Eutheria (Placental Mammals)


Eutheria includes one of three major clades of mammals, the extant members of which are referred to as placentals. With more than 5000 extinct and extant genera, including 1220 extant (living) genera and over 6000 extant species arrayed in 19 extant orders, placentals (extant eutherians) are the most taxonomically diverse of the three branches of extant mammals. Eutherians appear in the fossil record by 125 Ma (million years ago). Extant orders of eutherians (placentals) do not occur in the fossil record until after dinosaur extinction 65–66 Ma. Molecular studies generally agree with these dates for the origin of placental orders but place the origin of Eutheria at approximately 190 Ma and groups related to living orders of mammals by 100 Ma. Placentals vary greatly in size (whales down to shrews), in locomotion (flying, swimming, climbing, burrowing, running), and diet (meat, leaves, fruit, termites). They have a high resting temperature (homiothermy) and produce this heat internally (endothermy). They have a chorioallantoic placenta that allows a long gestation for development.

Key Concepts

  • Eutherian mammals are the most ecomorphological and taxonomically diverse group of mammals.
  • The evolutionary success of eutherian mammal is related to their mode of reproduction (euviviparity).
  • Eutherian mammals originated in the Early Cretaceous but modern placental orders radiated in the Cenozoic, after extinction of dinosaurs.
  • The four major groups of Placentalia are Xenarthra, Afrotheria, Laurasiatheria and Euarchontoglires.
  • The Cretaceous stem placentals mostly retained primitive therian dental formula which was further modified in the Cenozoic crown placentals.

Keywords: Mammalia; Eutheria; Placentalia; evolution; biodiversity; dental formula

Figure 1. Dental formula in therian ancestors (a), metatherians (b), Cretaceous Eutheria (c) and crown‐group Placentalia (d). Therian ancestors replaced all incisors, canines and all but first premolar. The last deciduous premolar is molariform in structure. Metatherians lost replacement in the fifth premolar locus and last premolar becomes the first molar. They also lost first premolar and all milk teeth except for the third premolar that becomes the last premolar. Cretaceous eutherians lost replacement of the third premolar. The crown‐group placentals lost two upper and one lower incisor and the third premolar.
Figure 2. The three kinds of reproduction in mammals.
Figure 3. Phylogeny of placental (extant eutherian) orders showing timing of splits between orders, based upon various molecular data.


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Further Reading

Agusti JD and Anton M (2005) Mammoths, Sabertooths, and Hominids: 65 Million Years of Mammalian Evolution in Europe. Columbia University Press: New York.

Attenborough D (2002) The Life of Mammals. Princeton University Press: Princeton.

Berta A (2012) Return to the Sea: The Life and Evolutionary Times of Marine Mammals. University of California Press: Berkeley.

Macdonald DW (2009) The Princeton Encyclopedia of Mammals. Princeton University Press: Princeton.

Prothero DR (2006) After the Dinosaurs: The Age of Mammals (Life of the Past). Indiana University Press: Bloomington.

Rose KD (2006) The Beginning of the Age of Mammals. Johns Hopkins University Press: Baltimore.

Ungar PS (2010) Mammal Teeth: Origin, Evolution, and Diversity. Johns Hopkins University Press: Baltimore.

Wallace DR (2005) Beasts of Eden: Walking Whales, Dawn Horses, and Other Enigmas of Mammal Evolution. University of California Press: Berkeley.

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How to Cite close
Averianov, Alexander O(Sep 2020) Eutheria (Placental Mammals). In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0029197]