Final answer:
Mammals replaced reptiles as the dominant form of land vertebrate during the Cenozoic Era, when the extinction of the dinosaurs opened up new ecological niches for mammals to occupy.
Step-by-step explanation:
The dominance of land vertebrates shifted from reptiles to mammals during the Cenozoic Era, also known as the age of mammals. The extinction of the dinosaurs around 65 million years ago allowed mammals to fill the ecological niches that were previously occupied by dinosaurs. As a result, mammals diversified and became the dominant form of land vertebrates.