Eutheria ("true beasts") are a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos). They are distinguished from non-eutherians by various features of the feet, ankles, jaws and teeth. One of the major differences between placental and non-placental eutherians is that placentals lack the epipubic bones, which are present in all other fossil and living mammals. The absence of these bones is probably an adaptation that helps them be faster on the run than their relatives.
These mammals have truly mastered all sorts of environments. They range from the polar caps(polar bears) to the equator(gorilla) to the oceans(blue whale). Among other things Eutherians support vast orders that have the ability to completley modify their body temperature and heart beat. This helps them beat the odds in adverse climes like deserts and ice caps.
Mildly recognizable eutherians broke away from the metatherian strain no later the Early Cretaceous in warm, probably moderately seasonal climates. Immediate ancestors were small, sharing many anatomical, physiological and reproductive features with small modern marsupials. Development of characteristically eutherian features involved interactions of body size and rates of metabolism(this means the rate of usage of energy).
Biological implications of this contrast are most pronounced at small body sizes. When resources are abundant, the relatively higher growth rates and earlier maturation of small eutherians (particularly those with high rates of metabolism) can lead to rapid population growth; among most marsupials, however, such high rates of reproduction are rarely possible. Also, only eutherians among mammals give birth to truly developed embryos and that has helped their spread across the globe.
You name it, they can do it! These mammals are known to live in very harsh climates and have adapted to eat anything that they can, at an extremely fast pace. They are so diversified that there is no one behavior that can be truly called eutherian aside from giving birth to well defined and very active babies.
Eutherians are the best communicators among mammals; well I am communicating well enough with you aren't I? They live in solitary arrangements, in groups and even in vast herds millions strong(think wildebeest).
Eutherians have adapted to be niche based eaters. They generally eat everything and each other when time and opportunity arises.
The most numerous eutherians are rodents and humans. They seem determined to have a population competition and the rodents are losing. We humans are eating and despoiling the planet for all the rest of the animals and so are the biggest threat to ourselves. Sometimes existential questions can be posed by , well, just being....
Orders within Eutheria
SuperOrder Afrotheria (Afrotherian mammals)
SuperOrder Euarchontoglires (Euarchontoglirean mammals)
SuperOrder Laurasiatheria (Laurasiatherian mammals)
SuperOrder Eulipotyphla (hedgehogs, shrews, and relatives)
SuperOrder Xenarthra (Xenarthrans)