NCBI Tree | Paleo Tree
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Mammals are characterized by animals with hair, three middle ear bones, and milk glands called mammary glands. The class Mammalia is split into three basic groups based on how the offspring spend the first part of their existence after conception.
Monotremata is the order of egg-laying mammals. There are only three species of monotremes in the world, and they are all native to Australia. The platypus is not even found in zoos outside of the island continent, but echidnas can be found at the San Diego Zoo. Echidnas are thought to be the most primitive mammals living today.
The classification Metatheria is used for marsupials. Marsupials are animals that give birth when the offspring is barely developed. Most marsupials develop further in a pouch the mother has. Again, many marsupials are native to Australia, but opposums can be found all over America. The San Diego Zoo also has a good selection of marsupials for viewing, such as kangaroos and koala bears.
The third group of mammals is called Eutheria. These are the placental mammals, which give birth to fairly well developed offspring.