What adaptations allowed mammals to dominate the Cenozoic era?

The Mesozoic Era gives evidence of being uniformly warm with high levels of Carbon Dioxide and humidity. Large cold blooded or poikilothermic organisms were able to thrive in the environment where there was little variation in temperature.

The Cenozoic Era which is the present Era has sometimes extreme temperature changes. Cold blooded animals are at a disadvantage when the temperature is cold. As the environment changed to having periods of extreme cold the warm blooded or homeothermic animals were able to better adapt to the environment.

Mammals being warm blooded were better able to adapt to the environmental conditions of the Cenozoic Era.

You can read more about mammals and the Cenozoic here.

Because of a variety of adaptations that allowed them to occupy a wide range of ecological niches, including the ability to regulate body temperature, diversified dentition for different diets, enhanced brain complexity, and reproductive strategies like live birth and parental care, mammals dominated the Cenozoic era.