Placentalia facts for kids

Quick facts for kids Placentals Temporal range: Maastrichtian-present 66.3–0Ma PreЄ Є O S D C P T J K Pg N Possible earlier Cretaceous records
Placentals from different orders.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Clade:	Eutheria
Infraclass:	Placentalia Owen, 1837
Magnorders and possible taxa
Atlantogenata Boreoeutheria †Palaeoryctida? †Taeniodonta? †Procerberinae? †Alveugena? †Ambilestes?
Synonyms
List of synonyms: Euplacentalia (Koenigswald, 2016) Eutheria (Huxley, 1880) Monodelphia (Gill, 1872) Placentaria (Fleming, 1822) Placentata (Turnbull, 1971)

Placental mammals are an amazing group of animals! This group includes most mammals alive today. Think of humans, dogs, cats, elephants, and whales – they are all placental mammals. What makes them special is how their babies grow.

Unlike other mammals, placental mothers carry their babies inside their bodies for a long time. The baby develops in the mother's uterus. A special organ called the placenta helps feed the baby and remove waste. This allows the baby to be much more developed when it is born.

Other mammal groups, like monotremes (egg-laying mammals like the platypus) and marsupials (like kangaroos with pouches), have different ways of raising their young. Marsupials also use a placenta, but for a shorter time. Their babies are born very tiny and then grow in a pouch.

What Makes Placental Mammals Unique?

Placental mammals have several special body features. These help them give birth to larger, more developed babies.

• They have a wide opening at the bottom of their pelvis. This allows a bigger baby to be born.
• They do not have certain bones called epipubic bones. These bones are found in other mammals. In placentals, not having them allows the mother's belly to expand easily during pregnancy.
• Their ankle bones fit together in a special way. This forms a strong and complete upper ankle joint.
• They have a small bone called a malleolus at the bottom of the fibula (a lower leg bone).
• Unlike many other animals, placental mammals have separate body openings. They have one opening for waste and another for reproduction.
• Their brains have a special connection called the corpus callosum. This connects the two halves of the brain.

Exploring the Diverse Groups of Placental Mammals

Scientists group placental mammals into three main branches. These groups are based on their shared features and DNA. They are called Boreoeutheria, Xenarthra, and Afrotheria. All these groups came from common ancestors a long, long time ago.

Scientists are always learning more about how these groups are related. Studies in 2022 helped identify some very early placental mammal groups. These include palaeoryctids and taeniodonts.

The 19 living orders of Placentalia in the three groups are:

• Magnorder Atlantogenata
  • Superorder Xenarthra
    • Order Cingulata (armadillos)
    • Order Pilosa (sloths and anteaters)
  • Superorder Afrotheria
    • Grandorder Afroinsectiphilia
      • Order Tubulidentata (aardvarks)
      • Mirorder Afroinsectivora
        • Order Afrosoricida (tenrecs, otter shrews, and golden moles)
        • Order Macroscelidea (elephant shrews)
    • Grandorder Paenungulata
      • Order Hyracoidea (hyraxes)
      • Mirorder Tethytheria
        • Order Proboscidea (elephants)
        • Order Sirenia (dugongs and manatees)
• Magnorder Boreoeutheria
  • Superorder Euarchontoglires
    • Grandorder Gliriformes
      • Mirorder Glires
        • Order Lagomorpha (rabbits, hares, and pikas)
        • Order Rodentia (rodents: mice, rats, hamsters, guinea pigs, chinchillas, capybaras, porcupines, voles, squirrels, beavers, etc.)
    • Grandorder Euarchonta
      • Order Scandentia (treeshrews)
      • Mirorder Primatomorpha
        • Order Dermoptera (colugos)
        • Order Primates (primates: monkeys, apes (including humans), lemurs, lorisids, galagos, and tarsiers)
  • Superorder Laurasiatheria
    • Order Eulipotyphla (hedgehogs, gymnures, shrews, moles, and solenodons)
    • Order Chiroptera (bats)
    • Grandorder Ferungulata
      • Mirorder Euungulata
        • Order Artiodactyla (even-toed ungulates: cattle, antelopes, sheep, deer, camelids, pigs, giraffes, cetaceans, hippopotamuses, goats, buffalo, etc.)
        • Order Perissodactyla (odd-toed ungulates: horses, asses, zebras, rhinoceroses, and tapirs)
      • Mirorder Ferae
        • Order Pholidota (pangolins)
        • Order Carnivora (carnivorans: dogs, cats, bears, pinnipeds, mongooses, raccoons, skunks, hyenas, weasels, etc.)

Scientists are still discussing the exact family tree of these three main groups. They use DNA and fossil evidence to figure out how they are related. These groups likely started to separate between 105 and 120 million years ago.

How Placental Mammals Evolved Over Time

The very first mammals that were ancestors of placentals appeared a long time ago. This was around 170 million years ago. These early mammals were small, ate insects, and were active at night. They often lived in trees.

The End of Dinosaurs and Rise of Mammals

Around 66 million years ago, a huge event changed the world. A large asteroid hit Earth, causing the extinction of most dinosaurs. This event left many empty spaces in nature. Mammals, including placentals, quickly took advantage of these new opportunities. This rapid growth and change is called an "adaptive radiation."

Mammals grew larger and filled roles that dinosaurs once held. They became big plant-eaters and big meat-eaters. Some mammals also explored completely new ways of life. For example, bats learned to fly and use echolocation. This made them great night hunters. Whales moved from land to water, first in rivers, then in the oceans. Primates developed special hands and feet for grasping branches. They also got better eyesight for seeing in the dark.

Evolution Across Continents

Placental mammals evolved differently on different continents. This is because large bodies of water made it hard for them to travel.

• In Africa, the Afrotheria group grew very diverse. This led to animals like elephants, aardvarks, and manatees.
• In South America, the Xenarthra group expanded. This gave us modern sloths, anteaters, and armadillos. It also included extinct giants like ground sloths.
• In other northern lands (called Laurasia), the Boreoeutheria group thrived. This group includes primates (like us!), rodents, carnivorans (like cats and dogs), and ungulates (hoofed animals).

Later, land bridges formed between continents. This allowed these different groups to spread even further.

Studies suggest that placental mammals became much more diverse after other mammal groups, like multituberculates, started to decline.