Eutheria (fromAncient Greek εὐ- (eú-), meaning "true, well", and θηρίον (thēríon), meaning "beast", and thus, "true beasts"), also calledPan-Placentalia, is theclade consisting ofplacental mammals and alltherian mammals that are more closely related to placentals than tomarsupials.
Eutherians are distinguished from non-eutherians by variousphenotypic traits of the feet, ankles, jaws and teeth. All extant eutherians lackepipubic bones, which are present in all other living mammals (marsupials andmonotremes). This allows for expansion of the abdomen during pregnancy,[2] though epipubic bones are present in a number of primitive eutherians.[3] Eutheria was named in 1872 byTheodore Gill; in 1880,Thomas Henry Huxley defined it to encompass a more broadly defined group than Placentalia.[4]
The earliest unambiguous eutherians are known from the Early CretaceousYixian Formation of China, dating around 120 million years ago.[5] Two tribosphenic mammals,Durlstodon andDurlstotherium from theBerriasian age (~145–140 million years ago) of theEarly Cretaceous in southernEngland have also been suggested to represent early eutherians.[6][7] Another possible eutherian speciesJuramaia sinensis has been dated at161 million years ago from the earlyLate Jurassic (Oxfordian) of China.[8] However some authors have consideredJuramaia as a stemtherian instead,[6][7] and some sources have doubted the dating of the specimen.[9]
an enlargedmalleolus ("little hammer") at the bottom of thetibia, the larger of the two shin bones[10]
thejoint between the firstmetatarsal bone and the entocuneiform bone (the innermost of the threecuneiform 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[10]
various features of jaws and teeth[10] including: having three molars in the halves of each jaw, each upper canine having two roots, the paraconid on the last lower premolar is pronounced, thetalonid region of the lower molars is narrower than thetrigonid.[11]
Some older systems contained an order calledCimolesta (sensu lato), which contains the above taxa Cimolestidae, Taeniodonta and Didymoconidae, but also (some or all of) the taxa †Ptolemaiidae, †Palaeoryctidae, †Wyolestidae, †Pantolesta, †Tillodontia, †Apatotheria, †Pantodonta,Pholidota and †Palaeanodonta. Those additional taxa (which are usually considered members of Placentaliasensu stricto) were thus also placed next to Cimolestidae, now classified as basal Eutheria.Creodonta andDinocerata have also been suggested as basal eutherians.
Some authors classify the taxa placed at the end of the above system as part of Placentaliasensu stricto. Depending on the author, this applies to taxa placed from (and inclusive of) Leptictida or Asioryctitheria or Adapisoriculidae down to (and inclusive of)Oxyprimus. Specifically, some older authors associated Cimolestidae withFerae, Zalambdalestidae withGlires, Zhelestidae withUngulata, and Leptictida and Adapisoriculidae withLipotyphla orArchonta. All of these are now considered basal Eutheria.
Eutheria contains severalextinctgenera as well as larger groups, many with complicatedtaxonomic histories still not fully understood. Members of theAdapisoriculidae,Cimolesta andLeptictida have been previously placed within the outdated placental groupInsectivora, whilezhelestids have been considered primitiveungulates.[34] However, more recent studies have suggested these enigmatic taxa representstem group eutherians, more basal to Placentalia.[35][36] Many non-placental eutherians are thought to have beeninsectivores, as is the case with many primitive mammals.[37] However, thezhelestids are thought to have been herbivorous.[38] Prior to theCretaceous–Paleogene (K-Pg) extinction event, most eutherians were small and exhibited low anatomical and ecological diversity. Fossil evidence shows that the K-Pg extinction event led to a widespread increase in mammalian size and morphological diversity, with many small and medium-sized species going extinct and the surviving lineages evolving to have larger body sizes. Studies suggest that this overall increase in eutherian body size happened within a few hundred thousand years of the extinction. The extinction event also proved to be selective, as mammals with more generalized diets were more likely to survive than species with more specialized diets. In the earlyPaleocene, eutherians experienced an increase in taxonomic diversification, another result of the extinction event, leading to the evolution of the stem-primatePurgatorius, stem carnivoransPristinictis and Ravenictis, and stemproboscideanEritherium. During this period, eutherians also expanded into new ecological roles, including specialized herbivores and carnivores. ThePaleocene–Eocene Thermal Maximum further impacted eutherian morphological diversity, although the extent of its impacts is less studied.[39]
The weakly favoured cladogram favours Boreoeutheria as a basal eutherian clade as sister to the Atlantogenata.[40][41][42]
^Rook, Deborah L.; Hunter, John P. (2013). "Rooting Around the Eutherian Family Tree: the Origin and Relations of the Taeniodonta".Journal of Mammalian Evolution.21:1–17.doi:10.1007/s10914-013-9230-9.
^Wilson Mantilla, Gregory P.; Renne, Paul R.; Samant, Bandana; Mohabey, Dhananjay M.; Dhobale, Anup; Tholt, Andrew J.; Tobin, Thomas S.; Widdowson, Mike; Anantharaman, S.; Dassarma, Dilip Chandra; Wilson Mantilla, Jeffrey A. (2022). "New mammals from the Naskal intertrappean site and the age of India's earliest eutherians".Palaeogeography, Palaeoclimatology, Palaeoecology.591. Elsevier BV.Bibcode:2022PPP...59110857W.doi:10.1016/j.palaeo.2022.110857.ISSN0031-0182.
^Scott, Craig S (2019-01-18). "Horolodectidae: a new family of unusual eutherians (Mammalia: Theria) from the Palaeocene of Alberta, Canada".Zoological Journal of the Linnean Society.185 (2):431–458.doi:10.1093/zoolinnean/zly040.ISSN0024-4082.
^Kynigopoulou, Zoi; Brusatte, Stephen; Fraser, Nicholas; Wood, Rachel; Williamson, Tom; Shelley, Steve (2023-06-12).Phylogeny, evolution, and anatomy of Taeniodonta (Mammalia: Eutheria) and implications for the mammalian evolution after the Cretaceous-Palaeogene mass extinction (Thesis). University Of Edinburgh.doi:10.7488/ERA/3414.hdl:1842/40653.
^Rose, Kenneth D. (2006).The beginning of the age of mammals. Baltimore: Johns Hopkins University Press.ISBN978-0-8018-9221-9.
^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.PMID17581585.
^abGheerbrant, Emmanuel; Teodori, Dominique (2021-03-24). "An enigmatic specialized new eutherian mammal from the Late Cretaceous of Western Europe (Northern Pyrenees)".Comptes Rendus Palevol (13).doi:10.5852/cr-palevol2021v20a13.ISSN1777-571X.
^Emmerson, R. M. (2022).Changes in morphological disparity in eutherian mammals across the K–Pg boundary and Palaeocene–Eocene Thermal Maximum using discrete morphofunctional characters (MSc by Research thesis, University of Salford, UK). University of Salford Repository. https://salford-repository.worktribe.com/output/1330684/changes-in-morphological-disparity-in-eutherian-mammals-across-the-k-pg-boundary-and-palaeocene-eocene-thermal-maximum-using-discrete-morphofunctional-characters
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