Placental mammals, the dominant terrestrial life form, have over 5,000 species including humans, dogs, cats, and horses. They are distinguished by their viviparousness and genitalia. They have occupied various ecological niches and have highly developed brains, with humans being the most successful.
Placental mammals, also known as the infraclass Eutheria (from the Greek eu- “well(-developed)” and ther “beast”), are currently the dominant terrestrial life form on Earth, like dinosaurs before them, and pelycosaurs before them. . Eutheria’s sister group is Metatheria, which includes marsupials and their extinct relatives. A eutherium is usually defined as any animal more closely related to living placental mammals than to living marsupials. This definition includes many extinct mammals.
Eutheria is one of three groups of mammals, the others being marsupials and monotremes. Monotremes include only six species, while marsupials include about 350. Placental mammals have over 5,000 species, which include humans and many animals of symbolic, practical and economic importance to us, including dogs, cats, horses, cows, pigs , mice, deer, etc. Placental mammals emerged as the dominant large animals on Earth after the extinction of non-avian dinosaurs 65.5 million years ago.
Like other mammals, placental mammals are hairy, endothermic (“warm-blooded”), and have an extra brain layer (cortex). Their body temperature is slightly higher than that of marsupials and monotremes. They are distinguished from these other mammalian groups by a number of characteristics, including their genitalia, the separation of the genitalia and anus, and their viviparousness (the young remain within the body until fully developed). While live birth is traditionally associated with these types of animals, many other organisms practice it, including some plants, fish, scorpions, some sharks, some snakes, and velvet worms. Vivipary is thought to have evolved independently on many different occasions.
As the dominant terrestrial vertebrates, placental mammals have occupied a number of ecological niches, from small scavengers to herbivores of all sizes to powerful carnivores such as wolves and bears. Many of the largest members of the Eutheria class, including giant rodents, mammoths, saber-toothed tigers, and many others, have gone extinct in the last few million years. The largest wave of extinction occurred about 50,000 years ago when our ancestors spread across the planet, hunting many species to extinction in their wake.
Placental mammals have some of the most highly developed brains in the animal kingdom, culminating in the order Carnivora (cats, dogs, and relatives) and primates (gorillas, monkeys, humans, etc.). Of course, of all the placental mammals, humans have been the most successful. Now numbering around 7 billion individuals worldwide, humans, their pets and livestock are estimated to account for 99% of all terrestrial vertebrate biomass.
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