On Earth today, mammals, particularly humans, are a common life form in many different habitats. But mammals were not always common or diverse. The first mammals, which evolved during the middle Triassic (about 240 million years ago), were small compared to many of their contemporaries, including the earliest dinosaurs. No larger than shrews or mice, these early mammals probably lived in burrows or other refuges when dinosaur and other reptilian predators roamed the Earth. Though they remained relatively small, mammals evolved to fulfill many ecological roles during the long reign of the dinosaurs. When dinosaurs (except birds) disappeared at the extinction event 66 million years ago, mammals survived and began to expand into ecological spaces (niches) vacated by the larger dinosaur herbivores and predators. By about 55 million years, ago, all modern groups of mammals had evolved. The Cenozoic (the Era in which we live) is known as the Age of Mammals, thanks to their successful radiation into a huge variety of habitats on Earth.

Tetrapods (from tetra = “four”, and pous = “foot”) include amphibians, reptiles (including dinosaurs), birds, and mammals. The first well-documented tetrapod remains are known from the latest Devonian (about 360 million years ago) although tracks belonging to this group are known from rocks that are 395 million years old. Scientists agree that tetrapods evolved from a group called the lobe-finned fishes. Fins turned into legs even before the first tetrapods moved onto land. Lobe-finned fishes developed the first lungs, allowing them to breathe air. The first tetrapods still had both gills and lungs, suggesting a gradual transition from breathing under water to air-breathing. The move to land also required a stronger backbone to support the body while walking. Among present-day animals, lungfishes are the closest living relatives of tetrapods.