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The Cretaceous-Paleogene (K-Pg) extinction event is a pivotal moment in Earth's history, marking the end of the Mesozoic Era approximately 66 million years ago. During this cataclysmic event, it is believed that an asteroid struck the planet, causing widespread environmental changes including forest fires, acid rain, and a significant drop in global temperatures due to the release of particulates into the atmosphere that blocked sunlight.

The immediate aftermath of this event saw the extinction of about three-quarters of the plant and animal species on Earth, most notably the non-avian dinosaurs. This massive loss of life opened the stage for one of the most significant evolutionary transformations: the rise of mammals in the early Cenozoic era, signaling the dawn of a new epoch wherein mammals would come to dominate the terrestrial ecosystem.

The evolutionary change in mammals following the K-Pg extinction event is a remarkable example of adaptive radiation, where a single or small group of ancestral species diversifies into a wide variety of forms. With the exit of the dinosaurs, many ecological roles were left unoccupied. Small, nocturnal, and generally inconspicuous mammals that had survived the extinction began to exploit these vacant niches.

As time progressed into the Cenozoic era, mammals started to experience dramatic changes. They grew in size, developed new dietary habits, and adapted to various environments. The evolutionary pressures following the extinction event allowed for diverse adaptations in mammals, including the development of different teeth structures for herbivores and carnivores, limbs adapted for climbing, running or swimming, and complex social behaviors, all exemplified in the array of species that are present today.

An ecological niche refers to the role and position a species has in its environment; it encompasses how a species meets its needs for food and shelter, how it survives, and how it reproduces. In the context of mammalian evolution in the Cenozoic era, the concept of ecological niches is crucial.

Following the K-Pg extinction event, the niches that had been dominated by dinosaurs for millions of years became available. Mammals began to occupy these niches, which led to significant diversification. Some took to the trees, others to the oceans, and many to different terrestrial habitats. This diversification was not random but a response to the selective pressures of their respective environments: food availability, predation pressure, and climatic conditions. Over millions of years, this led to the evolution of the vast array of mammalian species we see today, from the elephants and lions of the savannas to the bats in our night skies and the whales in the oceans.