Attenuation evolution of mammals in the Cenozoic era

Mammals are the most morphologically variable of vertebrates, from giant whales to bumblebee bats
.
How they evolved to this level of variation has been a long-standing question, with much of the debate focused on the timing and pace of
evolutionary change.
Goswami et al.
, in a large dataset of extinct and living mammalian skulls, find that the rate of evolutionary change peaked near the Cretaceous-Paleogene boundary and waned since then (see Santana and Grosnick's views).

Certain lifestyles, such as aquatic habitats or herbivory, lead to faster changes, while in some species, such as rodents, morphological changes appear to be decoupled from taxonomic diversification

The diversification of Cenozoic placental mammals is the prototype
of adaptive radiation.
However, the difference between molecular divergence estimates and fossil fuels is still debated
around the timing, rhythm, and drivers of radiation.
Analysis of three-dimensional skull datasets of live and extinct placental mammals shows that the rate of evolution peaks early and decays quickly
.
Over the past 66 million years, this long-term decline in pace has been accompanied by an explosion of innovation that is decreasing
in magnitude.
Social, precocious, aquatic and grass-fed species evolved the fastest, especially whales, elephants, krakens and extinct ungulates
.
The slow growth rate of rodents and bats indicates a separation
of taxonomic and morphological diversity.
Dishearteningly, estimates of highly similar ancestral shapes for superorders with placental mammals suggest that their earliest representatives may continue to be unambiguously unidentified
.