Skull variation reveals “attenuated evolution” of mammals across the Cenozoic

Mammal species diversified more rapidly early in their history – peaking roughly 66 million years ago – before a long-term decline in diversification punctuated by diminishing bursts of evolutionary innovation across the Cenozoic, according to a new study. This newly described macroevolutionary pattern, termed “attenuated evolution” by the authors, provides new insights into how placental mammals became one of the most morphologically diverse groups of animals on Earth. The first placental mammals to roam the Earth were small – likely no larger than a mouse or shrew. However, as we know them today, mammals have evolved into a stunning array of species with an immense variation of ecology and morphology, ranging from the bumblebee bat, which is no larger than a human thumb, to the ocean-bound, 100-foot-long blue whale. However, how this degree of variation came to be has been a longstanding question among evolutionary biologists and the timing, tempo, and evolutionary drivers remain poorly understood. Using a dataset comprised of three-dimensional skull shapes for 322 living and extinct mammal species, Anjali Goswami and colleagues show that the rate of evolutionary change among creatures peaked around the time of the Cretaceous-Paleogene extinction and, outside of punctuated bursts of evolutionary innovation, has largely tapered off over the last 66 million years. Goswami et al.’s analysis also revealed that certain lifestyles, including social, aquatic, and herbivorous species evolved the fastest, while other species, like rodents, primates, and bats diversified far slower. “By examining the skull shape, which encompasses many anatomical traits, the study simultaneously examined a multitude of ecological and functional drivers of diversification,” write Sharlene Santana and David Grossnickle in a related Perspective. “The breadth of the sample size analyzed by Goswami et al. is also impressive, containing species across all of Placentalia, rather than more restricted taxonomic groups, as is often the case for most studies on morphological evolution.”