Temporal range: Late Jurassic–present, 160–0Ma
|Life restoration of Juramaia sinensis, the earliest known eutherian|
Eutheria (//; from Greek ευ-, eu- "true/good" and θηρίον, thērion "beast" hence "true beasts") is one of two mammalian clades with extant members that diverged in the Early Cretaceous or perhaps the Late Jurassic. The other is the Metatheria, which includes marsupials, most of whom accommodate their neonates in pouches. Except for the Virginia opossum, which is a metatherian, all mammals indigenous to Europe, Africa, Asia, and North America north of Mexico are eutherians. Extant eutherians, their last common ancestor, and all extinct descendants of that ancestor are placentals, in the infraclass Placentalia.
Eutherians are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth. One of the major differences between placental and nonplacental eutherians is that placentals lack epipubic bones, which are present in all other fossil and living mammals (marsupials and monotremes).
"Eutheria" was introduced by Thomas Henry Huxley in 1880, meant to be broader in definition than "Placentalia", the term previously in use.
The features of Eutheria that distinguish them from metatherians, a group that includes modern marsupials, are:
- an enlarged malleolus ("little hammer") at the bottom of the tibia, the larger of the two shin bones.3
- the joint between the first metatarsal bone and the entocuneiform bone in the foot is offset further back than the joint between the second metatarsal and middle cuneiform bones – in metatherians these joints are level with each other.3
- various features of jaws and teeth.3
Placental mammals are distinguished from other eutherians by:
- the presence of a malleolus at the bottom of the fibula, the smaller of the two shin bones.3
- a complete mortise and tenon upper ankle joint, where the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula.3
- a wide opening at the bottom of the pelvis, which allows the birth of large, well-developed offspring. Marsupials and nonplacental eutherians have a narrower opening that allows only small, immature offspring to pass through.4
- the absence of epipubic bones extending forward from the pelvis, which are not found in any placental, but are found in all other mammals – nonplacental eutherians, marsupials, monotremes, and earlier mammaliaforms - as well as in other cynodonts that are closest to mammals. Their function is to stiffen the body during locomotion.5 This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.6
Eutheria contains several extinct genera and larger groups, many with complicated taxonomic history that is still unresolved. Members of the Adapisoriculidae, Cimolesta and Leptictida have been previously placed within the out-dated placental group "Insectivora", while Zhelestids have been considered primitive ungulates.7 However, more recent studies have suggested these enigmatic taxa represent stem group eutherians, more basal to Placentalia.89
Extant eutherians are divided into three major groups:
- Boreoeutheria, e.g. badgers, rabbits, guinea pigs, dogs
- Xenarthra, e.g. armadillos, anteaters
- Afrotheria, e.g. elephants, hyraxes, manatees
An analysis of transposable element insertions around the time of divergence of Boreoeutheria, Afrotheria, and Xenarthra has supported a near-concomitant origin (trifurcation) of these three superorders.1112 If accepted, this conclusion would eliminate the need to choose between the previously proposed groupings of Boreoeutheria and Xenarthra (Exafroplacentalia),13 Afrotheria and Xenarthra (Atlantogenata),131415 Afrotheria and Boreoeutheria (Epitheria).1216 The debate continues, however; a 2013 paper identifying Protungulatum donnae as one of the first placental mammals supports the Epitheria hypothesis using an analysis that combines molecular and phenomic considerations.1718
|The major lineages of placentals|
- Clade Afroinsectiphilia
- Clade Paenungulata
- Order Pilosa: sloths and anteaters (neotropical)
- Order Cingulata: armadillos and extinct relatives (Americas)
- Clade: Euarchontoglires (Supraprimates)
- Superorder Euarchonta
- Superorder Glires
- Clade Laurasiatheria
- Order Erinaceomorpha: hedgehogs
- Order Soricomorpha: moles, shrews, solenodons
- Clade Ferungulata
- Clade Cetartiodactyla
- Clade Pegasoferae
|The fossil eutherian species believed to be the oldest known is Juramaia sinensis, which lived about .2 Montanalestes was found in North America, while all other nonplacental eutherian fossils have been found in Asia. The earliest known placental fossils have also been found in Asia.3|
- Rook, Deborah L.; Hunter, John P. (April 2013). "Rooting Around the Eutherian Family Tree: the Origin and Relations of the Taeniodonta". Journal of Mammalian Evolution: 1–17. doi:10.1007/s10914-013-9230-9.
- Luo, Z.; C. Yuan, Q. Meng, and Q. Ji (2011). "A Jurassic eutherian mammal and divergence of marsupials and placentals". Nature 476 (7361): 42–45. Bibcode:2011Natur.476..442L. doi:10.1038/nature10291. PMID 21866158.
- Ji, Q., Luo, Z-X., Yuan, C-X.,Wible, J.R., Zhang, J-P. and Georgi, J.A. (April 2002). "The earliest known eutherian mammal". Nature 416 (6883): 816–822. doi:10.1038/416816a. PMID 11976675. Retrieved 2008-09-24.
- Weil, A. (April 2002). "Mammalian evolution: Upwards and onwards". Nature 416 (6883): 798–799. doi:10.1038/416798a. PMID 11976661. Retrieved 2008-09-24.
- Reilly, S.M., and White, T.D. (January 2003). "Hypaxial Motor Patterns and the Function of Epipubic Bones in Primitive Mammals". Science 299 (5605): 400–402. doi:10.1126/science.1074905. PMID 12532019. Retrieved 2008-09-24.
- Novacek, M.J., Rougier, G.W, Wible, J.R., McKenna, M.C, Dashzeveg, D.,and Horovitz, I. (October 1997). "Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia". Nature 389 (6650): 483–486. doi:10.1038/39020. PMID 9333234. Retrieved 2008-09-24.
- Rose, Kenneth D. (2006). The beginning of the age of mammals. Baltimore: Johns Hopkins University Press. ISBN 9780801892219.
- Wible, J. R.; Rougier, G. W.; Novacek, M. J.; Asher, R. J. (2007). "Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary". Nature 447 (7147): 1003–1006. doi:10.1038/nature05854.
- Wible, John R.; Rougier, Guillermo W.; Novacek, Michael J.; Asher, Robert J. (2009). "The Eutherian Mammal Maelestes gobiensis from the Late Cretaceous of Mongolia and the phylogeny of cretaceous eutheria". Bulletin of the American Museum of Natural History 327: 1–123. doi:10.1206/623.1.
- Archibald JD, Averianov AO, Ekdale EG (November 2001). "Late Cretaceous relatives of rabbits, rodents, and other extant eutherian mammals". Nature 414 (6859): 62–5. doi:10.1038/35102048. PMID 11689942.
- Nishihara, H., S. Maruyama, N. Okada (2009). "Retroposon analysis and recent geological data suggest near-simultaneous divergence of the three superorders of mammals". PNAS 106 (13): 5235–40. Bibcode:2009PNAS..106.5235N. doi:10.1073/pnas.0809297106.
- Churakov G., Kriegs J.O., Baertsch R., Zemann A., Brosius J., Schmitz J. (2009). "Mosaic retroposon insertion patterns in placental mammals". Genome Research 19 (5): 868–75. doi:10.1101/gr.090647.108. PMC 2675975. PMID 19261842.
- Murphy W.J., Pringle T.H., Crider T.A., Springer M.S., Miller W. (2007). "Using genomic data to unravel the root of the placental mammal phylogeny". Genome Research 17 (4): 413–421. doi:10.1101/gr.5918807. PMC 1832088. PMID 17322288.
- Wildman DE, Uddin M, Opazo JC, et al (2007). "Genomics, biogeography, and the diversification of placental mammals". PNAS 104 (36): 14395–400. doi:10.1073/pnas.0704342104. PMC 1958817. PMID 17728403.
- Schneider A, Cannarozzi GM (2009). "Support Patterns from Different Outgroups Provide a Strong Phylogenetic Signal". Mol. Biol. Evol. 26 (6): 1259–72. doi:10.1093/molbev/msp034. PMID 19240194.
- Shoshani J, McKenna MC (1998). "Higher taxonomic relationships among extant mammals based on morphology, with selected comparisons of results from molecular data". Mol Phylogenet Evol 9 (3): 572–584. doi:10.1006/mpev.1998.0520.
- O'Leary, Maureen A.; Bloch, Jonathan I.; Flynn, John J.; Gaudin, Timothy J.; Giallombardo, Andres; Giannini, Norberto P.; Goldberg, Suzann L.; Kraatz, Brian P.; Luo, Zhe-Xi; Meng, Jin; Ni, Michael J.; Novacek, Fernando A.; Perini, Zachary S.; Randall, Guillermo; Rougier, Eric J.; Sargis, Mary T.; Silcox, Nancy b.; Simmons, Micelle; Spaulding, Paul M.; Velazco, Marcelo; Weksler, John r.; Wible, Andrea L.; Cirranello, A. L. (8 February 2013). "The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals". Science 339 (6120): 662–667. doi:10.1126/science.1229237. PMID 23393258. Retrieved 9 February 2013.
- Wilford, John Noble (7 February 2013). "Rat-Size Ancestor Said to Link Man and Beast". New York Times. Retrieved 9 February 2013.
- Goloboff, Pablo A.; Catalano, Santiago A.; Mirande, J. Marcos; Szumik, Claudia A.; Arias, J. Salvador; Källersjö, Mari; Farris, James S. (2009). "Phylogenetic analysis of 73 060 taxa corroborates major eukaryotic groups". Cladistics 25 (3): 211–230. doi:10.1111/j.1096-0031.2009.00255.x.
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|Wikisource has the text of the 1911 Encyclopædia Britannica article Monodelphia.|