Short answers, pulled from the story.
The bowhead whale holds the record for the longest lifespan of any mammal, living up to 211 years. This longevity dwarfs the two-year existence of the common shrew, which represents the shortest lifespan among mammals.
The earliest clear evidence of a jaw joint formed solely by the squamosal and dentary bones comes from the fossil Hadrocodium, dated to approximately 195 million years ago in the early Jurassic. The oldest-known fossil among the Eutheria is the small shrewlike Juramaia sinensis, dated to 160 million years ago in the late Jurassic.
There are around 6,640 extant species of mammals, divided into 27 orders. The largest groups are rodents, bats, and eulipotyphlans, followed by primates, even-toed ungulates, and carnivores.
The monotremes are the only mammals that lay eggs, with five species existing today. These animals retain ancestral features such as the cloaca and produce leathery, uncalcified eggs similar to those of reptiles and birds.
Mammals are distinguished by the presence of milk-producing mammary glands, a broad neocortex region of the brain, fur or hair, and three middle ear bones. Their red blood cells lack a nucleus to make space for more haemoglobin, and their lungs are spongy and honeycombed with breathing achieved mainly by the diaphragm.
The word mammal is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758. This name is derived from the Latin mamma, meaning teat or pap, and Linnaeus initially defined the class during that year.