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3. The Microstructure of Bones and Teeth of Nonmammalian Therapsids
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65 3 The Microstructure of Bones and Teeth of Nonmammalian Therapsids Anusuya Chinsamy-Turan Bone is a specialized connective tissue. Except for extant jawless fishes (lampreys) and to some extent chondrichthyes (sharks), it composes the skeleton of all vertebrates. It is a composite material with an organic phase and an inorganic mineral phase consisting of carbonate hydroxyl apatite—Ca10 (PO4 Ca3 )6 (OH)2 . However, relatively soon after an animal dies, and becomes buried by sediment, the hydroxyl group is displaced from the bone mineral and is commonly replaced by fluorine to form carbonate fluorapatite. Thus, X-ray diffraction analysis of fossil bone often has a slightly different mineral spectrum than living bone (Fig. 3.1A). The organic phase of bone generally comprises different types of cells (osteoblasts, the bone forming cells; osteocytes, the bone cells; and osteoclasts, the bone resorbing cells), an extracellular matrix consisting mainly of Type 1 collagen, as well as other noncollagenous proteins such as osteocalcin, osteopontin, and osteonectin; polysaccharides; and a network of blood vessels and nerves (Currey 2002). In some vertebrates, namely, the teleost fishes and the heterostracans (the armored jawless fishes from the Ordovician) acellular bones occur. In all other vertebrates, bone cells (osteocytes) are located within small spaces in the bone matrix called lacunae (Fig. 3.1B). Neighboring osteocytes communicate with each other via cytoplasmic extensions which are located within extensions of the lacunae called canaliculi (Fig. 3.1B). During bone formation, blood vessels can be entrapped within the bone matrix as simple blood vessels, or, if the bone matrix is rapidly deposited, spaces are left around them (Fig. 3.2A). Subsequently, these channels are infilled centripetally with a more slowly formed bone tissue to form a bony ring (an osteon) around the blood vessel (Figs. 3.1B, 3.2B). Bone formation begins with the deposition of the osteoid (mainly composed of collagen), which later becomes impregnated with hydroxyapatite to form bone. Thus, the collagen fibers and the tiny bone mineral crystals (500 Å long, 280 Å wide, and 20 Å thick), the organic and inorganic phases of bone, respectively, are intimately associated during life (Weiner and Traub 1992). When an animal dies, the organic components soon decompose, but even after millions of years, the more resilient inorganic (mineral) part of bone often undergoes only minor changes and reflects the microstructure of the bone tissue (Chinsamy-Turan 2005, and references therein). Introduction Chinsamy-Turan 66 3.2. Fibrolamellar bone. (A) Dicynodon (SAM-PK-5576). Early stage of fibrolamellar bone formation showing the spaces that will later become infilled with a lamellar bone tissue. (B) Primary osteons (white arrows) in the fibrolamellar bone tissue in Tropidostoma (SAM-PK-K990). The microstructure of fossil bones has proven to be a powerful tool to decipher biological signals recorded therein. The same basic components of bone exist across all classes of vertebrates—they are simply organized in different ways to form different types of bone tissues (e.g., Ricqlés et al. 1991). Research on bone has shown that its microstructure largely reflects the rate at which the bone was formed (Amprino 1947) and that this is affected by a number of factors, such as ontogeny, phylogeny, pathology (e.g., Ricqlés et al. 1991; Chinsamy-Turan 2005; Erickson 2005; Erickson, Makovicky et al. 2009). This chapter serves as an atlas of bone tissues observed in nonmammalian therapsids. It also discusses some of the biological inferences that can be deduced from the occurrence of particular histological features. 50 mµ A 100µm B [44.200.175.46] Project MUSE (2024-03-29 16:17 GMT) Microstructure of Bones and Teeth of Nonmammalian Therapsids 67 Since the first step in any study of fossil bone microstructure is the preparation of thin sections (75–100 µm in thickness), this procedure is outlined. Fossil bones are generally rock-hard and indeed they are often petrographically thin sectioned in a similar way to rock samples (Chinsamy and Raath 1992). However, since fossil bones are generally brittle, the first step in the thin-sectioning process is to embed the bone in a polyester resin. The resin surrounds the bone and penetrates all the natural cavities and/or cracks. It is usually best to do this under vacuum for more porous bones, but most other bones can simply be left in the resin until the latter hardens. Once embedded, the specimen can be sectioned to remove the part of the bone that is of interest. From here onward, the process can vary depending on the...