Placentalia: Placental Mammals

In a Nutshell: What is Placentalia?

The infraclass Placentalia represents the most diverse and widespread group of mammals, including humans, whales, elephants, rodents, and carnivores. Placental mammals are distinguished by their complex placenta, which allows for extended fetal development within the mother’s womb. This key adaptation has contributed to their global success, enabling them to thrive in nearly every environment on Earth.

Placental mammals are found on every continent, including marine environments, where species like whales and seals have fully adapted to aquatic life. With over 6,500 species, Placentalia includes mammals of all sizes, from tiny shrews to the massive blue whale, the largest animal ever to exist.

Keep reading to learn more about Placentalia!

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What Are The Characteristics of Placental Mammals?

Placental mammals exhibit unique reproductive, anatomical, and physiological features that set them apart from other mammalian groups.

Reproductive Traits

Complex placenta – Provides oxygen, nutrients, and immune protection to the fetus, allowing for longer gestation periods than marsupials.
Live birth with well-developed young – Unlike monotremes (Yinotheria), which lay eggs, placental mammals give birth to relatively developed offspring.
Genetic diversity & chromosomal complexity – Placental mammals exhibit a greater variation in chromosomal structures, contributing to their adaptability.

Skeletal and Anatomical Features

Absence of epipubic bones – Unlike monotremes and marsupials, placental mammals lack epipubic bones, which allows for greater abdominal expansion during pregnancy.
Larger and more complex brains – Many placental mammals, particularly primates, cetaceans, and elephants, show high levels of intelligence and social behavior.
Specialized teeth – Placental mammals exhibit highly adapted dentition, reflecting their varied diets (carnivores, herbivores, omnivores, etc.).

Metabolic and Physiological Aspects

Higher metabolic rates – Placental mammals maintain a higher and more stable body temperature compared to marsupials and monotremes.
More efficient respiratory & circulatory systems – Enhanced lung capacity and oxygen transport support endurance and activity across diverse habitats.

Evolutionary Adaptations

Diverse ecological niches – Placental mammals inhabit every major terrestrial and aquatic biome, from deserts to deep oceans.
Immune system advantages – The longer gestation period allows for stronger immune system development in offspring, increasing survival rates.
Variety of locomotion methods – Placental mammals have evolved various movement adaptations, including flight (bats), swimming (whales), running (cheetahs), and digging (moles).

Placentalia vs. Eutheria: What’s the Difference?

The terms Placentalia and Eutheria are often used interchangeably, but they have distinct meanings in evolutionary biology and paleontology.

Eutheria: The Broadest Group (Extinct + Living)

Eutheria refers to all mammals more closely related to placentals than to marsupials. It includes extinct early mammals that lived during the Mesozoic era (e.g., Juramaia and Eomaia from the Jurassic and Cretaceous periods) and modern placental mammals (Placentalia).

Key Traits of Eutheria

Appeared around 160 million years ago (Jurassic period).
Developed placental-like reproduction but still had some primitive traits.
Some early eutherians laid eggs, while others gave birth to underdeveloped young.

Fossil Evidence:

Juramaia sinensis (~160 million years ago, China) – The earliest known eutherian.
Eomaia scansoria (~125 million years ago, China) – Had a long tail and limb proportions suited for climbing.

Placentalia: The Modern Group

Placental mammals (Placentalia) are the only surviving eutherians today. They evolved from early eutherians but developed a fully functional placenta to nourish embryos for long gestation periods, a more advanced brain and social behaviors, and diverse ecological adaptations—from deep-sea whales to high-speed cheetahs. Unlike some early eutherians, modern placentals do not lay eggs and give birth to well-developed young.

Revised Major Groups of Placentalia

Placental mammals (Placentalia) are classified into two major evolutionary magnorders:

Atlantogenata (Southern Placental Mammals)

This group includes mammals that likely originated in South America and Africa. It consists of two superorders:

Afrotheria – Mammals that evolved primarily in Africa (including elephants, manatees, aardvarks, elephant shrews, golden moles).
Xenarthra – Mammals that originated in South America with unique vertebral joints (including sloths, anteaters, armadillos).