Evolution of functional morphology of osteoderms across Crocodylomorpha
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The vertebrate integument is a complex anatomical system that has evolved a wide array of specialized structures that are related to an animal’s ecology and evolutionary history, including mineralized structures found in many clades and ecological niches called osteoderms. However, the development, functional morphology, and evolutionary history of osteoderms is still poorly understood in relation to other portions of the skeleton. Osteoderms have independently evolved multiple times and can be found in the skin of many vertebrate lineages, making up a significant portion of the evolutionary history of the tetrapod skeleton. Though several hypotheses of osteoderm function have been proposed based on their overall morphology, osteoderm function has rarely been the subject of experimental analyses and only in a few taxa. The major aims of this research are to test new and previously proposed hypotheses of osteoderms function and to better understand drivers of osteoderm morphological variation using Crocodylomorpha as the study group, both because of their long and rich history of ecological diversity and because of their exemplary fossil record compared with other osteoderm bearing taxa. Quantification of within-individual variation in extant crocodilians suggests that relaxed selection on osteoderm morphology only occurs in taxa thought to exhibit reduced intraspecific aggression, suggesting that osteoderms evolved in Crocodylia as a defensive structure in territorial disputes. This is corroborated by an ontogenetic analysis, which demonstrates that in at least some crocodilian species, osteoderm growth coincides with the onset of territorial behavior, contradicting previously proposed hypotheses that crocodilian osteoderms function in thermoregulation, in stabilization of the vertebral column, or as a defense against predators. In contrast with previous research that had demonstrated a reduction of body flexibility when osteoderms are present, a final comparative analysis across Crocodylomorpha found no correlation between osteoderm aspect ratio and locomotor ecology, suggesting that osteoderm dimensions have no impact on flexibility. This research disputes some previously proposed hypotheses of osteoderm function and when placed in context of other research, reveals that osteoderms may have different functions in different taxa.