Eutheria

Eutheria (from Ancient Greek εὐ- (eú-), meaning "true, well", and θηρίον (thēríon), meaning "beast", and thus, "true beasts"), also called Pan-Placentalia, is the clade consisting of placental mammals and all therian mammals that are more closely related to placentals than to marsupials. Placentalia is the only surviving taxon of eutherians, and Eutheria is often used as a synonym of Placentalia in zoological contexts. Eutherians are distinguished from non-eutherians by various phenotypic traits of the feet, ankles, jaws and teeth. All living placental mammals lack epipubic bones, which are present in all other living mammals (marsupials and monotremes). This allows for expansion of the abdomen during pregnancy, though epipubic bones are present in a number of primitive eutherians. Eutheria was named in 1872 by Theodore Gill; in 1880, Thomas Henry Huxley defined it to encompass a more broadly defined group than Placentalia. The earliest unambiguous eutherians are known from the Early Cretaceous Yixian Formation of China, dating around 120 million years ago. Two tribosphenic mammals, Durlstodon and Durlstotherium from the Berriasian age (~145–140 million years ago) of the Early Cretaceous in southern England have also been suggested to represent early eutherians. Another possible eutherian species Juramaia sinensis has been dated at 161 million years ago from the early Late Jurassic (Oxfordian) of China. However some authors have considered Juramaia as a stem therian instead, and some sources have doubted the dating of the specimen.