Nasal turbinates and the evolution of mammalian endothermy Public Deposited

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  • Although endothermy is one of the most significant evolutionary developments in the vertebrates, its origins among extinct taxa have traditionally been difficult to determine. Endothermy is primarily an attribute of the "soft anatomy," and its key features, such as complex lungs, elevated blood oxygen carrying capacity and mitochondrial density, do not fossilize. Previously, only one preservable morphological feature, the presence of complex nasal turbinates, has, anecdotally, been considered very suggestive of endothermy in the ancestry of mammals. This thesis examines the functions of the nasal turbinates of extant mammals with respect to physiological characteristics of endothermy. The fossil record of nasal turbinates is also studied. Nasal turbinates serve two distinct functions: olfaction and conditioning of respiratory air. The respiratory turbinates have previously been found to reduce respiratory water loss in desert mammals. Experimental data presented here indicate that these structures also substantially reduce respiratory water loss in non-desert mammals. These data support the conclusion that respiratory turbinates represent an adaptation to reduce desiccation associated with high pulmonary ventilation rates, and may have evolved in association with the origin of elevated ventilation rates and endothermy. Conversely, no particular correlation with endothermy exists for the olfactory turbinates. In extant mammals, the nasal turbinates attach to ridges along the lateral walls of the nasal cavity, which present a characteristic pattern. Studies of similar ridges in the nasal cavities of the ancient mammals and their ancestors, the mammal-like reptiles (Therapsida), indicate that respiratory turbinals first appear in two groups of advanced therapsids, Therocephalia and Cynodontia. This suggests that the evolution of "mammalian" oxygen consumption rates may have begun as early as the Late Permian, 260 million years ago, and developed largely independently in therocephalians and cynodonts. Full mammalian endothermy may have taken as much as 40 to 50 million years to develop.
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