Common characteristics of placental mammals?

Answer 1

bear live young, nourish young through mammary glands, usually have hair

Answer 2

wiki yay
Eutherians 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).[2]

There are no living non-placental eutherians, and so knowledge of their synapomorphies ("defining features") is entirely based on a few fossils – which means the reproductive features that distinguish modern placentals from other mammals cannot be used in defining the eutheria. The features of eutheria that distinguish them from metatherians, a group that includes modern marsupials, are:

    * an enlarged malleolus ("little hammer") at the bottom of the tibia, the larger of the two shin bones.[2]
    * the joint between the first metatarsal bone and the entocuneiform bone in the foot is offset further back than the joint between the second metatarsal and mesocuneiform bones – in metatherians these joints are level with each other.[2]
    * various features of jaws and teeth.[2]

Reproductive features are also of no use in identifying fossil placental mammals, which are distinguished from other eutherians by:

    * the presence of a malleolus at the bottom of the fibula, the smaller of the two shin bones.[2]
    * a complete mortise and tenon upper ankle joint, where the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula.[2]
    * a wide opening at the bottom of the pelvis, which allows the birth of large, well-developed offspring. Marsupials have and non-placental eutherians had a narrower opening that allows only small, immature offspring to pass through.[3]
    * the absence of epipubic bones extending forwards from the pelvis, which are not found in any placental, but are found in all other mammals – non-placental eutherians, marsupials, monotremes and mammaliformes – and even in the cynodont therapsids that are closest to mammals. Their function is to stiffen the body during locomotion.[4] This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.[5]