What is a Mammal?
Before exploring more extremes, visitors are introduced to the basics of mammal evolution and biology. There are more than 5,400 mammal species alive today, classified into 20 different groups, called orders.
About 300 million years ago, the evolutionary branch of the tree of life that includes mammals split off from the branch containing reptiles.
For over 130 million years, mammals lived side by side with dinosaurs. Some early mammal relatives have even been mistaken for dinosaurs, such as the sail-back synapsid Dimetrodon, a fossil featured in this section.
The fossilized skull of the more mammal-like Cynognathus shows the specialized teeth of early mammal ancestors, but not all the characteristics of the living groups of mammals.
To be classified as a mammal, a species must also nurse their young with milk; have three middle-ear bones; use a diaphragm for breathing; have a secondary palate that allows simultaneous eating and breathing; and maintain a warm, stable body temperature.
What is Extreme?
The skeletons of Uintatherium (the first giant mammal that evolved after large dinosaurs became extinct), an opossum, and a cast of a human skeleton illustrate a range of combinations of "normal" and "extreme" mammalian qualities.
While many opossum features are normal for mammals, like body size, the Virginia opossum's prehensile tail, which it uses like an extra limb to grasp or hang from branches is an exceptional feature.
Standing five-feet-tall at the shoulders, the Uintatherium's huge body, bony horns, dagger-like teeth, and tiny brain are all unique features compared to other mammals. And humans are out of the ordinary with their large brain-to-body ratio and the ability to walk upright on two legs. There are a few other bipedal mammals, but they are mostly hoppers like kangaroos.
Humans also have some normal features compared to other mammals, such as three middle ear bones and five digits on each hand and foot.
About the California Academy of Sciences:
The California Academy of Sciences is home to Steinhart Aquarium, Morrison Planetarium, Kimball Natural History Museum, and world-class research and education programs-all under one living roof.
Admission to the Academy is $24.95 for adults, $19.95 for youth ages 12 to 17, Seniors ages 65+ and students with valid ID, $14.95 for children ages 4 to 11 and free for children ages 3 and younger.
The Academy is free to the public on the third Wednesday of each month.
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