Short answers, pulled from the story.
Mammals are vertebrate animals of the class Mammalia characterized by milk-producing mammary glands, a broad neocortex region of the brain, fur or hair, and three middle ear bones. No animal other than a mammal has hair, making it a definitive characteristic of the class.
Around 6,640 extant mammal species have been described and divided into 27 orders. Research published in the Journal of Mammalogy in 2018 put the number of recognized mammal species at 6,495, including 96 recently extinct.
The first mammals in Kemp's sense appeared in the Late Triassic epoch, about 225 million years ago, 40 million years after the first therapsids. They originated from cynodonts, an advanced group of therapsids, during the Late Triassic to Early Jurassic.
The five species of monotreme, the platypus and the four species of echidna, lay eggs rather than giving birth to live young. Their eggs are leathery and uncalcified, and a platypus has sex chromosomes more like those of a chicken than those of a therian mammal.
Mammals range from the 30 to 40 millimeter bumblebee bat to the 30 meter blue whale, possibly the largest animal ever to have lived. Maximum lifespan ranges from two years for the shrew to 211 years for the oldest recorded bowhead whale.
The three largest mammal orders by number of species are the rodents, bats, and eulipotyphlans, which include hedgehogs, moles, and shrews. The next three are the primates, the even-toed ungulates, and the Carnivora.