The first true mammals appeared around 200 million years ago, during the Late Triassic period, while dinosaurs were just beginning their ascent. These early mammals were tiny -- most no larger than a modern shrew -- and largely nocturnal, skulking through the undergrowth to avoid the reptilian predators that dominated the daylight hours. Species like Morganucodon and Megazostrodon possessed the defining mammalian traits: fur, warm-blooded metabolism, and specialized teeth.
Living in the shadow of dinosaurs for over 130 million years, early mammals were far from passive. They diversified into a remarkable array of ecological niches. Castorocauda was a beaver-like swimmer. Volaticotherium was a glider, like a flying squirrel. Repenomamus was large enough to prey on small dinosaurs. The Mesozoic was no "dark age" for mammals -- it was a crucible of innovation that forged the traits we see across mammalian lineages today.
Three revolutionary adaptations defined the mammalian lineage. Warm-blooded (endothermic) metabolism allowed sustained activity regardless of ambient temperature. Fur provided insulation, making this metabolic strategy efficient. And mammary glands -- the defining feature of the class Mammalia -- enabled mothers to nourish their young with nutrient-rich milk, giving offspring a critical survival advantage in their earliest, most vulnerable days.
When the asteroid impact 66 million years ago wiped out the non-avian dinosaurs, mammals were perfectly positioned to fill the vacated ecological roles. Within just 10 million years of the extinction, mammals had exploded in size and diversity, evolving into the whales, bats, elephants, primates, and thousands of other forms that populate the planet today. The meek had inherited the Earth.