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,[1] though epipubic bones are present in many primitive eutherians.[2] Eutheria was named in 1872 byTheodore Gill; in 1880,Thomas Henry Huxley defined it to encompass a more broadly defined group than Placentalia.[3]
The earliest unambiguous eutherians are known from the Early CretaceousYixian Formation of China, dating around 120 million years ago.[4] 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.[5][6] Another possible eutherian speciesJuramaia sinensis has been dated at161 million years ago from the earlyLate Jurassic (Oxfordian) of China.[7] However some authors have consideredJuramaia as a stemtherian instead,[5][6] and some sources have doubted the dating of the specimen.[8]
an enlargedmalleolus ("little hammer") at the bottom of thetibia, the larger of the two shin bones[9]
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[9]
various features of jaws and teeth[9] 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.[10]
Some older systems contained an order calledCimolesta (sensu lato), which contains the above taxa Cimolestidae, Taeniodonta and Didymoconidae, but also (all or some of) the taxa †Ptolemaiidae, †Palaeoryctidae, †Wyolestidae, †Pantolesta (probably inclusive of the family †Horolodectidae), †Tillodontia, †Apatotheria, †Pantodonta,Pholidota and †Palaeanodonta. Those additional taxa (all of which are usually considered members of Placentaliasensu stricto today) were thus also placed among basal Eutheria in such older systems and were placed next to Cimolestidae.
Some systems also included the †Creodonta and/or †Dinocerata as basal Eutherians.
Some authors classify the taxa, which are at the end of the above system of basal Eutheria, as part of Placentaliasensu stricto. More specifically, depending on the author, this applies to the taxa of the above system that are placed from (and inclusive of) Leptictida or Asioryctitheria or Adapisoriculidae down to (and inclusive of)Oxyprimus.
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.[33] However, more recent studies have suggested these enigmatic taxa representstem group eutherians, more basal to Placentalia.[34][35]
The weakly favoured cladogram favours Boreoeutheria as a basal eutherian clade as sister to the Atlantogenata.[36][37][38]
Many non-placental eutherians are thought to have beeninsectivores, as is the case with many primitive mammals.[41] However, thezhelestids are thought to have been herbivorous.[40] Body size of eutherians was generally small during the Cretaceous period, but the range of body sizes increased dramatically after theK-Pg extinction, predominantly among placentals.[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.ISBN9780801892219.
^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.
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