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3. Selection of Specimens
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35 3 Selection of Specimens Kevin Padian, Ellen-Thérèse Lamm, and Sarah Werning For the student of histology, there is always something interesting to be found in the examination of the microscopic structure of any tissue. In consideration of the time and energy it takes to produce thin-section slides, as well as the use of a limited resource when sectioning fossil material, it is important to define your questions and plan the details and full scope of all projects in advance of slicing. Your investigation, when well planned, will also be more rewarding and more interesting to other researchers, as well as easier to justify to the curator or collection manager when asking for permission to section specimens. Here we consider several principal factors for you to define in the selection of specimens: the relevance to a given research question; which specimens to choose and why; where to make sections; and how to make best choices overall with respect to available materials. RELEVANCE TO RESEARCH QUESTIONS The research question directs the selection of specimens. When the research question is focused, the selection of specimens will become much clearer. For example, experience has shown that to determine the approximate age or ontogenetic stage of a given specimen, larger limb bones generally preserve the most complete record of skeletochronology (Stein & Sander 2009). In most tetrapods, the largest limb bones are the humerus, femur, tibia, and fibula. These elements tend to grow faster than smaller bones, and they undergo less reworking and often less erosion, particularly toward the medullary cavity (Stein & Sander 2009). Therefore, a more complete series of bone growth marks is often preserved. These features reflect 36 Selection of Specimens annual growth inflections from which age may be estimated or retrocalculated (e.g., Horner & Padian 2004; Bybee et al. 2006; Lee & Werning 2008). Generally speaking, the midshaft is the best place to take samples, because there will be less occurrence of other types of bone, such as metaphyseal bone, and other tissues such as calcified cartilage, as well as less remodeling because of muscle scarring . (A caveat: When midshaft trochanters develop through ontogeny, cortical drift and the centrifugal growth of the trochanter can produce unusual growth features, so it is important to examine a full section of this part of the shaft; sections adjacent to the trochanter may also be useful.) Moreover, the periosteal bone in this section of the shaft has been deposited for a longer time than the bone toward the ends. If the research question involves any sort of histological comparison among individuals or taxa, the researcher is best able to make comparisons when as many variables are constrained as possible. Bone-tissue type varies with many factors, including ontogenetic stage, skeletal element, and sampling position. Comparing “like to like” is the best way to ensure that one is clearly addressing the original biological question and that any signal detected is not conflated with others. For example, if a researcher is attempting to describe the tissue organization and growth rate differences between two taxa, sampling the same element of two similarly aged individuals is preferable because ontogenetic and positional differences are less likely. Variations between the taxa are therefore more likely the result of actual biological distinctions, rather than sampling differences. Sometimes, the use of rare specimens or taxa represented by one or only a handful of specimens is critical for a question, but the possibility of sampling homologous bones or similarly aged animals is eliminated because of preservation, completeness, or accessibility issues. In these cases, it may still be possible to make some limited comparisons, but interpretations need to be cautious and conservative , taking into account all possibilities for differences in data. As in all scientific fields, it is best for investigators to isolate the variable they wish to examine and to hold constant all other sources of variation that could affect their analysis. We now examine the most important sources of variation for the paleontologist to take into account when choosing to use histology as a tool to answer critical questions in paleontology. One must understand the individual and collective influences of these variables in order to select specimens that best answer the question being explored. Sources of Histological Variation Histologic variation within a single element. Thin sections taken at various places within a bone will help to characterize the kinds of hard tissues and other features that were formed in different regions of the bone, sometimes at different times. These tissues reflect...