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Mammals - Vertebrates Slideshow
We've finally gotten to the most advanced vertebrates on Earth. The class Mammalia includes over 5,000 species that include animals from the platypus to kangaroos to humans. As you can guess from our short list, living mammals come from one of three groups. There are egg-laying mammals, mammals that have pouches to raise their young, and mammals that have placentas. These placental mammals have their young develop inside their bodies before they give birth. Humans are placental mammals. Even though mammals have a few thousand living species, many more have become extinct. The first mammals appeared in fossils around 250 million years ago. They eventually developed many unique characteristics including control over their body temperature (homeotherms), hair, and seven cervical vertebrae in their necks. Since mammals can control their body temperature, they can be found across the entire planet. You might find small mice in the hottest deserts and polar bears in the arctic cold. There are also flying mammals that live in trees and caves. There are even groups of mammals that have returned to the oceans of the world. They have developed characteristics that allow them to hold their breath for long periods and fins that make them strong swimmers. Never forget that they still have lungs and always return to the surface for a quick breath of air. Image Credit: Andrew Rader Studios
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