Eutheria

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Eutheria
Temporal range: Late JurassicHolocene, 160–0  Ma
Juramaia NT.jpg
Juramaia , the oldest known eutherian
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Subclass: Theria
Clade: Eutheria
Gill, 1872
Subgroups

Eutheria ( /jˈθɪəriə/ ; from Greek εὐ-, eú- 'good, right' and θηρίον, thēríon 'beast'; lit.'true beasts') is the clade consisting of all therian mammals that are more closely related to placentals than to marsupials.

Contents

Eutherians are distinguished from noneutherians by various phenotypic traits of the feet, ankles, jaws and teeth. All extant eutherians lack epipubic bones, which are present in all other living mammals (marsupials and monotremes). This allows for expansion of the abdomen during pregnancy. [2]

The oldest-known eutherian species is Juramaia sinensis , dated at 161  million years ago from the early Late Jurassic (Oxfordian) of China. [3]

Eutheria was named in 1872 by Theodore Gill; in 1880 Thomas Henry Huxley defined it to encompass a more broadly defined group than Placentalia. [4]

Characteristics

The entocuneiform bone Cambridge Natural History Mammalia Fig 068.png
The entocuneiform bone

Distinguishing features are:

Evolutionary history

Eutheria contains several extinct genera as well as larger groups, many with complicated taxonomic histories still not fully understood. 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. [6] However, more recent studies have suggested these enigmatic taxa represent stem group eutherians, more basal to Placentalia. [7] [8]

The weakly favoured cladogram favours Boreoeuthearia as a basal eutherian clade as sister to the Atlantogenata. [9] [10] [11]

Eutheria  
Atlantogenata

Xenarthra

Afrotheria

Boreoeutheria

Laurasiatheria

Euarchontoglires

The fossil eutherian species believed to be the oldest known is Juramaia sinensis , which lived about 160  million years ago. [3] 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. [5]
Cynodonts

Tritylodontids

Mammaliaformes

Other mammaliaformes

Hadrocodium

Mammals
Australosphenids

Other
Australosphenids

Monotremes

Theria

Metatheria

Eutheria

Simplified, non-systematic, outline of evolution of eutheria from cynodont therapsids. [5]
† = extinct

Related Research Articles

Marsupial Infraclass of mammals in the clade Metatheria

Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a pouch. Well-known marsupials include kangaroos, wallabies, koalas, opossums, wombats, Tasmanian devils, and the extinct thylacine. Some lesser-known marsupials are the dunnarts, potoroos, and the cuscus.

Placentalia Infraclass of mammals in the clade Eutheria

Placentalia is one of the three extant subdivisions of the class of animals Mammalia; the other two are Monotremata and Marsupialia. The placentals are partly distinguished from other mammals in that the fetus is carried in the uterus of its mother to a relatively late stage of development. The name is something of a misnomer considering that marsupials also nourish their fetuses via a placenta, though for a relatively briefer period, giving birth to less developed young who are then kept for a period in the mother's pouch.

<i>Eomaia</i> Extinct family of mammals

Eomaia is a genus of extinct fossil mammals containing the single species Eomaia scansoria, discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower Cretaceous about 125 million years ago. The single fossil specimen of this species is 10 centimetres (3.9 in) in length and virtually complete. An estimate of the body weight is between 20–25 grams (0.71–0.88 oz). It is exceptionally well-preserved for a 125-million-year-old specimen. Although the fossil's skull is squashed flat, its teeth, tiny foot bones, cartilages and even its fur are visible.

Metatheria Clade of marsupials and close relatives

Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a slightly more inclusive group than the marsupials; it contains all marsupials as well as many extinct non-marsupial relatives.

<i>Zalambdalestes</i> Genus of shrew-like mammal from the Upper Cretaceous period

Zalambdalestes was a eutherian mammal, most likely not a placental due to the presence of an epipubic bone, living during the Upper Cretaceous in Mongolia.

<i>Pucadelphys</i> Extinct genus of mammals

Pucadelphys is an extinct genus of non-marsupial metatherian species. The genus contains a single species, P. andinus. Fossils of Pucadelphys have been found in the Santa Lucía Formation in Tiupampa in Bolivia.

<i>Sinodelphys</i> Extinct family of mammals

Sinodelphys is an extinct mammal from the Early Cretaceous, estimated to be 125 million years old. It was discovered and described in 2003 in rocks of the Yixian Formation in Liaoning Province, China, by a team of scientists including Zhe-Xi Luo and John Wible.

<i>Protungulatum</i>

Protungulatum is a extinct genus of pan-euungulate mammals within extinct family Protungulatidae, and also one of the earliest known placental mammal in the fossil record, that lived in North America from the Late Cretaceous to early Paleocene.

Cimolestes is a genus of early eutherians with a full complement of teeth adapted for eating insects and other small animals. Paleontologists have disagreed on its relationship to other mammals, in part because quite different animals were assigned to the genus, making Cimolestes a grade taxon of animals with similar features rather than a genus of closely related ones. Fossils have been found in North America, South America, Europe and Africa. Cimolestes first appeared during the Late Cretaceous of North America. According to some paleontologists, Cimolestes died out at the start of the Paleocene, while others report the genus from the early Eocene.

Evolution of mammals Derivation of mammals from a synapsid precursor, and the adaptive radiation of mammal species

The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked like mammals. The lineage leading to today's mammals split up in the Jurassic; synapsids from this period include Dryolestes, more closely related to extant placentals and marsupials than to monotremes, as well as Ambondro, more closely related to monotremes. Later on, the eutherian and metatherian lineages separated; the metatherians are the animals more closely related to the marsupials, while the eutherians are those more closely related to the placentals. Since Juramaia, the earliest known eutherian, lived 160 million years ago in the Jurassic, this divergence must have occurred in the same period.

Maelestes is a prehistoric shrew-like mammal discovered in 1997 in the Gobi Desert. The animal lived in the late Cretaceous Period, around 71-75 million years ago, and was a contemporary of dinosaurs such as Velociraptor and Oviraptor. According to some scientists, the discovery and analysis of this species suggests that true placental mammals appeared near the time the non-avian dinosaurs became extinct 66 million years ago, not much earlier in the Cretaceous as thought by others. However, the presence of an epipubic bone, among other characteristics, place it as a non-placental eutherian.

Epipubic bone

Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials and most non-placental fossil mammals: multituberculates, monotremes, and even basal eutherians. They first occur in non-mammalian cynodonts such as tritylodontids, suggesting that they are a synapomorphy between them and Mammaliformes.

<i>Vincelestes</i> Extinct family of mammals

Vincelestes is an extinct genus of actively mobile mammal, that lived in what would be South America during the Early Cretaceous from 130—112 mya, existing for approximately 18 million years .

Dryolestida Extinct order of mammals

Dryolestida is an extinct order of mammals; most of the members are mostly known from the Jurassic to Paleogene, with one member, Necrolestes, surviving as late as the early Miocene. They are considered members of the clade Cladotheria, close to the ancestry of therian mammals. It is also believed that they developed a fully mammalian jaw and also had the three middle ear bones. Other than that, not much is known about them, this is because their fossils are made up mostly of jaw and tooth remains.

Leptictida Extinct order of mammals

Leptictida is a possibly invalid extinct order of placental mammals. Their classification is contentious: according to cladistic studies, they may be (distantly) related to Euarchontoglires, although they are more recently regarded as the first branch to split from basal eutherians. One recent large-scale cladistic analysis of eutherian mammals favored lepictidans as close to the placental crown-clade; and several other recent analyses that included data from Cretaceous non-eutherian mammals found Leptictis to belong to the superorder Afrotheria.

<i>Necrolestes</i> Extinct family of mammals

Necrolestes is an extinct genus of non-therian mammals, which lived during the Early Miocene in what is now Argentine Patagonia. It contains two species, N. patagonensis and N. mirabilis, and is the most recent known genus of dryolestoid. The type species N. patagonensis was named by Florentino Ameghino in 1891 based on remains found by his brother, Carlos Ameghino in Patagonia. Fossils of Necrolestes have been found in the Sarmiento and Santa Cruz Formations.

Holotheria Extinct clade of mammals

Holotheria are a diverse group of mammals that are descendants of the last common ancestor of Kuehneotherium and Theria.

Marsupionta is a hypothesised subclass within the Mammalia group. The existence of Marsupionta is a postulation by some researchers as a category devolving upon a notional unification between marsupials with the egg-laying monotremes. Under this suggested classification, placental mammals would be the sister subclass to Marsupionta. The Marsupionta hypothesis was proposed in 1947 by W.K. Gregory and has since been the subject of multiple studies. This merging of marsupials and monotremes into the hypothesized subclass of Marsupionta is contrary to the widespread belief that pouch and placental mammals share the common subclass Theria that excludes monotremes.

Zhelestidae is a lineage of extinct eutherian mammals. Occurring in the Late Cretaceous from the Turonian to the Maastrichtian, they were an extremely successful group, with representatives present in Europe, Asia, India, Africa and North America, ostensibly rendering them a cosmopolitan clade. They were specialised towards an herbivorous lifestyle and were in fact initially considered stem-ungulates, but the presence of epipubics and "archaic" dental characters render them as non-placental eutherians.

Zalambdalestidae Extinct family of mammals

Zalambdalestidae is a clade of Asian eutherians occurring during the Late Cretaceous. Once classified as Glires, features like epipubic bones and various cranial elements have identified these animals as outside of Placentalia, representing thus a specialised clade of non-placental eutherians without any living descendants, and potentially rather different from modern placentals in at least reproductive anatomy.

References

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  2. Reilly, Stephen M.; White, Thomas D. (2003-01-17). "Hypaxial Motor Patterns and the Function of Epipubic Bones in Primitive Mammals". Science. 299 (5605): 400–402. Bibcode:2003Sci...299..400R. doi:10.1126/science.1074905. ISSN   0036-8075. PMID   12532019.
  3. 1 2 Luo, Z.; C. Yuan; Q. Meng; 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.
  4. Eutheria (Placental Mammals) by J David Archibald, San Diego State University, San Diego, California, USA. PDF file from sdsu.edu
  5. 1 2 3 4 5 Ji, Q.; Luo, Z-X.; Yuan, C-X.; Wible, J.R.; Zhang, J-P. & Georgi, J.A. (April 2002). "The earliest known eutherian mammal". Nature. 416 (6883): 816–822. Bibcode:2002Natur.416..816J. doi:10.1038/416816a. PMID   11976675.
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  7. 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. Bibcode:2007Natur.447.1003W. doi:10.1038/nature05854. PMID   17581585.
  8. 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" (PDF). Bulletin of the American Museum of Natural History. 327: 1–123. doi:10.1206/623.1. hdl:2246/6001.
  9. Foley, Nicole M.; Springer, Mark S.; Teeling, Emma C. (2016-07-19). "Mammal madness: is the mammal tree of life not yet resolved?". Phil. Trans. R. Soc. B. 371 (1699): 20150140. doi:10.1098/rstb.2015.0140. PMC   4920340 . PMID   27325836.
  10. Tarver, James E.; Reis, Mario dos; Mirarab, Siavash; Moran, Raymond J.; Parker, Sean; O'Reilly, Joseph E.; King, Benjamin L.; O'Connell, Mary J.; Asher, Robert J. (2016-02-01). "The Interrelationships of Placental Mammals and the Limits of Phylogenetic Inference". Genome Biology and Evolution. 8 (2): 330–344. doi:10.1093/gbe/evv261. PMC   4779606 . PMID   26733575.
  11. Esselstyn, Jacob A.; Oliveros, Carl H.; Swanson, Mark T.; Faircloth, Brant C. (2017-08-26). "Investigating Difficult Nodes in the Placental Mammal Tree with Expanded Taxon Sampling and Thousands of Ultraconserved Elements". Genome Biology and Evolution. 9 (9): 2308–2321. doi:10.1093/gbe/evx168. PMC   5604124 . PMID   28934378.