<P> The time of appearance of the earliest eutherians has been a matter of controversy . On one hand, recently discovered fossils of Juramaia have been dated to 160 million years ago and classified as eutherian . Fossils of Eomaia from 125 million years ago in the Early Cretaceous have also been classified as eutherian . A recent analysis of phenomic characters, however, classified Eomaia as pre-eutherian and reported that the earliest clearly eutherian specimens came from Maelestes, dated to 91 million years ago . That study also reported that eutherians did not significantly diversify until after the catastrophic extinction at the Cretaceous--Paleogene boundary, about 66 million years ago . </P> <P> Eomaia was found to have some features that are more like those of marsupials and earlier metatherians: </P> <Ul> <Li> Epipubic bones extending forwards from the pelvis, which are not found in any modern placental, but are found in all other mammals--early mammaliaforms, non-placental eutherians, marsupials, and monotremes--as well as in the cynodont therapsids that are closest to mammals . Their function is to stiffen the body during locomotion . This stiffening would be harmful in pregnant placentals, whose abdomens need to expand . </Li> <Li> A narrow pelvic outlet, which indicates that the young were very small at birth and therefore pregnancy was short, as in modern marsupials . This suggests that the placenta was a later development . </Li> <Li> Five incisors in each side of the upper jaw . This number is typical of metatherians, and the maximum number in modern placentals is three, except for homodonts, such as the armadillo . But Eomaia's molar to premolar ratio (it has more pre-molars than molars) is typical of eutherians, including placentals, and not normal in marsupials . </Li> </Ul> <Li> Epipubic bones extending forwards from the pelvis, which are not found in any modern placental, but are found in all other mammals--early mammaliaforms, non-placental eutherians, marsupials, and monotremes--as well as in the cynodont therapsids that are closest to mammals . Their function is to stiffen the body during locomotion . This stiffening would be harmful in pregnant placentals, whose abdomens need to expand . </Li>

The fossils of eutheria are recognized and distinguished by what
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