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265 10 Research Applications and Integration Kevin Padian, Maria de Boef Miara, Hans C.E. Larsson, Laura Wilson, and Timothy Bromage The point of providing all the information in the previous chapters is to stimulate readers with questions that fuel new research insights and integrative collaborations . Our goal has been to provide information about selecting, processing, recording , and interpreting samples of fossil bone and other tissues that can help us understand the variation displayed in fossil tissues and structure and to test hypotheses that explain this variation. In this final chapter, we begin by paralleling the structure of the first. The basic influences of the “four signals” that the macroand microstructure of bone tissue were described in the first chapter. In the chapters that followed, we unpacked some of these influences. Here we address some of the questions and issues that are associated with the “four signals” with the aim of actuating further research. THE “FOUR SIGNALS” THAT ARE REFLECTED IN BONE-TISSUE DEPOSITION Ontogeny, phylogeny, biomechanics, and environment have been treated in a variety of publications on fossil bone tissues (e.g., Horner et al. 1999, 2000, 2005; Horner & Padian 2002, 2004; Padian et al. 2001; de Ricqlès et al. 2000; de Margerie 2002; de Margerie et al. 2002, 2004, 2005; Cubo et al. 2005, 2008). The consensus is that these four “signals” are reflected in the micromorphology of tissues deposited at any time during the life of a given section of bone. Indeed, in some way or another, these signals are central to the origin and maintenance of all morphologies (see Chapter 1). They are not entirely separate from one another. For example, growth in ectothermic animals is closely tied to the environment. Other 266    Research Applications and Integration signals, such as environmental and reproductive stresses, have some effect on bone microstructure, but these influences are less well known. Other signals are also influence bone morphology. Physiology is certainly an important component of the rates at which bone tissues grow and was no doubt important to extinct vertebrates as well. Physiology can serve as a synonym for all of the processes of growth and metabolism that figure in the formation and expression of bone tissue. The problem is that it is difficult to measure traditional physiological factors (such as endothermy, homeothermy, and metabolic rate) as directly in the bones of extinct vertebrates as it is to assess other factors. In fact, inferences of physiology in ancient animals that have been made directly from their bone tissues have been strongly criticized (e.g. Padian & Horner 2004; Chinsamy & Hillenius 2004). Consequently, here we prefer to view signals of physiology as inferences that proceed from the assessment of growth rates in bone tissues. There is a very strong correlation between growth rates and metabolic rates in the tissues of living animals, and in our view this correlation has a causal basis because to date there is no known literature that establishes how high growth can be sustained in vertebrates without high metabolic rates. For the purposes of these discussions and because metabolic rates and rates of tissue formation are so much a function of growth stage and overall growth rates, we fold considerations of physiology mostly within the concept of ontogeny. However, there is no perfect or clear-cut way to name or parse the “signals” that we recognize in bone tissue or to separate them cleanly from each other. We’ve chosen to highlight and isolate these four “signals” to help account for histological variation in bone tissues. Bone-tissue types may change as an animal ages; among species or morphs in an evolutionary lineage; among related species that differ in size; among bones in a skeleton that grow at different rates because they differ in size or mechanical load; and among individuals that differ in nutrition or other sorts of environmental stress. Hypotheses that attempt to explain an apparently peculiar histomorphology in a fossil bone or tooth should be formed in this context. These four signals are expected to have osteological correlates that are more easily examinable than other, more subtle signals. Once predictable associations are discovered among these more obvious divisions of signals , advances in less obvious signals of osteology can be made. These include how overall bone size and shape may influence its cross-sectional histology and even how population demographics and behavior may influence bone biology. This integrative approach is intended to stimulate new research programs that assess all of these “signals” and their influences...

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