Placentalia

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Placental Mammals 2021 Montage

Placentalia is a taxonomic group (or clade) within the larger class of mammals known as Mammalia. It includes all living mammals except for marsupials and monotremes, as well as many extinct groups. Members of the Placentalia are distinguished by the presence of a placenta, a specialized organ that forms during pregnancy to facilitate the exchange of nutrients, gases, and waste products between the mother and the developing fetus. This evolutionary adaptation allows for a longer gestational period, which in turn leads to more fully developed offspring at the time of birth.

Characteristics

Placental mammals share several key characteristics beyond the presence of a placenta. These include:

• A complex structure of the brain, with a well-developed neocortex.
• The presence of a diaphragm, which aids in respiration.
• A four-chambered heart, which efficiently separates oxygenated and deoxygenated blood.
• Endothermy (the ability to regulate body temperature through metabolic processes).
• Viviparity (giving birth to live young, as opposed to laying eggs).

Evolution

The earliest known placental mammals date back to the end of the Cretaceous period, around 66 million years ago, shortly before the Cretaceous–Paleogene extinction event that led to the demise of the non-avian dinosaurs. This timing has led to the hypothesis that the extinction of the dinosaurs and many other species provided ecological niches that the early placentals and their descendants could exploit, leading to a rapid diversification and evolution of the group.

Classification

Placentalia is divided into several major groups, known as orders. These include, but are not limited to:

• Primates, which includes humans, apes, monkeys, and related species.
• Carnivora, which includes predators such as lions, tigers, bears, and seals.
• Cetacea, which includes marine mammals like whales, dolphins, and porpoises.
• Rodentia, which is the largest order and includes mice, rats, squirrels, and beavers.
• Chiroptera, which includes all species of bats.

Ecological Impact

Placental mammals occupy a wide range of ecological niches across the globe, from deep oceans (whales and dolphins) to the highest mountains (certain species of mice and rats). Their adaptations have allowed them to become one of the most successful groups of animals on the planet in terms of diversity, population, and impact on the environment.

Conservation

Many species within the Placentalia are currently facing threats from habitat destruction, climate change, hunting, and other human activities. Conservation efforts are critical to preserving the biodiversity and ecological roles of these mammals.

See Also

• Mammalia
• Marsupialia
• Monotremata
• Evolution of mammals