“Respiratory” turbinates in endotherms and ectotherms

For thirty years respiratory turbinates have been hailed as unequivocal indicators of endothermy in mammals and birds, but their validity as osteologic correlates of metabolic rate is limited to extant mammals. The nasal cavity contributes to heat and water conservation in extant birds, but their trachea also plays an important role. Efficiency of countercurrent heat exchange in the avian trachea increases when cartilaginous conchae are surgically removed. Respiratory turbinates and ectothermy are not mutually exclusive ‐ recent re‐examination of the nasal cavity of extant crocodilians reveals presence of a prominent turbinate‐like preconcha. Vascularised surface area of the preconcha in the American alligator scales with positive allometry. A larger‐than‐predicted preconcha provides additional respiratory surface for evaporative cooling in bigger animals, in which cutaneous heat loss is made difficult by the reduced surface area:volume ratio. Thermographic imaging and in vivo measurements of upper respiratory tract temperatures in juvenile alligators provide experimental evidence that the nasal cavity acts as a site of heat dissipation. Heat loss and selective brain cooling, not heat and water conservation, may have been the original selective pressure driving the evolution of respiratory turbinates in archosaurs.