Eutheria (from Greek εὐ-, 'good, right' and θηρίον, 'beast'; lit. 'true beasts'), also called Pan-Placentalia, is the clade consisting of placentals and all therian mammals that are more closely related to placentals than to marsupials. Eutherians are distinguished from non-eutherians by various phenotypic traits of the feet, ankles, jaws and teeth. All extant eutherians 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 many 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 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.

Characteristics

Distinguishing features are:

Taxonomy

Eutheria (i.e. Placentalia sensu lato, Pan-Placentalia): Notes:

Evolutionary history

Eutheria contains several extinct genera as well as larger groups, many with complicated taxonomic histories still not fully understood. Members of the Adapisoriculidae, Cimolesta and Leptictida have been previously placed within the outdated placental group Insectivora, while zhelestids have been considered primitive ungulates. However, more recent studies have suggested these enigmatic taxa represent stem group eutherians, more basal to Placentalia. The weakly favoured cladogram favours Boreoeutheria as a basal eutherian clade as sister to the Atlantogenata. Phylogeny after Wang & Wang, 2023. Below is a phylogeny from Gheerbrant & Teodori (2021):

Ecology

Many non-placental eutherians are thought to have been insectivores, as is the case with many primitive mammals. However, the zhelestids are thought to have been herbivorous. Body size of eutherians was generally small during the Cretaceous period, but the range of body sizes increased dramatically after the K-Pg extinction, predominantly among placentals.