Mammals are unique in the animal kingdom by possessing endothermy (warm-bloodedness), as well as a region of the brain known as the neocortex, the ossicle bones of the ear, the bodily integument known as "hair," and specialized sweat glands that produce milk in females for their offspring. These glands are called "mammaries," and it is from this word that mammals get their name. Mammals are most basically broken down into three groups: the primitive, egg-laying monotremes (the smallest group), the marsupials (which give birth to undeveloped live young that further facilitate development within a pouch on its mother's abdomen), and the largest group, the placentals (which give birth to developed live young). Mammals owe their origin to the "mammal-like reptiles" of the Carboniferous Period known as synapsids. It would be during the Triassic period that the synapsid group known as mammals would silently evolve alongside and in the shadow of the ruling vertebrates known as dinosaurs. Following the extinction event at the end of the Cretaceous period that extinguished the non-avian dinosaurs (approximately 66 million years ago), the mammals finally gained the opportunity to fulfill newly opened ecological niches and evolve into several new forms. Today they rule much of our world; flying in the form of bats, swimming in the form of whales, and dominating the earth as the most successful mammal of all time: humans.