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There are three different types of mammals, and I wanted to explain these three groups.

The first group is placental mammals.  These are mammals that have placentas.  The second are marsupials, which have a pouch to nourish their young.  The third are monotremes, which lay eggs.

I wanted to find out if all three live in North America.  If you want to know, then please read on.

Phylogenetic chart of mammals

Placental Mammals

The placental mammals give birth to live young. They are the most diverse mammals and comprise over 4,000 known species. There are several different forms of placental mammals. These include whales, bats, elephants, tigers, and humans.

They are referred to as placental mammals because they have an actual placenta. This is a significant and dominant characteristic of this group of mammals. 

The fetus stays in the womb until it reaches full development and is ready for birth. During this time, the fetus receives nourishment through the placenta. This organ facilitates the exchange of oxygen, nutrients, and waste products between the fetus’s and the mother’s blood.

Marsupial and placental mammals differ, although some consider marsupials similar to placentals. The marsupial may have a placenta, but it is short-lived and does not significantly contribute to the fetus’s development. The placenta is efficient and well-developed to support the young to full-term growth.

Placental mammals feed their young ones exclusively on milk during the first few days after birth. They have nipples from which their babies suckle to get the necessary nourishment to grow and thrive.

True placental mammals come from the clade Eutheria’s stem-group members, thought to have existed in the Jurassic period. Modern placental orders might have originated in the Paleogene period.

They gradually evolved and took over different ecological niches left bare when the dinosaurs disappeared. They also increased in size, with some mammals developing specialized hands and feet, such as primates.

With time, the groups expanded to various continents, such as Africa, North America, and South America.



There are three Superorders of placental mammals. These result from a geographically separate evolution a long time ago.

The first is Afrotheria, which is made up of several Orders from Africa. These comprise the dugongs and manatees, elephants and tenrecs from Madagascar, shrew-like creatures, elephant shrews, the aardvark, the golden mole, and the rock dassies.

The second Superorder is named Xenarthra. These mammals come from South and Central America. Xenarthra is made up of anteaters, armadillos, and slots. All of these creatures of this Superorder contain additional joints that connect the vertebrae.

The third placental Superorder is Boreoeutheria. The countries that these are associated with are now North America, Greenland, and Eurasia. However, these were once part of a single landmass.

Boreoeutherians consist of primates, rabbits, rodents, flying lemurs, and tree shrews. These are part of the Superorder Euarchontoglires.

A separate branch consists of hedgehogs, bats, moles, dogs, cats, mongooses, hyenas, bears, and pandas. This second branch also includes horses, rhinos, and other odd-toed ungulates. Even-toed ungulates are also in this Superorder.

These include mammals such as bighorn sheep, camels, and giraffes. These are part of the Superorder Laurasiatherians.

Want to know how mammals are classified? I have you covered. Read this article here.

Marsupial mammals

The name marsupial is derived from marsupium, which means pouch. They are a group of mammals that are characterized by premature birth.  The newborn continues to develop while attached to the nipples on the lower belly of the mother. 

The pouch, or the marsupium, is a flap of skin covering the nipples. Some species have the nipples exposed or covered with just the remnants of a pouch.

The young marsupials are born very early, even before they have fully developed. The baby is born only one month after conception and is usually a mere embryo.

The young baby remains attached to the milk-filled teats for a period corresponding to the remaining gestation period in a placental mammal.


The reason why marsupials give birth early is they lack a placenta. The placenta connects the embryo to the mother’s blood supply.

The placenta works to provide the embryo with much-needed nutrients as well as oxygen. The placenta also takes away all the waste products. 

Since these animals lack a placenta, they are born at an early stage in their development. Throughout the remaining part of their early lives, the baby marsupial is nourished by milk from the mother. 


Marsupials are divided into two groups consisting of over 500 species of animals. The two primary groups include the American and Australian marsupials.

The American marsupials can be found in most parts of north, central, and South America. North America has only two species of marsupials, with the rest believed to have gone extinct. 

The most common marsupial is the Virginia opossum. Australia has the highest number of species, with about 120. They include kangaroos, possums, koalas, wallabies, and wombats.

The smallest marsupial in the entire group is the long-tailed planigale. It is tiny, measuring just 2.3 inches and weighing approximately 4.3 grams. This tiny mammal inhabits most parts of northern Australia, including grasslands and woodlands.

On the other hand, the largest animal in this group is the red kangaroo. These mammals have a rusty red color and can weigh up to 200 pounds. On average, they measure 3.25 to 5.25 feet in length.

Reproduction and Pregnancy

When a pregnant marsupial expects a young one, she prepares for birth by preening the pouch. She does so to make way for the incoming embryo. The cleaning involves licking off pee and droppings from the prior baby.

The reproductive systems of this group of mammals are pretty complex. They have a double reproductive tract. Female marsupials possess two uteruses. Each of the uteri has its lateral vagina, and they both join up to form a birth canal that is centrally located.

In some species, males have a two-headed penis behind the scrotum.

Did you know that only one type of mammal can fly? Find out what it is in an article I have written here.


A monotreme refers to a group of unique mammals that don’t give birth to young ones but instead lay eggs. The monotremes have only five species of animals.

These include the duck-billed platypus and four species of spiny anteaters or echidna. Both animals are native to the islands of Australia and New Guinea, with none in North America.

Aside from the fact that they lay eggs, monotremes are more or less similar to other mammals. They produce milk to feed their newborns. Unlike other mammals with nipples, monotremes have mammary gland openings in their skin.

The openings secrete a fluid that accumulates in the mother’s abdomen—the young feed on the milk from here by lapping it up. 

Do Monotremes Lay Eggs?

Monotremes are the only mammals that lay eggs, and in this way, they are entirely different from marsupials and placental mammals. The body temperature of a monotreme is also very different from most warm-blooded animals.

They have a lower body temperature, which is more familiar to a reptile.  

The name of a monotreme is due to the vast difference between them and the other types of mammals. Whereas placental mammals and marsupials have two openings, one for waste and one for eggs to pass through, monotremes only have one. Through this opening, both eggs and waste pass.

Want to know what the ten most deadly mammals in North America are? Please read this article I have written to find out.

Besides the fact that monotremes lay eggs and have some technical features in the skeleton, monotremes are not very different from other mammals in their habitats.

The echidna and the platypus are both specialized in their respective ways. They both have short limbs, which extend sideways rather than downwards.

Duck-billed Platypus

The platypus uses these for swimming down to the bottoms of lakes to collect food, such as invertebrates. The echidna uses its limbs to burrow and dig out its termites.

The duck-billed platypus is an odd-looking type of mammal. They have a broad bill similar to a duck, a tail that resembles beavers, and webbed feet. They spend time both on land and in the water. The platypus is an exceptional swimmer and can dive underwater for up to 30 seconds when looking for food.  

The platypus lays between one and three small eggs. They develop in the uterus for 28 days and then are incubated by the mother for ten days. The mother then curls around them to keep them warm. 

Although they have mammary glands, they lack teats, something that most other mammals do have. They release the milk to the young through pores in the skin.


The echidna is a monotreme with a body covered with spines except for the belly and face. The spines are mainly for protection from predators.

There are two types of echidnas; the short-beaked and the long-beaked. The short-beaked ones are smaller than their counterparts and possess short beaks, just as their name suggests. The long-beaked ones have longer and down-curving beaks. They also have shorter spines.

The echidnas have long snouts with long front claws to plow through the forest litter, break ant and termite mounds, and even turn over hollow logs.

It uses its long and sticky tongue to suck them up into its mouth when it finds insects. The echidna mainly feeds on ants, which has earned it the name spiny anteater.

A female echidna lays an egg directly into a temporary pouch that develops on her abdomen during breeding. The bag incubates the egg for about ten days until the baby hatches. The echidna only lays one egg.  

After hatching, the baby echidna is tiny and helpless and stays in the pouch to grow further. For about eight weeks, the baby remains there, sucking the yellowish milk seeping out of the mother’s pores.

Once the baby echidna has fully developed, they are deposited out of the pouch and into a nursery burrow. From here, the baby will spend six to eight weeks in the bag.

After this, the spines grow, irritating the mother and forcing her to eject them from the pouch. The mother frequently returns to nourish them until the young are five months old. 

Want to know more about the different orders of mammals? Please read this article I have written to find out.