There are three living groups of mammals: placental mammals, monotremes, and marsupials. Placental mammals are the largest and best known, using a placenta during gestation. Marsupials do not use a placenta and have a pouch for their young. Monotremes are unique, reproducing using eggs.
There are three main living groups of mammals: placental mammals, monotremes, and marsupials. A fourth group, the multituberculates, is now extinct. Of the three living groups, the placental mammals are by far the largest and probably the best known; after all, you are a placental mammal and if you have mammalian pets, chances are high that they too are placental mammals. These groups of mammals are known as infraclasses taxonomically, meaning they are ranked below class and subclass, but above order.
No matter what infraclass a mammal is in, it shares a number of traits with other mammals. All mammals have a neocortex, a brain region that is unique to mammals, and they also have three middle ear bones and a single jaw bone. Mammals also possess sweat glands and are able to produce milk through the use of specialized glands. Mammals also possess specially designed hair and skulls that connect to their neck vertebrae to protect them.
Both placental mammals and marsupials belong to the theria subclass, which includes mammals that give birth to live young. Placental mammals are more properly classified as belonging to the infraclass eutheria, but many people use “placental” to describe a mammal in this infraclass in reference to its distinguishing feature. All placental mammals use a placenta during gestation. Some examples of placental mammals besides humans and pets include bats, goats, sloths, and dolphins.
Marsupials, on the other hand, do not use the placenta. Instead, they develop a small yolk sac that feeds the young marsupial for a short time before it is born in a very underdeveloped state. At this stage, the young animal crawls into its mother’s pouch to feed and develop further. Marsupials are sometimes known as pocket mammals, referring to their interesting method of development. Some famous marsupials include opossums, kangaroos and koalas.
Monotremes are quite unique, and there are only six living species today. They reproduce using eggs, rather than giving birth to live young, and have a number of other intriguing physiological traits that distinguish them from other members of the mammalian class. The platypus is a well-known example of a monotreme; the earliest reports of the platypus were so incredible to Europeans that they believed the animal to be a hoax!
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