They argued that mammalian homeothermy evolved in two steps.
Similarly, Heinrich (1977) suggests that homeothermy may allow enzyme specialization, which results in greater metabolic efficiency (see also Hochachka and Somero 1984).
In any case, as body size decreased and mass-specific metabolism increased, homeothermy based on thermal inertia was supplanted by endothermic homeothermy.
For example, if it could be shown from fossil indicators that high resting metabolic rate and endothermic homeothermy preceded high maximal aerobic capacities (and associated high stamina), then the aerobic capacity model would be falsified.
Standard metabolism of monotremes and the evolution of homeothermy.