Eutheria

Further information: Evolution of mammals

Eutheria Temporal range: 165–0 Ma PreЄ Є O S D C P T J K Pg N Mid Jurassic – Recent
Fossil of Eomaia, an early eutherian
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Subclass:	Theria
Infraclass:	Eutheria Thomas Henry Huxley, 1880
Superorders and genera[1]
Placentalia Superorder Xenarthra Superorder Afrotheria Superorder Euarchontoglires Superorder Laurasiatheria †Bobolestes †Eomaia †Montanalestes †Murtoilestes †Prokennalestes and others

Eutheria (pronounced /juːˈθɪəriə/; Greek ευθήριον, pronounced euthérion and meaning "true/good beasts") is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos). They 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 (monotremes and marsupials).

The oldest known eutherian species is Juramaia sinensis, dated at 160 million years ago from the Jurassic in China.^[2] The previously earliest known fossil eutherian, Eomaia scansoria, was also from China and is dated to the Early Cretaceous period, about 125 million years ago.

Eutheria was introduced by Thomas Henry Huxley in 1880, meant to be broader in definition than its precursor Placentalia.

Definition

Eutherians are a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos).^[3]

There are no living nonplacental eutherians, and so knowledge of their synapomorphies ("defining features") is entirely based on a few fossils, which means the reproductive features that distinguish modern placentals from other mammals cannot be used in defining Eutheria. 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 mesocuneiform bones – in metatherians these joints are level with each other.^[3]
• various features of jaws and teeth.^[3]

Reproductive features are also of no use in identifying fossil placental mammals, which 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 have and nonplacental eutherians had 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 mammaliformes – and even in the cynodont therapsids 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]

Subgroups

Eutheria Euarchontoglires Laurasiatheria Xenarthra Afrotheria

These are the subgroups of extant members of Eutheria:

• Boreoeutheria, e.g. badgers, rabbits, guinea pigs, dogs, dolphins
  • Euarchontoglires, e.g. humans, monkeys, rats, hares
  • Laurasiatheria, e.g. cattle, whales, bats, cats
• Xenarthra, e.g. armadillos, anteaters
• Afrotheria, e.g. elephants, manatees

These groups together make up the crown group Placentalia (placental mammals). Eutheria also includes now extinct lineages that lie outside of Placentalia (see below).^[7]

Analysis of transposable element insertions around the time of divergence of Boreoeutheria, Afrotheria, and Xenarthra strongly support a near-concomitant origin (trifurcation) of these three superorders, making further subdivision impractical and meaningless.^[8][9] These observations eliminate the need to choose between the previously proposed groupings of Boreoeutheria and Xenarthra (Exafroplacentalia)^[10], Afrotheria and Xenarthra (Atlantogenata)^[11][12][13], Afrotheria and Boreoeutheria (Epitheria).^[14][15]

Evolutionary history

The fossil eutherian species believed to be the oldest known is Juramaia sinensis, said to be about 160 million years ago.[2] Another early eutherian mammal is the nonplacental Eomaia scansoria from the Lower Cretaceous of China, dated to about 125 million years ago. Some of its fossils show thick fur. 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]

Millions of years ago — = Placentals     — = Other eutheria Origin of eutheria     = Asian fossils         = N American fossils     = Period when placental classes diverged according to molecular phylogenetics estimates Murtoilestes Prokennalestes Eomaia Montanalestes Ukhaatherium Asioryctes Kennalestes Zalambdalestes Daulestes Aspanestes Eoungulatum Protungulatum Gypsonictops Cimolestes Fossil record of Cretaceous eutheria[3]

Cynodonts † Other Cynodonts † Tritylodontids Mammaliformes † Other mammaliformes † Hadrocodium Mammals † Other mammals Australosphenids † Other Australosphenids Monotremes Theria † Other Theria Metatheria † Other Metatheria Marsupials Eutheria † Other Eutheria Placentals Simplified, non-systematic, outline of evolution of eutheria from cynodont therapsids.[3] † = extinct

References

1. ^ "Eutheria phylogeny". Mikko's Phylogeny Archive. http://www.helsinki.fi/~mhaaramo/metazoa/deuterostoma/chordata/synapsida/eutheria/eutheria_index.html. Retrieved 2008-07-24. 
2. ^ ^{a b} Luo Z, Yuan C, Meng Q & Ji Q (2011), "A Jurassic eutherian mammal and divergence of marsupials and placentals", Nature 476(7361): p. 42–45.
3. ^ ^{a b c d e f g h i} 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. http://www.nature.com/nature/journal/v416/n6883/full/416816a.html. Retrieved 2008-09-24. 
4. ^ Weil, A. (April 2002). "Mammalian evolution: Upwards and onwards". Nature 416 (6883): 798–799. doi:10.1038/416798a. PMID 11976661. http://www.nature.com/nature/journal/v416/n6883/full/416798a.html. Retrieved 2008-09-24. 
5. ^ 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. http://www.sciencemag.org/cgi/content/full/299/5605/400. Retrieved 2008-09-24. 
6. ^ 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. http://www.nature.com/nature/journal/v389/n6650/full/389483a0.html. Retrieved 2008-09-24. 
7. ^ 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. 
8. ^ Nishihara, H., Maruyama, S. & Okada, N. 2009. Retroposon analysis and recent geological data suggest near-simultaneous divergence of the three superorders of mammals. PNAS 106: 5235–40.
9. ^ Churakov, G., Kriegs, J.O., Baertsch, R., Zemann, A., Brosius, J. & Schmitz, J. 2009. Mosaic retroposon insertion patterns in placental mammals. Genome Research 19: 868–75.
10. ^ 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, pp. 413–421.
11. ^ 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. http://www.pnas.org/content/104/36/14395.full. 
12. ^ Murphy WJ, Pringle TH, Crider TA, Springer MS, Miller W (2007). "Using genomic data to unravel the root of the placental mammal phylogeny". Genome Research 17 (4): 413–21. doi:10.1101/gr.5918807. PMC 1832088. PMID 17322288. http://genome.cshlp.org/content/17/4/413.long. 
13. ^ 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. 
14. ^ 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: 572–584.
15. ^ Churakov G, Kriegs JO, 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. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2675975. 

Further reading

• Goloboff, P.A.; Catalano, S.A.; Mirande, J.M.; Szumik, C.A.; Arias, J.S.; Källersjö, M & Farris, J.S. 2009. Phylogenetic analysis of 73 060 taxa corroborates major eukaryotic groups. Cladistics 25 (3): 211–230