Final answer:
Mammal diversity increased after the Cretaceous due to their ability to fill vacant ecological roles left by dinosaurs. The cooler climate favored warm-blooded mammals, and new food sources like flowering plants aided in their diversification. This adaptive radiation resulted in the numerous mammalian species present today.The correct option is D.
Step-by-step explanation:
The increase in mammal diversity shortly after the Cretaceous was primarily due to Option D: Mammals diversified to take advantage of ecological roles that were previously filled by the dinosaurs.
When the dinosaurs went extinct at the end of the Cretaceous period, about 65 million years ago, many ecological niches became vacant.
Mammals, being endothermic, were better suited to the cooler climates that followed the mass extinction event caused by a meteor impact and ensuing volcanic activity, which led to widespread environmental changes.
Additionally, the evolution of flowering plants provided new food sources, and changes in Earth's geography created more isolated environments, reducing competition.
During the Cenozoic Era, mammals began filling niches once occupied by dinosaurs. Early mammals were already diversifying due to the less competitive environments and available resources.
With dinosaurs no longer dominating the ecosystems, mammals could evolve into various forms, including larger sizes, and exploit new food sources and habitats. This adaptive radiation led to the wide variety of mammalian species we see today.The correct option is D.