Placentalia

Placental mammals, known scientifically as Placentalia, are one of the three main groups of living mammals. The other two groups are Monotremata and Marsupialia. Placentalia includes most of the mammals we see today. These animals are special because their babies, or fetuses, grow inside the mother's body in a place called the uterus for a longer time compared to other mammal groups.

The name "placental" can be a little confusing. This is because marsupials also have a structure called a placenta that helps feed their babies. However, marsupials give birth to their babies earlier, and the babies continue to grow and develop inside a pouch on the mother's belly. Placental mammals, on the other hand, carry their babies inside until they are more developed.

Placentalia is the only living group that belongs to a larger group called Eutheria. This group includes all mammals that are more closely related to placentals than to marsupials. Because of this, placental mammals are very diverse and can be found all around the world, living in many different environments and having many different lifestyles.

Anatomical features

Placental mammals have some special body features that set them apart from other mammals. They have a wide opening at the bottom of the pelvis to help them give birth to larger babies. They also lack certain bones called epipubic bones that are found in other mammals.

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 communicate.

Subdivisions

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

There are 19 orders of placental mammals grouped into these three main lines. 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 to each other.

Genomics

As of 2020, scientists have mapped the full genetic code, or genome, for at least one species in every living group of placental mammals. They have also studied the genomes of about 83% of placental mammal families, which means they looked at 105 out of the 127 families that are alive today.

See list of sequenced animal genomes.

Evolutionary history

True placental mammals, which include all modern placentals, evolved from a group called eutherians that lived as far back as the Middle Jurassic period, about 170 million years ago. These early mammals were small, active at night, and ate insects, often living in trees.

Placental mammals likely began to appear in the Late Cretaceous period around 90 million years ago, but the earliest clear fossils come from right after the time when a huge event changed Earth's history. As dinosaurs and many other animals disappeared, mammals quickly began to evolve and fill 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 roles left empty by the loss of dinosaurs, 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 mammals 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.