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Osteoderms are uncommon inmammals, although they have occurred in manyxenarthrans (armadillos and the extinctglyptodonts andmylodontidground sloths). The heavy, bony osteoderms have evolved independently in many different lineages.[1] The armadillo osteoderm is believed to develop in subcutaneous dermal tissues.[2] These varied structures should be thought of as anatomical analogues, nothomologues, and do not necessarily indicatemonophyly. The structures are however derived fromscutes, common to all classes ofamniotes and are an example of what has been termeddeep homology.[3] In many cases, osteoderms may function as defensivearmor. Osteoderms are composed ofbone tissue, and are derived from a scleroblastneural crest cell population during embryonic development of the organism. The scleroblastic neural crest cell population shares some homologous characteristics associated with thedermis.[4] Neural crest cells, through epithelial-to-mesenchymal transition, are thought to contribute toosteoderm development.[2]
The osteoderms of modern crocodilians are heavily vascularized,[5] and can function as both armor and as heat-exchangers,[6] allowing these large reptiles to rapidly raise or lower their temperature. Another function is to neutralizeacidosis, caused by being submerged under water for longer periods of time and leading to the accumulation of carbon dioxide in the blood.[7] The calcium and magnesium in the dermal bone will release alkaline ions into the bloodstream, acting as abuffer against acidification of the body fluids.[8]
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