Placentals (Placentalia or Eutheria) are the largest group of mammals belonging to the subclass of live animals. The terms Placentalia and Eutheria are synonymous only for recent mammals, but in general Eutheria is a broader group, including many other extinct lines in addition to placentals.
Most living mammals (including humans) belong to the placental mammals, the remaining two groups represent the superorder of marsupials, which also belong to the subclass of live-bearing mammals, and the order of avian beetles, which belongs to the subclass of oviparous. In the past, however, there were other, extinct groups of mammals. The main feature that distinguishes placentals from other mammals is the feeding of the fetus using the placenta during pregnancy in the uterus (except for marsupials, bandicuts that have a primitive placenta). Only females have a placenta. Due to the placenta through which the fetus is nourished during development, the pup is much better developed at birth at the time of the newborn marsupial pup. The placentals controlled the country on all continents, except Australia, which separated from the other continents before their expansion.
Eutheria is a taxon introduced by Thomas Henry Huxly in 1880 that contains all placental mammals and the nearest ancestors of placentals (known only from fossil records). The name literally means "real animal". The closest living relatives are marsupials.
The oldest known species of placental mammal is probably the Jurassic Juramaia sinensis, which comes from 160 million years old.
Longer known is Eomaia (literally "ancient mother") from the Lower Cretaceous, about 125 million years old. The fossil was found in the sediments of a Chinese lake. She is undoubtedly a placental, but her hips are too narrow to give birth to a fully developed pup. This suggests that the placenta played only a minor role in the development of pups in this species. The rapid development of placentals occurred mainly in the period 66 million years ago, after a large mass extinction at the end of the Cretaceous. However, phylogenomic data show that true placentals probably appeared in the Cretaceous period, ie 145 to 66 million years ago. Placentals probably lived as early as the last dinosaurs, as shown by some discoveries in present-day India.
A. V. Lopatin & A. O. Averianov (2018). Earliest Placentals: at the Dawn of Big Time. Nature 2018 (4): 34-40. (Russian)
Thomas J. D. Halliday, Mario dos Reis, Asif U. Tamuri, Henry Ferguson-Gow, Ziheng Yang and Anjali Goswami (2019). Rapid morphological evolution in placental mammals post-dates the origin of the crown group. Proceedings of the Royal Society B: Biological Sciences. 286 (1898): 20182418. doi: https://doi.org/10.1098/rspb.2018.2418
Mark S. Springer (2022). Afrotheria. Current Biology. 32 (5): PR205-R210. doi: https://doi.org/10.1016/j.cub.2022.02.001
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Encyclopedia Keyword Placentalia in Otto's Dictionary of Teaching in Wikisource