Several mutually incompatible theories exist about how and why endothermy evolved in mammals and birds. Some take the primary function to have been thermoregulation, selected for one adaptive purpose or another. Others take the high aerobic metabolic rate to have been primary. None of these theories is incontrovertibly supported by evidence, either from the fossil record of the synapsid amniotes or from observations and experiments on modern organisms. Furthermore, all are underpinned by the tacit assumption that endothermy must have evolved in a stepwise pattern, with an initial adaptive function followed only later by the addition of further functions. It is argued that this assumption is unrealistic and that the evolution of endothermy can be explained by the correlated progression model. Each structure and function associated with endothermy evolved a small increment at a time, in loose linkage with all the others evolving similarly. The result is that the sequence of organisms maintained functional integration throughout, and no one of the functions of endothermy was ever paramount over the others. The correlated progression model is tested by the nature of the integration between the parts as seen in living mammals, by computer simulations of the evolution of complex, multifunctional, multifactorial biological systems, and by reference to the synapsid fossil record, which is fully compatible with the model. There are several potentially important implications to be drawn from this example concerning the study of the evolution of complex structure and the new higher taxa that manifest it.

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The study concerns an incomplete skull and postcranial skeleton of the traversodontid cynodont Luangwa drysdalli Brink, from the Middle Triassic Ntawere Formation of Zambia. The incisor teeth had cutting edges maintained by tooth abrasion, and the post‐canines functioned more by a puncture‐crushing mechanism than by shearing. The jaw muscles, in conjunction with the teeth, were arranged to avoid stresses at the jaw articulation. Both the region of the orbit, and the braincase show certain tendencies towards mammalian structure. The forelimb operated in a primitive, sprawling fashion. The equivalent of the therian supraspinatus muscle had evolved, but not the infraspinatus. The hindlimb was more or less erect, and the hip musculature was very close to mammalian. Expanded costal plates were still present on the posterior thoracic and lumbar ribs, and functioned to maintain the vertebral column virtually rigid.