Eutheria (Placental Mammals)

Eutheria includes one of three major clades of mammals, the extant members of which are referred to as placentals.

With almost 4800 extinct and extant genera, including 1135 extant (living) genera and over 5000 extant species arrayed in 20 extant orders, placentals (extant eutherians) are the most taxonomically diverse of the three branches of extant mammals. Eutherians appear in the fossil record by 105 Ma (million years ago) and possibly by 160 Ma. 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, etc.), and diet (meat, leaves, fruit, termites, etc.). 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 arose somewhere between 105 and 190 Ma.
  • Extant eutherian mammals, known as placentals, may have originated as long as 100 Ma based on molecular studies.
  • Placental orders first began to appear and diversify approximately 65 Ma (±10 My) based on both molecular and fossil studies.
  • There are 20 orders of placentals comprised of 1135 living genera and over 5000 species.
  • Placentals have a high resting temperature (homiothermy) and produce this heat internally (endothermy).
  • Placentals have a chorioallantoic placenta that allows a long gestation for development, hence their name.
  • Placentals are arguably the most ecologically diverse group of living vertebrates as they vary greatly in size, locomotory types, and diet.

Keywords: placenta; chorion; allantois; amnion; incisor; canine; premolar; molar; corpus callosum; endothermy

Figure 1. The maximum number of upper and lower teeth normally found in an extant marsupial such as opossum (left) and in an extant placental such as a pig (right). First generation teeth are shown in grey and second generation teeth are shown in white. Marsupials suppress their anterior first generation teeth. Molars are first generation teeth that erupt only as the jaws increase in size (by the author following Archibald and Averianov, 2012).
Figure 2. The three kinds of reproduction in mammals (by the author after a variety of sources).
Figure 3. Phylogeny of placental (extant eutherian) orders showing timing of splits between orders, based upon various molecular data (ordinal relationships–modified after Meredith et al., 2011. Copyright by American Association for the Advancement of Science).
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 References
    Archibald JD (2003) Timing and biogeography of the eutherian radiation: fossils and molecules compared. Molecular Phylogeny and Evolution 28: 350–359.
    book Archibald JD (2011) Extinction and Radiation: How the Fall of the Dinosaurs Led to the Rise of the Mammals. Baltimore: Johns Hopkins University Press.
    Archibald JD and Averianov AO (2012) Phylogenetic analysis, taxonomic revision, and dental ontogeny of the Cretaceous Zhelestidae (Mammalia: Eutheria). Zoological Journal of the Linnean Society 164: 361–426.
    Cifelli RL (1999) Tribosphenic mammal from the North American Early Cretaceous. Nature 401: 363–366.
    Cifelli RL and Gordon C (2011) New information on tribosphenic mammals from the Cloverly formation, Montana and Wyoming. Journal of Vertebrate Paleontology Program and Abstracts 31(suppl. 6): 89.
    Ji Q, Luo Z‐X, Wible JR, Zhang J‐P and Georgi JA (2002) The earliest known eutherian mammal. Nature 416: 816–822.
    Kielan‐Jaworowska Z and Dashzeveg D (1989) Eutherian mammals from the Early Cretaceous of Mongolia. Zoologica Scripta 18: 347–355.
    Luo Z‐X, Yuan C‐X, Meng Q‐J and Ji Q (2011) A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature 476: 442–445.
    book McKenna MC and Bell SK (1997) Classification of Mammals. New York: Columbia University Press.
    Meredith RW, Janecka JE, Gatesy J et al. (2011) Impacts of the cretaceous terrestrial revolution and KPg extinction on extant mammal diversification. Science 334: 521–524.
    Murphy WJ, Eizirik E, O'Brien SJ et al. (2001) Resolution of the early placental mammal radiation using Bayesian phylogenetics. Science 294: 2348–2351.
    book Rose KD and Archibald JD (eds) (2005) The Rise of Placental Mammals: Origin and Relationships of the Major Extant Clades. Baltimore: Johns Hopkins University Press.
    Rougier GW, Wible JR and Novacek MJ (1998) Implications of Deltatheridiun specimens for early marsupial history. Nature 396: 459–463.
    book Wilson DE and Reeder DM (2005) Mammal Species of the World: A Taxonomic and Geographic Reference. Baltimore: Johns Hopkins University.
 Further Reading
    book Agusti JD and Anton M (2005) Mammoths, Sabertooths, and Hominids: 65 Million Years of Mammalian Evolution in Europe. New York: Columbia University Press.
    book Attenborough D (2002) The Life of Mammals. Princeton: Princeton University Press.
    book Berta A (2012) Return to the Sea: The Life and Evolutionary Times of Marine Mammals. Berkeley: University of California Press.
    book Macdonald DW (2009) The Princeton Encyclopedia of Mammals. Princeton: Princeton University Press.
    book Prothero DR (2006) After the Dinosaurs: The Age of Mammals (Life of the Past). Bloomington: Indiana University Press.
    book Rose KD (2006) The Beginning of the Age of Mammals. Baltimore: Johns Hopkins University Press.
    book Ungar PS (2010) Mammal Teeth: Origin, Evolution, and Diversity. Baltimore: Johns Hopkins University Press.
    book Wallace DR (2005) Beasts of Eden: Walking Whales, Dawn Horses, and Other Enigmas of Mammal Evolution. Berkeley: University of California Press.
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Archibald, J David(May 2012) Eutheria (Placental Mammals). In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001555.pub3]