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Mammals evolved from reptile ancestors, just like birds. Mammals, however, came first and they came from a different reptilian stock: therapsids, descendants of the synapsids. Small mammal ancestors bided their time during the time of the dinosaurs. Once some of the major competition was gone, mammals diversified and spread.
Bat. Image from here.
The name of the class Mammalia comes from one of the major features of mammals, mammary glands. ("Mamma" in Latin means breast.) You may have noticed these before. Only mammals feed their young from a rich, nutritious fluid they create.
Hound. Image from here.
What else makes a mammal?
Bonobo. Image from here.
We forgot one, right? Mammals don't lay eggs. Actually there is an exception, though the vast majority doesn't lay eggs. There are three subclasses of mammals:
Monotremes are the egg-laying mammals. There are only two monotremes, platypuses and echidnas, and they are found only in Australia and New Guinea. Since they lay eggs, they don't need placentas, another unique feature of all other mammals. A placenta is an organ created along with a developing fetus inside the uterus. It connects the fetus to the mother's bloodstream, providing food and oxygen. (Eggs have their own food source and air pocket inside.)
See an echidna here.
The folks down under have a very unique group of animals. The next group, marsupials, is mostly found in Australia. This includes the famous kangaroos and koalas. There is also one marsupial, the opossum, in North America and a few others in South America. Marsupials give birth to a very tiny, undeveloped baby after only a few weeks on the inside. The fetus then squirms up the mom's belly and into a pouch. Inside the pouch are the teats for nursing and a snug place to hang out for a while and grow.
Kangaroo. Image from here.
Now, drum roll please . . . us. Or, at least, this is our subclass, Eutheria. Marsupials do produce a simpler and short-lived placenta, but eutherians (sometimes called placentals) have a more complex and developed placenta and keep the entire development of a fetus in the uterus. This developmental period is called gestation and—ask your mother—it takes some time and lots of resources.
Wolf. Image from here.
Orca. Image from here.
Eutherians include over 4,000 species and are some of the most familiar to us humans – dogs, elephants, rabbits, bats, squirrels, whales and so on. Mammals live on land and in water and a few even fly. It also includes the primates and we'll get to those next.
Watch the amazing birth journey of a kangaroo.