The Evolution of Endothermy in the Phylogeny of Mammals

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Abstract
The evolution of the endothermy of mammals can be traced to the ectothermy of early reptiles. It is suggested that small endotherms cannot be directly derived from small ectotherms because of the requirement for the simultaneous change in thermal conductance and the rate of metabolism. Instead, small ectotherms probably gave rise to large ectotherms (thereby increasing thermal inertia), a fur coat may have been added to increase thermal stability, and a decrease in size with only a modest decrease in the total rate of metabolism converted "inertial" homoiothermy to endothermy. Measurements of skull size in cynodont therapsids conform to this suggestion. A secondary palate is formed in cynodonts during the decrease in body size and is interpreted as a correlate of increased ventilation rates produced by a decrease in size and a conversion of ectothermy to endothermy. A decrease in size and the formation of a secondary palate also occurred in bauriamorphs, which suggests that they too independently evolved endothermy, although it may not have been as effective as that developed in cynodonts. Other therapsids apparently were ectotherms, although a large body size may have contributed to an "inertial" homoiothermy. It is concluded that many of the characteristics that distinguish mammals from reptiles, including endothermy, viviparity, and even lactation, may be related to the marked decrease in body size that occurred in the evolution of mammals from advanced therapsids. The reduction in body size combined with an appreciable temperature independence may have permitted cynodonts to exploit nocturnal insects as a food resource.