Eutherians, the true beasts, lost a pair of bones to make room for pregnancy
Marsupials and monotremes, every living mammal outside the placentals, carry a pair of small epipubic bones. You do not. Losing them let the belly expand during pregnancy, one of the quiet anatomical shifts that define the eutherians, the lineage whose name means true beasts.
Eutheria, from Greek words for true and beast, takes in the placental mammals plus every extinct relative closer to them than to marsupials. Since placentals are the only survivors, zoologists often treat the two names as interchangeable. Theodore Gill coined the term in 1872, and Thomas Henry Huxley broadened it in 1880 to cover more than the placentals alone.
Telling an early eutherian from a marsupial relative comes down to fine print in the skeleton. Eutherians have an enlarged malleolus, the little hammer at the base of the tibia; a joint in the foot set farther back than its neighbour, where in marsupial relatives the two sit level; three molars in each half of the jaw; and upper canines with two roots. Even the epipubic bones are not a perfect guide, since several primitive eutherians still had them.
The oldest undisputed members come from China's Yixian Formation, roughly 120 million years old. Two tiny mammals from southern England, Durlstodon and Durlstotherium, may push the record back to around 145 to 140 million years ago. A more dramatic candidate, Juramaia sinensis, has been dated to 161 million years ago in the Jurassic, although some researchers think it sits outside the group and others doubt the date.
For most of their early history these animals were small and fairly uniform, many of them insect eaters, although the zhelestids seem to have grazed on plants. Families once lumped into an outdated group called Insectivora now look like early branches outside the placentals proper. The mass extinction that ended the Cretaceous changed everything: many small and medium species vanished, and the surviving lineages grew larger and far more varied in form. Among living placentals, the weakly favoured family tree splits them into two great branches, Boreoeutheria and Atlantogenata.
Source: Eutheria
