Hall of Primitive Mammals
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.”
Dimetrodon was one of the earliest relatives of mammals. The large "sail" on its back may have been used for temperature regulation, to attract mates, or to frighten off other animals.
For millions of years, enormous, plant-eating glyptodonts lumbered across the Americas. Some species were as large as cars: up to 10 feet long. They were covered with thick armor, made from bone that grew from within their skin.
Giant ground sloths such as Lestodon lived in South America and became extinct 30,000 years ago. There is evidence that ground sloths and early humans used the same caves, but not necessarily at the same time.
Mammals have evolved to live on land, in the sea, and in the air. They move through their environments in many different ways: by hopping, running, flying, swimming, gliding, walking, climbing, and swinging from tree limb to tree limb.
Numerous remains of the Smilodon, or saber-toothed cat, have been found at the La Brea Tar Pits in Los Angeles. Scientists are unsure why this site attracted so many saber-toothed cats, but some suggest these cats may have been scavengers rather than hunters.