Eutheria

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Eutheria is one of the two major groups of living therian mammals, the other being Metatheria, which includes marsupials. Eutherians are distinguished from noneutherians by various features of the feet, ankles, jaws, and teeth. The term "Eutheria" was introduced by Thomas Henry Huxley in 1880. Eutherians are also known as placental mammals because they possess a complex placenta that allows a long gestation period within the mother's womb.

Characteristics

Eutherians are characterized by several unique features:

• A complex placenta that facilitates nutrient and waste exchange between the mother and the developing embryo.
• A longer gestation period compared to marsupials.
• The presence of a corpus callosum, a structure that connects the two hemispheres of the brain.
• Differentiated teeth, including incisors, canines, premolars, and molars.

Evolution

Eutherians are believed to have diverged from other mammals during the Late Jurassic period. The earliest known eutherian is Juramaia, which lived about 160 million years ago. The diversification of eutherians accelerated during the Cretaceous period, leading to the wide variety of placental mammals we see today.

Classification

Eutheria is divided into several orders, including:

• Primates (e.g., humans, monkeys, and apes)
• Carnivora (e.g., dogs, cats, and bears)
• Cetartiodactyla (e.g., whales, dolphins, and even-toed ungulates like deer and cows)
• Chiroptera (e.g., bats)
• Rodentia (e.g., mice, rats, and squirrels)
• Lagomorpha (e.g., rabbits and hares)
• Proboscidea (e.g., elephants)
• Perissodactyla (e.g., horses, rhinoceroses, and tapirs)

Fossil Record

The fossil record of eutherians is extensive, with numerous well-preserved specimens that provide insight into their evolutionary history. Notable fossil sites include the Jehol Biota in China and the Hell Creek Formation in North America.

Importance

Eutherians are of significant interest to scientists because they include the most diverse and widespread group of mammals. They play crucial roles in various ecosystems and have significant interactions with humans, both as domesticated animals and as wildlife.

See also

• Theria
• Metatheria
• Marsupial
• Placenta
• Juramaia
• Mammal