Theria: Marsupials & Placental Mammals

In a Nutshell: What is Theria?

The subclass Theria is a major group of mammals that includes all marsupials and placental mammals. This means that nearly all living mammals, from kangaroos to humans, belong to Theria. The defining trait of therian mammals is that they give birth to live young, unlike their egg-laying relatives, the monotremes (e.g., platypuses and echidnas).

Therians have successfully adapted to nearly every ecosystem on Earth, from the deep oceans (whales and dolphins) to the highest mountains (snow leopards and mountain goats). This incredible diversity makes Theria the dominant group of mammals on the planet today.

Keep reading to learn more about Theria!

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What Are The Characteristics of Theria?

Reproductive Traits

Viviparity (Live Birth): Therians give birth to live young, a significant evolutionary advancement over egg-laying (oviparity). This adaptation enhances offspring survival by providing a protected developmental environment within the mother’s body.
Placental Development: Both marsupials and placental mammals possess a placenta, an organ facilitating nutrient and gas exchange between mother and embryo. In marsupials, the placenta is rudimentary, leading to the birth of underdeveloped young that continue development externally, often in a pouch. In contrast, placental mammals have a more complex placenta, allowing extended in-utero development, resulting in more mature offspring at birth.
Separate Excretory and Reproductive Openings: Unlike monotremes, which have a single cloacal opening, therians have distinct anal and urogenital openings. This separation is due to the presence of a perineum, a tissue region dividing these openings, which aids in more efficient waste elimination and reproductive processes.

Anatomical Features

Absence of the Coracoid Bone: Therians lack the coracoid bone found in the shoulder girdle of monotremes. This absence leads to a more streamlined and flexible shoulder structure, enhancing mobility and adaptability in various environments.
Presence of External Ears (Pinnae): Most therian mammals possess external ear structures, known as pinnae, which aid in sound direction and amplification. While some aquatic or burrowing therians have secondarily lost visible pinnae, the genetic blueprint for this feature remains a unique characteristic of the group.
Specialized Ankle Joint: Therians exhibit an ankle joint adapted for enhanced power and a wide range of motion. This specialization contributes to their diverse locomotive abilities, from running and climbing to swimming and digging.

Sensory and Neural Attributes

Highly Developed Brains: Therian mammals generally possess larger and more complex brains compared to other vertebrates, facilitating advanced behaviors, learning capabilities, and social interactions.
Presence of Whiskers (Vibrissae): Almost all therians have whiskers, which serve as tactile sensory organs, allowing them to detect and respond to environmental stimuli effectively.

Genetic Characteristics

Syncytin Proteins for Placentation: Therians utilize specific proteins called syncytins, derived from ancient viral genes, to facilitate the formation of the syncytiotrophoblast layer of the placenta. This layer is crucial for nutrient exchange and immune protection during embryonic development.
SRY Gene for Sex Determination: The presence of the SRY gene in therians plays a pivotal role in initiating male sex determination, leading to the development of male-specific anatomical features.

What Are the Main Groups of Theria?

The subclass Theria is divided into two major groups:

Placentalia (Placental Mammals)

Placental mammals (Placentalia) are the dominant group of mammals today, comprising over 6,000 species, including primates, carnivores, rodents, whales, and ungulates. The defining feature of placental mammals is the complex placenta, which allows offspring to develop inside the womb for an extended period before birth.

Key Traits of Placental Mammals

Advanced placenta – Transfers nutrients, oxygen, and antibodies from mother to fetus.
Longer gestation periods – Offspring are born at a more developed stage compared to marsupials.
High intelligence & social structures – Many placental mammals exhibit advanced problem-solving skills and cooperative behavior.
Wide geographic distribution – Placental mammals have adapted to nearly every environment on Earth.

Placentalia vs. Eutheria

Placentalia refers to modern placental mammals.
Eutheria includes Placentalia + extinct stem relatives (some of which were not fully placental).
Paleontologists use Eutheria when discussing fossil mammals, while Placentalia is used for living species.

Marsupialia (Marsupial Mammals)

Marsupial mammals (Marsupialia) are unique to Australia, South America, and North America, with species such as kangaroos, koalas, opossums, and Tasmanian devils. Unlike placental mammals, marsupials give birth to underdeveloped young, which then continue growing inside a pouch or attached to a teat.

Key Traits of Marsupial Mammals

Short gestation, extended lactation – Offspring are born tiny and immature and must crawl to the mother’s pouch (marsupium).
Mostly found in Australia & South America – Marsupials dominate Australia’s ecosystem, while opossums are the only marsupials in North America.
Different skeletal structure – Marsupials have more teeth than placental mammals and a paired reproductive system.
Distinct reproductive strategy – Many species have delayed implantation or embryonic diapause, allowing them to pause pregnancy in tough conditions.

Marsupialia vs. Metatheria

Marsupialia refers to modern marsupial species.
Metatheria includes Marsupialia + extinct relatives.
Metatheria is mainly used in fossil studies, whereas Marsupialia is the preferred term for living species.