The Hall of Primitive Mammals, one of two halls in the Lila Acheson Wallace Wing of Mammals and Their Extinct Relatives, traces the lower branches of the evolutionary tree of mammals, including monotremes, marsupials, sloths, and armadillos.

The roots of the mammalian line reach back almost 300 million years. Some of the very early mammal relatives dominated the landscape millions of years before dinosaurs appeared, and most of these species became extinct. During the age of dinosaurs, most mammals were not much bigger than small rodents. It was after the extinction of the large dinosaurs that the great diversity of mammals arose.

This hall highlights the development of such key mammalian physical features as the synapsid opening in the skull, a large hole behind the eye socket for muscles that extend to the jaw; three middle ear bones; and the placenta. These traits correspond to eating, hearing, and reproduction, and each trait represents the splitting off of an evolutionary branch. Animal groups represented in this hall include monotremes, multituberculates, triconodonts, edentates, and extinct relatives of mammals, such as the Dimetrodon and glyptodonts. Some living animals from these groups, such as the egg-laying mammal platypus, a monotreme, are called “living fossils.”