Theria: Characteristics, Diversity, Types, Examples

Theria is a subclass of mammals, including placental mammals (Eutheria) and marsupials (Metatheria). Placental mammals have a placenta, nourish their young internally, and include diverse groups like primates and rodents. Marsupials give birth to undeveloped young and continue their development in a pouch.

It evolved around 160 million years ago from non-therian mammals. The exact transition is still under study. The earliest therian fossils date back to the Late Jurassic and Early Cretaceous periods. 

Further, it underwent diversification and expansion during the Cretaceous and Paleogene periods and represented the majority of mammalian species.

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Characteristics of Theria

Live birth and nurturing of young through mammary glands

Theria mammals give birth to live young instead of laying eggs. They nourish their offspring through mammary glands, which produce milk for feeding. The reproductive strategy allows for direct parental care and the survival and development of the young.

Key anatomical and physiological features

  1. Development of a placenta in placental mammals: Placental mammals have evolved a specialized organ called the placenta. The placenta develops during pregnancy and facilitates the exchange of nutrients and waste products between the mother and the developing fetus. This adaptation allows for more extended gestation periods and the birth of more developed young.
  2. Unique reproductive strategies in marsupials: Marsupials, another group, have distinct reproductive strategies. They give birth to relatively undeveloped young, continuing their development outside the womb. The underdeveloped young crawl into a pouch on the mother’s abdomen, where they attach to a nipple and receive nourishment until they are more mature and independent.

Diversity of Theria

1. Placental Mammals

Placental mammals are a diverse group within Theria characterized by the presence of a placenta during pregnancy. This adaptation allows for a more extended gestation period and more advanced offspring at birth.

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Major groups and their characteristics

  • Primates: Primates include humans, apes, monkeys, and prosimians. They are known for their complex social structures, grasping hands, and forward-facing eyes.
  • Carnivores: Carnivores encompass animals like lions, tigers, wolves, and domestic cats. They have specialized teeth and claws for hunting and feeding on other animals.
  • Ungulates: Ungulates consist of hoofed mammals, such as horses, cows, deer, and giraffes. They are herbivores and have specialized adaptations for efficient grazing or browsing.
  • Rodents and lagomorphs: This group includes rodents like mice, rats, squirrels and lagomorphs like rabbits and hares. They have continuously growing incisor teeth and are known for their prolific breeding.
  • Bats are the only in the group of mammals capable of sustained flight. They have adapted wings and are known for their echolocation abilities.
  • Insectivores, such as shrews, moles, and hedgehogs, are specialized in feeding on insects and other invertebrates.

The placental mammals also include diverse species like elephants, seals, whales, anteaters, and many more.

Examples of remarkable species within each group

  • Primates: Humans (Homo sapiens), chimpanzees, gorillas, orangutans.
  • Carnivores: Lions (Panthera leo), tigers (Panthera tigris), wolves (Canis lupus), domestic cats (Felis catus).
  • Ungulates: Horses (Equus ferus caballus), cows (Bos taurus), giraffes (Giraffa camelopardalis), deer (family Cervidae).
  • Rodents and lagomorphs: Mice (Mus musculus), rats (Rattus norvegicus), rabbits (Oryctolagus cuniculus), hares (Lepus europaeus).
  • Bats: Little brown bat (Myotis lucifugus), fruit bat (Artibeus jamaicensis).
  • Insectivores: European hedgehog (Erinaceus europaeus), eastern mole (Scalopus aquaticus).

2. Marsupials

Marsupials are a group within Theria that have a unique reproductive strategy. They give birth to relatively undeveloped young, then continue their development outside the womb, typically in a pouch.

They give birth to relatively undeveloped young, then continue their development outside the womb, typically in a pouch. This distinctive feature sets them apart from placental mammals.

Marsupials Distribution and Habitats

Marsupials are a group within Theria that have a unique reproductive strategy. They give birth to relatively undeveloped young, then continue their development outside the womb, typically in a pouch.

Examples of notable marsupial species

Kangaroos and wallabies: This group includes iconic species like the red kangaroo (Macropus rufus) and the agile wallaby (Macropus agilis). They are known for their hopping locomotion and are well-adapted to the Australian landscape.

Koalas: Koalas (Phascolarctos cinereus) are arboreal marsupials found in Australia. They primarily feed on eucalyptus leaves and have a specialized diet and unique adaptations for tree-dwelling.

Tasmanian devils: Tasmanian devils (Sarcophilus harrisii) are carnivorous marsupials native to Tasmania. They have a stocky build and powerful jaws and are known for their aggressive scavenging behavior.

Opossums: Opossums belong to the Didelphidae family and are found in the Americas. They are recognized for their adaptability and ability to thrive in various habitats.

Others: Other notable marsupials include the numbat (Myrmecobius fasciatus), quokka (Setonix brachyurus), sugar glider (Petaurus breviceps), and the bilby (Macrotis lagotis).

Evolutionary History of Theria

Fossil evidence and transitional forms

The evolutionary history of these mammals is supported by fossil evidence and the presence of transitional forms. Fossils of early Therian mammals have been discovered, providing insights into the evolutionary transition from non-therian mammals to Therians.

These fossils show intermediate features between non-therian and therian mammals, shedding light on the critical evolutionary changes that occurred.

Major evolutionary milestones and adaptations

  1. Radiations and diversifications: Theria underwent significant radiations and diversifications throughout their evolutionary history. It led to various orders and families within placental mammals and marsupials. These radiations allowed for the occupation of diverse ecological niches and the evolution of various adaptations.
  2. Impact of environmental changes on Theria evolution: Environmental changes played a crucial role in shaping their evolution. Throughout Earth’s history, there have been significant shifts in climate, habitat availability, and the emergence of new ecological opportunities. These changes provided selective pressures that influenced their evolution, leading to adaptations suited to different environments and lifestyles.
  3.  Coexistence with non-therian mammals: During their evolutionary history, Theria coexisted with non-therian mammals, including monotremes and other now-extinct lineages. This coexistence allowed for competition and potential interactions between different mammalian groups. Their evolutionary success and their ability to thrive and diversify can be attributed, in part, to their adaptations and ecological strategies.


Theria mammals, which encompass placental mammals and marsupials, exhibit remarkable variation in their methods of reproduction, physical characteristics, and adaptations to their environments. 

Over millions of years, they have undergone extensive evolutionary changes, giving rise to many species inhabiting diverse habitats. Preserving them is of utmost importance as it plays a critical role in upholding ecological equilibrium, biodiversity, and the overall health of ecosystems.

Summary on Theria

  • Theria mammals give birth to live young instead of laying eggs like monotremes or carrying eggs in a pouch like marsupials.
  • Placental mammals are the dominant group within Theria. Their unique placenta forms and is a specialized organ that supplies essential nutrients and oxygen to the developing fetus.
  • Unlike placental mammals, Marsupials have a short gestation period, after which the underdeveloped young are born and continue their development in a pouch.
  • Likewise, they exhibit remarkable species, habitats, and adaptations diversity.
  • As predators, prey, seed dispersers, pollinators, and ecosystem engineers, these mammals are vital in ecosystems.
  • Fossil evidence provides insights into the origin, diversification, and adaptations of different therian lineages.



About Author

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Krisha Karki

Krisha Karki is a B.Sc. microbiology student with a passion for writing, and expanding her knowledge is a unique blend of scientific curiosity and creative expression. Likewise, she deeply understands the microscopic world and its intricacies. With a deep-rooted curiosity and an insatiable thirst for knowledge, she eagerly explores the realms of microbiology. Her love for science and technology permeates the writing, infusing it with a unique blend of intellectual rigor and imaginative flair.

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