Dinosaur Physiology and the Origin of Mammals

Locomotion, activity patterns, and thermoregulation were closely related in the evolution of tetrapods. The short, stocky limbs of the first Carboniferous reptiles suggest that they were not heliophilic and were active in the shaded areas of the coal forests. The first archosaurs retained the sprawling locomotion and possibly the thermal preferences of primitive reptiles. However, by the Mid Triassic large, predatory archosaurs with more erect posture and very gracile, quick-action limbs appeared, and in these animals the metabolic scope and endogenous heat production during activity may have exceeded that of large living monitor lizards. Fully Erect Posture, permitting support of the body free of the ground for long periods of time, appeared in dinosaurs at the end of the Triassic. Dinosaurs probably combined large body size, high body temperature, large hatchling size, and continuous high levels of activity with an efficient aviantype air-sac system for loss of excess heat. Thus dinosaurs could achieve homeothermy in the warm Mesozoic climates. Birds were derived from dinosaurs, and, with the development of feathered insulation, could apply the dinosaurian mechanisms for homeothermy to small body size. Advanced mammal-like reptiles may not have been heliophilic, but specialized in reducing surface area to volume ratios, possibly to conserve heat. In the competition with Triassic archosaurs, the retention of sprawling locomotion and possibly the lack of efficient heat-loss mechanisms in therapsids caused their extinction. However, with the development of superficial insulation of hair and high endogenous heat production, the first mammals could maintain their activity temperatures all through the night and could exploit nocturnality more extensively than nocturnal lizards, while avoiding activity during the day and thus reducing the dangers of overheating and of contact with the large predatory diurnal lizards and dinosaurs. Mesozoic mammals were restricted to small body size partly because of the difficulty of losing excess body heat in a large mammal lacking efficient evaporative cooling, but mostly because mammals were competitively inferior to dinosaurs during the day and were forced to seek secure diurnal shelters in trees and burrows to escape the great reptiles.