gigantic and strutlike upper and lower jaws linked to the back of the skull to allow support of and feeding with baleen. Such differences in form mainly reflect evolutionary modifications for different feeding habits, but other factors (such as digging behavior) may also A Primer on Skeletal Anatomy 357 Appendix 1.3. Skull of (a) the sheep and (b) the beaver in left lateral views. Images © Dino Pulerà reproduced by kind permission of Elsevier Inc. play a role in shaping the form of the skull. Indeed, the jaws, particularly the mandible, and the rest of the skull may need to respond to different pressures. The mandible is clearly affected by feeding behavior, but the upper jaw and the skull may face conflicting demands, including the size of the brain, the support of antlers or horns...

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