Weighing in on miniaturization: New body mass estimates for Triassic eucynodonts and analyses of body size evolution during the cynodont‐mammal transition

Abstract

Body size influences most aspects of an animal's biology, consequently, evolutionarydiversification is often accompanied by differentiation of body sizeswithin a lineage. It is accepted that miniaturization, or the evolution ofextremely small body sizes, played a key role in the origin and early evolutionof different mammalian characters in non-mammaliaform cynodonts. However,while there are multiple studies on the biomechanical, behavioral, andphysiological consequences of smaller sizes, few explore the evolutionary processesthat lead to them. Here, we use body mass as a universal size measure-ment in phylogenetic comparative analyses to explore aspects of body sizeevolution in Cynodontia, focusing on the cynodont-mammal transition, andtest the miniaturization hypothesis for the origin of Mammaliaformes. We estimated the body masses of 29 species, ranging from Theriocephalia to Mammaliaformes,providing the largest collection of Triassic cynodont body massestimates that we know of, and used these estimates in analyses of disparitythrough time and RR . Unexpectedly, our results did not support the miniaturizationhypothesis. Even though cynodont body size disparity fell during theLate Triassic, and remained lower than expected under a purely Brownianmotion model of evolution up until the Early Jurassic, we found that rates ofbody size evolution were significantly lower in prozostrodontians leading tothe first Mammaliaformes than in other lineages. Evolution rates were higherin medium and large-sized taxa, indicating that size was changing more rapidlyin those lineages and that small sizes were probably a persistent plesiomorphiccharacter-state in Cynodontia.