Looking cross a large dataset of extinct and extant mammalian skulls, the rate of evolutionary change peaked around the time of the Cretaceous-Paleogene boundary and has general tapered off since then

Attenuated evolution of mammals through the Cenozoic. Anjali Goswami et al. Science, Oct 27 2022, Vol 378, Issue 6618, pp. 377-383. DOI: 10.1126/science.abm7525

Becoming diverse: Mammals have the greatest degree of morphological variation among vertebrate classes, ranging from giant whales to the tiny bumblebee bat. How they evolved this level of variation has been a persistent question, with much debate being centered around the timing and tempo of evolutionary change. Goswami et al. looked across a large dataset of extinct and extant mammalian skulls and found that the rate of evolutionary change peaked around the time of the Cretaceous-Paleogene boundary and has general tapered off since then (see the Perspective by Santana and Grossnickle). Certain lifestyles, such as aquatic habitats or herbivory, led to faster change, whereas in some species such as rodents, morphological change appeared to be decoupled from taxonomic diversification. —SNV

Abstract: The Cenozoic diversification of placental mammals is the archetypal adaptive radiation. Yet, discrepancies between molecular divergence estimates and the fossil record fuel ongoing debate around the timing, tempo, and drivers of this radiation. Analysis of a three-dimensional skull dataset for living and extinct placental mammals demonstrates that evolutionary rates peak early and attenuate quickly. This long-term decline in tempo is punctuated by bursts of innovation that decreased in amplitude over the past 66 million years. Social, precocial, aquatic, and herbivorous species evolve fastest, especially whales, elephants, sirenians, and extinct ungulates. Slow rates in rodents and bats indicate dissociation of taxonomic and morphological diversification. Frustratingly, highly similar ancestral shape estimates for placental mammal superorders suggest that their earliest representatives may continue to elude unequivocal identification.