he Lila Acheson Wallace Wing of Mammals and Their Extinct Relatives—which includes the Hall of Primitive Mammals and the Paul and Irma Milstein Hall of Advanced Mammals—tells a fascinating tale of great diversification, sudden extinctions, and the forces that determine the success and obliteration of life. Mammals evolved at nearly the same time as the first dinosaurs, and the roots of the mammalian line reach back almost 300 million years. Some of the very early relatives of mammals, creatures resembling enormous lizards with giant fins along their backs, actually lived millions of years before the dinosaurs and dominated the land. Most of them then became extinct, and during the age of dinosaurs none of the mammals got much larger than small rodents. After the extinction of the large dinosaurs, the great diversity of mammals arose that we see today, including both primitive and advanced species.

The Hall of Primitive Mammals traces the lower branches of the evolutionary tree of mammals, including such features as the synapsid opening in the skull (a large hole behind the eye socket for muscles that extend to the jaw, found also in early relatives of mammals), the three middle ear bones (used to classify all mammals), and the placenta. These traits correspond to eating, hearing, and reproduction functions, and each represents an evolutionary branch. Included in this hall are monotremes, marsupials, sloths, and armadillos. Some living animals from these groups, such as the platypus, have so many primitive features that they are called "living fossils."

Learn more about the Fossil Halls.