Placentalia

Placental mammals, called Placentalia, are one of the three main types of living mammals. The other two types are Monotremata and Marsupialia. Placentalia includes most mammals we see today.

These animals are special because their babies grow inside the mother’s body in a place called the uterus for a longer time than in other mammal types.

The word “placental” can be confusing. This is because marsupials also have something called a placenta to help feed their babies. But marsupials have their babies earlier, and the babies grow in a pouch on the mother’s belly. Placental mammals carry their babies inside until they are more developed.

Placentalia is the only living type in a bigger group called Eutheria. This means placental mammals are very diverse and live all over the world, in many different places and with many different ways of living.

Anatomical features

Placental mammals have special body features that make them different from other mammals. They have a wide opening at the bottom of the pelvis to help them give birth to bigger babies. They do not have certain bones called epipubic bones that other mammals have.

These mammals have a special joint in their ankles and a small bone at the end of a lower leg bone. Unlike some other animals, they do not have a cloaca. Instead, their body has separate openings for digestion and reproduction. They also have a special structure in their brains called a corpus callosum that helps the two sides of the brain talk to each other.

Subdivisions

Studies of genes have shown scientists how placental mammals are related. Today, we know there are three main groups of placental mammals: Boreoeutheria, Xenarthra, and Afrotheria. These groups came from common ancestors long ago.

There are 19 orders of placental mammals in these three groups. Some examples include armadillos, sloths, anteaters, rabbits, rodents, bats, cattle, sheep, horses, dogs, and cats. Scientists are still learning more about how these groups are related.

Genomics

Scientists have studied the full genetic code, or genome, for many placental mammals. They have looked at the genomes of most placental mammal families.

See list of sequenced animal genomes.

Evolutionary history

True placental mammals, including all modern placentals, evolved from a group called eutherians. They lived in the Middle Jurassic period, about 170 million years ago. These early mammals were small, active at night, and ate insects. Many lived in trees.

Placental mammals likely began to appear in the Late Cretaceous period around 90 million years ago. The earliest clear fossils come from after a big event changed Earth's history. When dinosaurs and many other animals disappeared, mammals quickly evolved and filled new roles in nature. This led to the development of many groups we see today, such as primates, rodents, and carnivores. Mammals grew larger and took on new roles, like eating plants and hunting other animals. Some even developed new skills, like bats, who evolved to fly and use echolocation to hunt insects at night.

Different groups of placental mammals developed in different parts of the world. In Africa, some evolved into elephants, elephant shrews, and other unique animals. In South America, others evolved into sloths, anteaters, and armadillos. Smaller groups, like rodents and primates, were able to travel between continents.