Bursts in skull evolution weakened with time

The skull shapes of mammals diversified more rapidly early in their history Evolutionary biologists generally agree that placental mammals began as small-sized animals that ate insects, and later evolved to become more varied in size and morphology. Today, the morphologies of placental mammals are incredibly diverse, ranging from the bumblebee bat to the blue whale ( 1 – 3 ). There is considerable debate over how the small, insect-eating ancestors of placental mammals gave rise to such incredibly divergent lineages. Among the competing hypotheses are varying opinions on the timing and pace of early placental mammal evolution, especially in the context of major environmental changes such as the Cretaceous-Paleogene (K-Pg) mass extinction event 66 million years ago and the Paleocene-Eocene Thermal Maximum (PETM) 56 million years ago. On page 377 of this issue, Goswami et al. ( 4 ) contribute to this discussion by reconstructing the patterns and possible drivers of placental mammal morphological diversification with a quantitative analysis of skull shape spanning over 70 million years of evolution.