Placental mammals are the dominant group of terrestrial vertebrates, with 20 orders in four superorders. The largest orders are Chiroptera, Rodentia, Carnivora, and Cetartiodactyla. Humans and their pets are the majority of placental mammals, causing extinction of many species through hunting and habitat destruction.
The placental mammals, the infraclass Eutheria (which is Greek for “true/good beast”) are the dominant group within mammals in general (classification which also includes marsupials and monotremes) and the dominant group of terrestrial vertebrates. This has been the case since the extinction of non-avian dinosaurs 65.5 million years ago. Although placental mammals consist of fewer species (about 4,900 in total) than reptiles (8,200 species), amphibians (6,100 species), or birds (10,000 species), placental mammals are dominant as they are the most numerous, occupy the most niches, are the largest, and a placental mammal almost always occupies the highest positions in the terrestrial food chain, Australia being the main counterexample.
Placental mammals consist of 20 orders contained in four superorders: Xenarthra (an early breakaway placental mammal family includes armadillos, sloths, and anteaters); Laurasiatheria (including most mammalian species), Afrotheria (smaller group of animals native to Africa including tenrecs, aardvarks, hyraxes, golden moles, elephant shrews, elephants, and manatees); and Euarchontoglires (a sister group to Laurasiatheria that includes rabbits, hares, rodents, primates, shrews and colugos). These superorder arrangements are based on varying levels of support from molecular genetics and fossil evidence, although some are quite controversial, substantially contradicting previous morphology-based classifications.
The four largest orders of placental mammals are Chiroptera (bats), Rodentia (mice, rats), Carnivora (dogs, cats, bears, other placental carnivores), and Cetartiodactyla (all ungulates, such as pigs and buffaloes, and cetaceans, which includes whales and dolphins). Despite a large number of species, gross morphological variation is somewhat limited in the first two orders, but large in the second two. It’s hard to imagine that pigs and goats are part of the same order as whales and dolphins, but it’s true. These groups split about 60 million years ago, during the first major waves of diversification of placental mammals.
Today, the majority of all placental mammals, on an individual basis, are humans, our pets, livestock, and animals adapted to live near us, especially rats and mice. In our short 200,000-year stint on the world stage, humans have radically reshaped the biodiversity picture of placental mammals. Hundreds if not thousands of placental mammals went extinct as humans spread across the globe, bringing with them hunting tools like spears and axes, something the fully biological world couldn’t adapt to fast enough to defend itself against. As a result, many of the impressive placental mammal megafauna of ancient times, such as saber-toothed cats and mammoths, are now completely extinct. Numerous placental mammals continue to become extinct due to habitat destruction and other factors.
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