Theria

Theria is a subclass within the class Mammalia, which encompasses the vast majority of mammals. The group is divided into two infraclasses: Eutheria, which includes placental mammals, and Metatheria, which comprises the marsupials. This classification is based on the presence of certain shared anatomical features and reproductive mechanisms. The term "Theria" itself is derived from the Greek word for "beast" and was first introduced in the late 19th century to distinguish these live-bearing mammals from the egg-laying Monotremata, the other subclass of mammals.

Characteristics

Therians are distinguished by several key characteristics, including the presence of three middle ear bones (the malleus, incus, and stapes), a neocortex region in the brain, and mammary glands for feeding their young. Additionally, therians give birth to live young, a trait that separates them from monotremes, which lay eggs. The reproductive system of therians is also unique, with females possessing a uterus and placenta in eutherians, which facilitates a longer gestation period within the mother's body, or a pouch in marsupials where the newborn continues to develop after a short gestation.

Evolution

The earliest known therians are believed to have appeared during the Mesozoic era, with evidence suggesting their presence as early as the Jurassic period. The divergence between the two infraclasses of Theria, Eutheria and Metatheria, is thought to have occurred approximately 160 million years ago. This evolutionary split led to the diverse range of mammalian species observed today, with eutherians encompassing the majority of mammal species, including humans, while metatherians are primarily represented by the marsupials of Australasia and the Americas.

Classification

The classification of Theria has evolved over time as new fossil evidence and genetic data have become available. Currently, Theria is recognized as one of the major groups within Mammalia, with its members characterized by their advanced reproductive and physiological traits. The subclass is further divided into the infraclasses Eutheria and Metatheria, with numerous orders under each infraclass that reflect the diversity of species within this group.

Conservation

Many therian species are currently facing threats from habitat destruction, climate change, and human activities. Conservation efforts are crucial to protect these animals, as they play vital roles in their ecosystems. Efforts include habitat preservation, legal protection, and breeding programs for endangered species.

See Also

• Mammalia
• Eutheria
• Metatheria
• Monotremata
• Evolution of mammals

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