Difference between revisions of "Placentalia" - New World Encyclopedia
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| + | '''Placentalia''' is a term used in some [[taxonomy|taxonomic classification schemes]] for one of the three major branches of [[mammal]]s that includes extant members, the others being [[Monotremata]] (montremes) and [[Marsupial]]ia (marsupials). With its members known as '''placental mammals''' or '''placentals,''', extant members are characterized by live birth of offspring and having the [[fetus]] nourished during [[gestation]] via a well-developed [[placenta]]. Marsupials also have a extra-embryonic, temporary vascular organ that forms in the uterus of females that is known as a placenta, but is is of different structure. | ||
| − | + | In some taxonomic schemes, Placentalia is placed within the clade [[Eutheria]] within the class Mammalia. In this case, extant eutherians, their last common ancestor, and all extinct descendants of that ancestor are placentals. | |
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==Overview== | ==Overview== | ||
Revision as of 01:20, 1 May 2014
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 Fossil specimen of Eomaia scansoria, an early eutherian
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Placentalia is a term used in some taxonomic classification schemes for one of the three major branches of mammals that includes extant members, the others being Monotremata (montremes) and Marsupialia (marsupials). With its members known as placental mammals or placentals,, extant members are characterized by live birth of offspring and having the fetus nourished during gestation via a well-developed placenta. Marsupials also have a extra-embryonic, temporary vascular organ that forms in the uterus of females that is known as a placenta, but is is of different structure.
In some taxonomic schemes, Placentalia is placed within the clade Eutheria within the class Mammalia. In this case, extant eutherians, their last common ancestor, and all extinct descendants of that ancestor are placentals.
Overview
Mammals (class Mammalia) are a clade of endothermic amniotes distinguished from the reptiles and the birds by the possession of hair, three middle ear bones, mammary glands in females, and a neocortex (a region of the brain). The mammalian brain regulates body temperature and the circulatory system, including the four-chambered heart. The basic body type is a four-legged land-borne animal, but some mammals are adapted for life at sea, in the air, in the trees, or on two legs.
In some classifications, the mammals are divided into two subclasses (not counting fossils): the Prototheria (order of Monotremata) and the Theria, the latter composed of the infraclasses Metatheria and Eutheria. The marsupials constitute the crown group of the Metatheria and therefore include all living metatherians as well as many extinct ones; the placentals likewise constitute the crown group of the Eutheria.
While the classification of mammals at the family level has been relatively stable, different treatments at higher levels—subclass, infraclass, and order—appear in contemporaneous literature, especially for the marsupials. Much recent change has reflected the results of cladistic analysis and molecular genetics. Results from molecular genetics, for example, have led to the adoption of new groups such as the Afrotheria and the abandonment of traditional groups such as the Insectivora.
Eutheria comes from the Greek ευ-, eu-, meaning "true/good" and θηρίον, thērion, meaning "beast," and thus "true beasts." Gill named the group in 1872 but applied the term to both placentals and marsupials; Huxley in 1880 recognized the term as used today where placentals are included and marsupials excluded and where the term is meant to encompass a more broadly defined group than Placentalia (Archibald 2001).
Characteristics
Mammals are all endotherms and amniote vertebrates. As with other mammals, eutherians are endotherms and amniote vertebrates, and have at most two generations of teeth, unlike reptiles in which there can be many generations of teeth. In mammals, the articular and quadrate, which composes the jaw joint in reptiles, is incorporated into the middle ear as the outermost two ear ossicles (malleus and incus; reptiles have only one ear ossicle, thte columnella). While in birds and reptiles, the large aorta leaving the heart bends to the right, in mammals it bends to the left (Archibald 2001).
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 (Ji et al. 2002).
- 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 (Ji et al. 2002).
- various features of jaws and teeth (Ji et al. 2002).
Description
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.[1]
- 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.[1]
- 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.[2]
- 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.[3] This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.[4]
Classification
Placentals are divided into three major groups:[5]
- Boreoeutheria, e.g. badgers, rabbits, guinea pigs, dogs
- Xenarthra, e.g. armadillos, anteaters
- Afrotheria, e.g. elephants, hyraxes, manatees
Molecular studies based on DNA analysis have revised the understanding of relationships among placental groups during the 21st century.[6] Classification systems based on molecular studies reveal three major groups or lineages of placental mammals: Afrotheria, Xenarthra, and Boreoeutheria, all of which diverged from common ancestors in the Cretaceous. The exact relationships between these three lineages is currently a subject of debate, and three different hypotheses have been proposed with respect to which group is basal, or diverged first from other placentals. These hypotheses are Atlantogenata (basal Boreoeutheria), Epitheria (basal Xenarthra), and Exafroplacentalia (basal Afrotheria).[7] Boreoeutheria in turn contains two major lineages- Euarchontoglires and Laurasiatheria.
Estimates for the divergence times among these three placental groups range from 105 to 120 million years ago, depending on type of DNA (e.g. nuclear or mitochondrial)[8] and varying interpretations of paleogeographic data.[7]
Evolution
The earliest known placental mammal species (including Protungulatum donnae, Purgatorius coracis, and Leptacodon proserpinae) are all dated to the early Paleogene period, about 65 million years ago. True placental mammals appeared shortly after the K-Pg extinction event and quickly diversified into ecological niches previously occupied by various groups of dinosaurs and other prehistoric reptiles.[9] True placental mammals (the crown group of all modern placentals) arose from stem-group members of the clade Eutheria, which had existed since at least the mid-Jurassic period.[1]
ReferencesISBN links support NWE through referral fees
- ↑ 1.0 1.1 1.2 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.
- ↑ Weil, A. (April 2002). Mammalian evolution: Upwards and onwards. Nature 416 (6883): 798–799.
- ↑ 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.
- ↑ 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.
- ↑ Archibald JD, Averianov AO, Ekdale EG (November 2001). Late Cretaceous relatives of rabbits, rodents, and other extant eutherian mammals. Nature 414 (6859): 62–5.
- ↑ Kriegs, Jan Ole and Churakov, Gennady; Kiefmann, Martin; Jordan, Ursula; Brosius, Jürgen; Schmitz, Jürgen (2006). Retroposed Elements as Archives for the Evolutionary History of Placental Mammals. PLoS Biology 4 (4): e91.
- ↑ 7.0 7.1 Nishihara, H. and Maruyama, S.; Okada, N. (2009). Retroposon analysis and recent geological data suggest near-simultaneous divergence of the three superorders of mammals. Proceedings of the National Academy of Sciences 106 (13): 5235–5240.
- ↑ Template:Cite doi
- ↑ (8 February 2013)The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals. Science 339 (6120): 662–667.
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