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The Hall of Mammalian Evolution
at the Yale Peabody Museum
The fossils in this hall, from the Yale Peabody Museum’s collections in the Division of Vertebrate Paleontology, belong to the main groups of mammals that evolved during the Cenozoic Era (65 million years ago to the present), after the extinction of the dinosaurs. During this time mammals diversified into a wide variety of carnivorous, herbivorous and omnivorous forms. Overlooking the hall is the 60-foot mural The Age of Mammals, painted by Rudolph F. Zallinger in 1964, which dramatically illustrates the evolution of North American mammals during the Cenozoic Era.
Some of the largest fossils on display in Hall of Mammalian Evolution include the Otisville Mastodon (Mammut americanus) and the huge Brontotherium, both acquired in the 19th century by Yale’s O.C. Marsh.
At right: The Otisville Mastodon (skull)
Mammut americanus
YPM catalog no. 12600
The discovery of this skeleton in 1872 near Otisville, New York, recevied considerable attention in the newspapers of the time.
Other outstanding specimens include Smilodon californicus, a saber-toothed cat from the La Brea Tar Pits in California, and an 11,000-year-old ground sloth (Nothrotherium shastense) from New Mexico that still retains pieces of skin and hair, a rarity in a fossil of that age.
Left: Skull of the saber-toothed
Smilodon californicus
According to the fossil record the first mammals, shrew-like and probably nocturnal, appeared about 200 million years ago during the Mesozoic Era (245 to 65 million years ago). Throughout this era the dominant terrestrial animals were dinosaurs, and mammals remained small, such as Cimolestes depicted in the Cretaceous Period section of The Age of Reptiles mural in the Museum’s Great Hall. While the specimens in the Hall of Mammalian Evolution show that some lines of mammals evolved to great sizes, still nearly half the 4,000 or so living mammal species are small rodents.
Present-day mammals are distinguished from other groups of living animals by several readily observable features, including milk glands and hair or fur. But such evidence is usually not preserved in fossilized remains, so paleontologists mostly use unique skeletal characteristics to separate mammals from other types of ancient animals. Among these features, common to all mammals whether living or extinct, are a lower jaw that is a single bone, and the presence of 3 bones in the middle ear.
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