THE MAMMALIAN SKELETON HAS BEEN evolving for more than 200 million years, since it originated from that of nonmammalian cynodonts, resulting in variations as different in size and adaptation as those of bats, moles, horses, elephants, and whales. Therefore, to assume that there is a living species that displays the "typical" mammalian skeleton would be naive and misleading. Nevertheless, all mammalian skeletons represent variations on a fundamental theme, and in terms of the addition or loss of skeletal elements, mammals have, in general, remained rather conservative. The objective of this chapter is to review the skeleton of generalized mammals as a foundation for the discussion of mammalian dentition and osteology throughout this book, and to briefly survey some of the variations on this theme.
Compared to the skeletons of lower tetrapods, those of mammals are simpler (with fewer elements, because of fusion or loss of bones) and better ossified (with more bone and less cartilage in adults). Both conditions probably contribute to greater mobility and speed of movement. One of the most important consequences of thorough ossification is more precisely fitting limb joints. The articular ends of reptile limbs are covered in cartilage. Because reptile bones grow in length throughout life by gradual ossification of this cartilage, a distinct articular surface never forms. By contrast, the articular ends, or epiphyses, of mammalian limb bones (and certain bony features associated with muscle attachment, such as the femoral tro-chanters) develop from separate centers of ossification from the one that forms the shaft, or diaphysis. Growth in length occurs at the cartilaginous plates between the shaft and the epiphyses, thus allowing the formation of well-defined articular surfaces, even in animals that are still growing.
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