![]() Another is dissolution of the mineral by storage in formalin or by prior use of an acidic cartilage stain ( Wagemans and Vandewalle, 2001 Grünbaum et al., 2003 Walker and Kimmel, 2007 Werneburg et al., 2015). One explanation for no staining is that the bone is still osteoid or not yet mineralized. However, μCT fails to resolve cartilage and small bones ( Werneburg et al., 2015), and early appearing bones and portions of otherwise stained bones can also fail to stain with Alizarin red ( Rieppel, 1993a Cubbage and Mabee, 1996 Mabee and Trendler, 1996 Mitgutsch et al., 2011). Sectioned specimens ( Hanken and Hall, 1988a) and calcein-stained whole mounts ( Schreiber, 2006) can give earlier results, and micro-computed tomography (μCT) gives comparable results ( Polachowski and Werneburg, 2013), but with the advantages of producing digital 3D images, and being faster, less size-limited and nondestructive, though caution must be taken to not dry out small specimens. Caveats About Collecting and Interpreting Ossification Sequencesĭetermining when exactly a bone appears depends on how it is visualized, which has been done typically in Alizarin red-stained, glycerin-cleared whole mounts. to present hypotheses and evidence that advocate for a new “whole-animal, whole-ontogeny” approach to understand the developmental and evolutionary significances of these data. to use frogs and salamanders to question the prevailing adult function- and embryogeny-based approaches for interpreting ossification sequences, and 3. to summarize caveats about collecting and using ossification sequence data, 2. While summarizing this research is beyond the scope of this paper, amphibians stand apart from other vertebrates for having exceptionally variable and late-occurring bone formation, as well as highly plastic rates of larval growth and development and high ranges in larval period. Amphibians feature prominently in this research owing to the large number of species for which these data have been collected (over 60 at last count) and their biphasic life cycles, which entail dramatic transformations in morphology and behavior midway through life and have supported remarkable behavioral and ecological diversification. Ossification sequence, meaning the order by which bones appear in a species, has recently become the subject of much evo-devo study and phylogenetic analysis. I also argue that understanding plasticity in ossification requires shifting focus away from embryonic development and adult function, and toward postembryonic mechanisms of regulating skeletal growth, especially ones that respond directly to midlife environments and behaviors. I discuss evidence and hypotheses for how hormone mediation and calcium physiology might elicit non-adaptive variability in ossification sequence, and for adaptive strategies to partition larval habitats using bone to offset the buoyancy created by lung use. Amphibians demonstrate a need for a whole-animal, whole-ontogeny approach that integrates the entire ossification process with physiology, behavior and ecology. Amphibians accentuate these concerns because of their highly specialized biphasic life histories and the exceptionally late timing, and high variability of their ossification sequences. Also, the singular focus on bone appearances and the omission of other tissues and behavioral, ecological and life history events limit the relevance of such analyses. However, questions remain about how to detect and order bone appearances, the adaptive significance of ossification sequences and their relationship to adult function, and the utility of categorizing bones by embryonic origin and type. Recent evo-devo research has used ossification sequences to compare skeletal development among major groups, to identify conserved and labile aspects of a sequence within a group, to derive ancestral and modal sequences, and to look for modularity based on embryonic origin and type of bone. Skeleton plays a huge role in understanding how vertebrate animals have diversified in phylogeny, ecology and behavior. ![]() Biology Department, James Madison University, Harrisonburg, VA, United States.
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