Placentals
| Placental mammals Temporal range: Paleocene-Holocene 66–present Ma Suspected, but not confirmed, Cretaceous origin |
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| a sample of placental diversity | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Clade: | Eutheria |
| Infraclass: |
Placentalia Owen, 1837 |
| Subgroups | |
Placentalia ("Placentals") is one of the three extant subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupialia. The placentals are primarily distinguished from other mammals in that the fetus is carried in the uterus of its mother where it is nourished via a placenta, until the live birth of a fully developed offspring occurs.
Placental mammals are anatomically distinguished from other mammals by:
Analysis of retroposon presence/absence patterns has provided a rapid, unequivocal means for revealing the evolutionary history of organisms: this has resulted in a revision in the classification of placentals. There are now thought to be three major subdivisions or lineages of placental mammals: Boreoeutheria, Xenarthra, and Afrotheria, all of which diverged from common ancestors.
The orders of placental mammals in the three groups are:
The exact relationships among these three lineages is currently a subject of debate, and three different hypotheses have been proposed with respect to which group is basal or diverged first from other placentals. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra), and Exafroplacentalia (basal Afrotheria). Estimates for the divergence times among these three placental groups range from 105 to 120 million years ago (MYA), depending on the type of DNA (e.g. nuclear or ) and varying interpretations of paleogeographic data.
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