Morphological disparity of mammalian limb bones throughout the Cenozoic: the role of biotic and abiotic factors

Mammals exhibit ecology-related diversity in long bone morphology, revealing an ample spectrum of adaptations both within and between clades. Their occupation of unique ecological niches in postcranial morphology is thought to have occurred at different chronological phases in relation to abiotic factors such as climate and biotic interactions amongst major clades. Mammalian morphologies rapidly evolved throughout the Cenozoic, with several orders following different paths in locomotory adaptations. We assessed morphological variation in limb proportions for a rich sample of extant and fossil large mammalian clades (mainly carnivores and ungulates) to test associations with ecological adaptations and to identify temporal patterns of diversification. Phylogenetic relationships among species were incorporated into the analysis of limb bone proportions, showing significant morphological changes in relation to species substrate preference. Major climatic events appeared to have no temporal impact on patterns of morphological diversification, expressed as morphological disparity, in either clades or ecological groups. Linear stochastic differential equations supported a double-wedge diversification model for limb proportions of carnivorous clades ('Creodonta' and Carnivora). The concomitant increase in morphological disparity throughout the Cenozoic for the orders Carnivora and Artiodactyla had a significative impact on the disparity of Perissodactyla supporting biotic interaction as primary driver of mammalian morphological diversification. Our findings challenge the classic idea of abiotic factors as primary driving forces in the evolution of postcranial morphologies for large terrestrial mammals, and propose clade competition as a key factor in temporal diversification.