Eutheria

Eutheria (from Ancient Greek εὐ- (eú-), meaning "true, well", and θηρίον (thēríon), meaning "beast", and thus, "true beasts"), also called Pan-Placentalia, is the clade consisting of placental mammals and all therian mammals that are more closely related to placentals than to marsupials.

Eutherians are distinguished from non-eutherians 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,<ref>Template:Cite journal</ref> though epipubic bones are present in a number of primitive eutherians.<ref>Template:Cite journal</ref> Eutheria was named in 1872 by Theodore Gill; in 1880, Thomas Henry Huxley defined it to encompass a more broadly defined group than Placentalia.<ref>Template:Cite book</ref>

The earliest unambiguous eutherians are known from the Early Cretaceous Yixian Formation of China, dating around 120 million years ago.<ref>Template:Citation</ref> Two tribosphenic mammals, Durlstodon and Durlstotherium from the Berriasian age (~145–140 million years ago) of the Early Cretaceous in southern England have also been suggested to represent early eutherians.<ref name=Berriasian/><ref name=":2">Template:Cite journal</ref> Another possible eutherian species Juramaia sinensis has been dated at Template:Ma from the early Late Jurassic (Oxfordian) of China.<ref name="Juramaia_Nature_2011">Template:Cite journal</ref> However some authors have considered Juramaia as a stem therian instead,<ref name=Berriasian>Template:Cite journal</ref><ref name=":2" /> and some sources have doubted the dating of the specimen.<ref>Template:Cite journal</ref>

Characteristics

The entocuneiform bone

Distinguishing features are:

• an enlarged malleolus ("little hammer") at the bottom of the tibia, the larger of the two shin bones<ref name="JiLuoYuan2002EarliestEutherian">Template:Cite journal</ref>
• the joint between the first metatarsal bone and the entocuneiform bone (the innermost 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<ref name="JiLuoYuan2002EarliestEutherian" />
• various features of jaws and teeth<ref name="JiLuoYuan2002EarliestEutherian" /> 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, the talonid region of the lower molars is narrower than the trigonid.<ref name=":1" />

Taxonomy

Eutheria (i.e. Placentalia sensu lato, Pan-Placentalia):<ref>Template:Cite book</ref><ref>Template:Cite book</ref><ref>Template:Cite book</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name=":1">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite book</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite web</ref><ref>Template:Cite web</ref>Template:Excessive citations inline

• incertae sedis:
  • ?Family †Holoclemensiidae<ref>Template:Cite journal</ref> (often considered a basal metatherian)
  • ?Genus †Hyotheridium<ref>Template:Cite book</ref><ref>Template:Cite journal</ref>
  • ?Genus †Endotherium<ref>Template:Cite journal</ref>
  • Genus †Durlstodon
  • Genus †Durlstotherium
  • Genus †Microtherulum
  • Genus †Sinodelphys
  • Genus †Cokotherium
  • Genus †Ambolestes
  • Genus †Montanalestes
  • Genus †Indoclemensia<ref>Template:Cite journal</ref>
  • ?Family †Horolodectidae (might belong somewhere within Placentalia sensu stricto)<ref>Template:Cite journal</ref>
  • ?Genus †Juramaia
  • Genus †Eomaia
• Genus †Acristatherium
• Clade †Tamirtheria ? (monophyly disputed<ref>Template:Cite journal</ref>)
  • Genus †Prokennalestes
  • ?Genus †Hovurlestes
  • Genus †Murtoilestes
  • Genus †Bobolestes (inclusive of the former genus Otlestes)
  • Family †Adapisoriculidae (inclusive of the genus †Sahnitherium)
  • Genus †Paranyctoides
  • Family †Zhelestidae (inclusive of the genus Eozhelestes)
  • Family †Cimolestidae (inclusive of the genera †Maelestes and †Batodon)
  • Order †Asioryctitheria
  • Family †Zalambdalestidae
  • Order †Leptictida
  • ?Order †Taeniodonta<ref>Template:Cite thesis</ref>
  • ?Family †Didymoconidae
• ?Genus †Purgatorius
• ?Genus †Protungulatum
• ?Genus †Oxyprimus
• Infraclass Placentalia sensu stricto

Notes:

• Some older systems contained an order called Cimolesta (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 Placentalia sensu stricto) were thus also placed next to Cimolestidae, now classified as basal Eutheria. Creodonta and Dinocerata have also been suggested as basal eutherians.
• Some authors classify the taxa placed at the end of the above system as part of Placentalia sensu 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 with Ferae, Zalambdalestidae with Glires, Zhelestidae with Ungulata, and Leptictida and Adapisoriculidae with Lipotyphla or Archonta. All of these are now considered basal Eutheria.

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 outdated placental group Insectivora, while zhelestids have been considered primitive ungulates.<ref name=Rose>Template:Cite book</ref> However, more recent studies have suggested these enigmatic taxa represent stem group eutherians, more basal to Placentalia.<ref name=Wibleetal2007>Template:Cite journal</ref><ref name=Wibleetal09>Template:Cite journal</ref> Many non-placental eutherians are thought to have been insectivores, as is the case with many primitive mammals.<ref>Template:Cite journal</ref> However, the zhelestids are thought to have been herbivorous.<ref name="gheerbrant&teodori2021">Template:Cite journal</ref> Prior to the Cretaceous–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 early Paleocene, eutherians experienced an increase in taxonomic diversification, another result of the extinction event, leading to the evolution of the stem-primate Purgatorius, stem carnivorans Pristinictis and Ravenictis, and stem proboscidean Eritherium. During this period, eutherians also expanded into new ecological roles, including specialized herbivores and carnivores. The Paleocene–Eocene Thermal Maximum further impacted eutherian morphological diversity, although the extent of its impacts is less studied.<ref>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</ref>

The weakly favoured cladogram favours Boreoeutheria as a basal eutherian clade as sister to the Atlantogenata.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Phylogeny after Wang & Wang, 2023.<ref name=":0">Template:Cite journal</ref>

Template:Clade Below is a phylogeny from Gheerbrant & Teodori (2021):<ref name="gheerbrant&teodori2021"/> Template:Clade