Eutheria

Eutheria (/juːˈθɪəriə/; from Greek εὐ-, eu- "good" or "right" and θηρίον, thēríon "beast" hence "true beasts") is one of two mammalian clades with extant members that diverged in the Early Cretaceous or perhaps the Late Jurassic. Except for the Virginia opossum, from North America which is a metatherian, all post-Miocene 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 members of Placentalia.

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 Jurassic in 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]

Eutheria
Temporal range: Late Jurassic or Early CretaceousHolocene, 161 or 125–0 Ma
Juramaia NT
Juramaia, the oldest known eutherian
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Subclass: Theria
Clade: Eutheria
Gill, 1872
Subgroups

Characteristics

Cambridge Natural History Mammalia Fig 068
The entocuneiform bone

Distinguishing features are:

  • an enlarged malleolus ("little hammer") at the bottom of the tibia, the larger of the two shin bones[5]
  • the joint between the first metatarsal bone and the entocuneiform bone (the outermost of the three cuneiform bones) in the foot is offset farther back than the joint between the second metatarsal and middle cuneiform bones—in metatherians these joints are level with each other[5]
  • various features of jaws and teeth[5]

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

References

  1. ^ 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.
  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. ^ a b 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. ^ a b c d e 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. Retrieved 2008-09-24.
  6. ^ Rose, Kenneth D. (2006). The beginning of the age of mammals. Baltimore: Johns Hopkins University Press. ISBN 9780801892219.
  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". Bulletin of the American Museum of Natural History. 327: 1–123. doi:10.1206/623.1.
  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.
Atlantogenata

Atlantogenata is a proposed clade of mammals containing the cohorts or superorders Afrotheria and Xenarthra. These groups originated and radiated in the South American and African continents, presumably in the Cretaceous. Together with Boreoeutheria, they make up Eutheria. The monophyly of this grouping was supported by some genetic evidence.Alternative hypotheses are that Boreoeutheria and Afrotheria combine to form Epitheria (as generally supported by anatomical and other physiological evidence) or that Boreoeutheria and Xenarthra combine to form Exafroplacentalia or Notolegia.Updated analysis of transposable element insertions around the time of divergence strongly supports the fourth hypothesis of a near-concomitant origin (trifurcation) of the three superorders of mammals.

Below shows the phylogeny of the extant atlantogenate families.

Exafroplacentalia

Exafroplacentalia or Notolegia is a clade of placental mammals proposed in 2001 on the basis of molecular research.Exafroplacentalia places Xenarthra as a sister group to the Boreoeutheria (comprising Laurasiatheria and Euarchontoglires), thus making Afrotheria a primitive group of placental mammals (the group name roughly means "those which are not African placentals").

List of mammals of Samoa

This is a list of the mammal species recorded in Samoa. There are 9 mammal species in Samoa, of which 1 is endangered and 2 are vulnerable.

Prototheria

Prototheria (; from Greek πρώτος, prōtos, first, + θήρ, thēr, wild animal) is the subclass to which the order Monotremata belongs.

Most of the animals in this group are extinct. The egg-laying monotremes are known from fossils of the Cretaceous and Cenozoic periods; they are represented today by the platypus and several species of echidna.

The names Prototheria, Metatheria, and Eutheria (meaning "first beasts", "changed beasts", and "true beasts", respectively) refer to the three mammalian groupings of which we have living representatives. Each of the three may be defined as a total clade containing a living crown-group (respectively the Monotremata, Marsupialia and Placentalia) plus any fossil species which are more closely related to that crown-group than to any other living animals.

The threefold division of living mammals into monotremes, marsupials and placentals was already well established when Thomas Huxley proposed the names Metatheria and Eutheria to incorporate the two latter groups in 1880. Initially treated as subclasses, Metatheria and Eutheria are by convention now grouped as infraclasses of the subclass Theria, and in more recent proposals have been demoted further (to cohorts or even magnorders), as cladistic reappraisals of the relationships between living and fossil mammals have suggested that the Theria itself should be reduced in rank.Prototheria, on the other hand, was generally recognised as a subclass until quite recently, on the basis of a hypothesis which defined the group by two supposed synapomorphies: (1) formation of the side wall of the braincase from a bone called the anterior lamina, contrasting with the alisphenoid in therians; and (2) a linear alignment of molar cusps, contrasting with a triangular arrangement in therians. These characters appeared to unite monotremes with a range of Mesozoic fossil orders (Morganucodonta, Triconodonta, Docodonta and Multituberculata) in a broader clade for which the name Prototheria was retained, and of which monotremes were thought to be only the last surviving branch.The evidence which was held to support this grouping is now universally discounted. In the first place, examination of embryos has revealed that the development of the braincase wall is essentially identical in therians and in 'prototherians': the anterior lamina simply fuses with the alisphenoid in therians, and therefore the 'prototherian' condition of the braincase wall is primitive for all mammals while the therian condition can be derived from it. Additionally, the linear alignment of molar cusps is also primitive for all mammals. Therefore, neither of these states can supply a uniquely shared derived character which would support a 'prototherian' grouping of orders in contradistinction to Theria.In a further reappraisal, the molars of embryonic and fossil monotremes (living monotreme adults are toothless) appear to demonstrate an ancestral pattern of cusps which is similar to the triangular arrangement observed in therians. Some peculiarities of this dentition support an alternative grouping of monotremes with certain recently discovered fossil forms into a proposed new clade known as the Australosphenida, and also suggest that the triangular array of cusps may have evolved independently in australosphenidans and therians.The Australosphenida hypothesis remains controversial, and some taxonomists (e.g. McKenna & Bell 1997) prefer to maintain the name Prototheria as a fitting contrast to the other group of living mammals, the Theria. In theory, the Prototheria is taxonomically redundant, since Monotremata is currently the only order which can still be confidently included, but its retention might be justified if new fossil evidence, or a re-examination of known fossils, enables extinct relatives of the monotremes to be identified and placed within a wider grouping.

Theria

Theria (; Greek: θηρίον theríon, wild beast) is a subclass of mammals amongst the Theriiformes (the sister taxon to Yinotheria). Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials).

Zatheria

Zatheria is a group (sublegion) of mammals that includes the common ancestor of Arguimuridae, Vincelestidae, Peramuridae and Tribosphenida (living therians plus all of its ancestors).

Extant mammal orders
Yinotheria
Theria

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