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1 1 Why Study the Bone Microstructure of Fossil Tetrapods? Kevin Padian In the nineteenth century, when morphology was the queen of the biological sciences, every student of the living world had to know the intimate details of plant and animal anatomy, including microscopic anatomy, as well as the theories of the generation and determination of form and structure that underpinned the science of morphology (Sloan 1992; Desmond 1982, 1989). As a matter of routine, Richard Owen took a thin section of a bone of Scelidosaurus when he described the dinosaur in 1861. However, with the twentieth-century advent of genetics, and later the great advances in cellular and molecular biology, histology—the study of tissues—was eclipsed by these many new fields of study. There simply was not enough time for everything. Besides, in this new world order, histology was regarded as old-fashioned; it wasn’t telling us anything new or anything more than human anatomy was. People had to learn these subjects in order to do medicine or surgery or functional morphology, and then they needed to move on to more active fields. In fact, the analysis of the microscopic structure of fossil bone was never a huge field. We think of pioneers such as Quekett, Gross, Foote, Ørvig, Enlow, Peabody, de Ricqlès, and Reid in the field of fossil tetrapod bone histology , but we do not always realize that they were usually rather lonely. Although fossil bones have been sectioned for centuries, they have never been systematically sampled until recently. By “systematically,” we mean not only phylogenetically but also in the sense of having an explicit, standard approach to sampling with the goal of identifying and controlling as many variables in your sample as possible. A simple illustration will show how much and how quickly things have changed. In the 1960s, when Armand de Ricqlès was assembling his samples for his dissertation, which ultimately became the great series of works that he published in Annales de Paléontologie in the 1960s and 1970s, curators of museum collections were reticent to 2    Why Study the Bone Microstructure of Fossil Tetrapods? give him perfectly good bones to sacrifice to the saw. Generally, he was able to work with incomplete or fragmentary bones, occasionally with dubious identifications of element and taxon. So it was very difficult for him to standardize his samples and control his variables. Nevertheless, he managed to induce a series of generalizations about patterns of bone growth strategies in tetrapods that in its general outlines and many of its details holds up very well nearly half a century later (Padian 2011). Paleohistologists have historically faced this main problem: an adventitious, capricious sample set. It was only in the 1980s and 1990s, when John R. (“Jack”) Horner and contemporaries became interested in what paleohistology could tell us about the ages and growth rates of dinosaurs, that samples could be adequately standardized. The reason was that Jack collected his own dinosaurs, mostly from the Cretaceous of Montana. As Curator of Paleontology at the Museum of the Rockies, he was able not only to cut up whatever he wanted but also to standardize his sections so that he could get a range of sizes and ages of specimens, always sampling the same bone in the same place. Along with pioneering studies by Anusuya Chinsamy and others , Jack, Armand, and their students and colleagues were soon applying the same approaches to a great many extinct amphibians, reptiles, and birds. Jack employed a full-time technician to process thin sections of fossil and recent bones. The burgeoning of this field and the great insights that it has given us into the paleobiology of dinosaurs and other fossil vertebrates are the main reasons for this book. THE “FOUR SIGNALS” OF FOSSIL BONE HISTOLOGY The microscopic structure of fossil bone generally reveals four influences or “signals ” (Horner et al. 1999, 2000; de Ricqlès et al. 2001; Padian et al. 2001). These are ontogeny, phylogeny, mechanics, and environment. These signals hold sway to different degrees in different bones in different taxa at different times of life, as well as in different environments. Reading these signals requires comparative information; without it, inferences about age, growth rate, and other factors can be mistaken. And it is important to note from the start that more than one of these signals can be manifested synergistically in any given section of bone tissue. Ontogeny As a bone grows, it changes its histological features in several ways...

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