One of the most common classes of animals are the mammals. It includes animals that many people know well: cats, dogs, bats, whales, etc. Most people have a pretty good idea of what constitutes a mammal.
Mammalia comes from the Latin word ‘mamma‘ which means breast. Mammary glands, or milk producing glands, are one of the key features of mammals.
Besides mammary glands, all mammals also have hair, a single lower jaw bone (reptiles have 3 bones), 3 inner ear bones, cheek teeth with a divided root and an aortic arch (main artery in the heart) that bends to the left. Some traits are easier to observe than others and some of these traits aren’t as straightforward as they seem. For example, platypuses don’t have nipples like most mammals. Instead, they secrete milk through patches on their abdomen. Porcupines have modified hairs (quills) that they use to defend themselves from predators. And even whales and dolphins are some hair!
There are 3 types of living mammals: marsupials, monotremes, and placental mammals. Monotremes are the most primitive mammals and lay eggs. Today, the only monotremes–platypuses and echidnas–are found in Australia. Marsupials are slightly more derived than monotremes and give birth to underdeveloped young that mature inside a pouch. Modern marsupials live in Australia as well as South America and Eastern North America. Finally, placental mammals, including humans, are a relatively recent development. They give birth to live, relatively well-developed, young.
To learn more about mammals, check out the link below!