Eutheria Explained

Eutheria (from Greek Greek, Ancient (to 1453);: εὐ-, 'good, right' and Greek, Ancient (to 1453);: θηρίον, 'beast';), 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 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,^[1] though epipubic bones are present in many primitive eutherians.^[2] Eutheria was named in 1872 by Theodore 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 Cretaceous Yixian Formation of China, dating around 120 million years ago. Two tribospenic 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.^[4] Another possible eutherian species Juramaia sinensis has been dated at from the early Late Jurassic (Oxfordian) of China.^[5] However some authors have considered Juramaia as a stem therian instead,^[6] and some sources have doubted the dating of the specimen.^[7]

Characteristics

Distinguishing features are:

• an enlarged malleolus ("little hammer") at the bottom of the tibia, the larger of the two shin bones^[8]
• 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
• various features of jaws and teeth 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.

Taxonomy

Eutheria (i.e. Placentalia sensu lato, Pan-Placentalia):^{[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]}

• incertae sedis:
  • ?Family †Holoclemensiidae^[24] (often considered a basal metatherian)
  • ?Genus †Hyotheridium^{[25] [26]}
  • ?Genus †Endotherium^[27]
  • Genus †Durlstodon
  • Genus †Durlstotherium
  • Genus †Microtherulum
  • Genus †Sinodelphys
  • Genus †Cokotherium
  • Genus †Ambolestes
  • Genus †Montanalestes
  • Genus †Indoclemensia^[28]
  • ?Family †Horolodectidae (might belong somewhere within Placentalia sensu stricto)^[29]
  • ?Genus †Juramaia
  • Genus †Eomaia
• Genus †Acristatherium
• Clade †Tamirtheria ? (monophyly disputed^[30])
  • 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 †Taeniodonta^[31]
    • • Order †Asioryctitheria Family †Zalambdalestidae
  • Order †Leptictida
  • ?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 (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 Placentalia sensu 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 Placentalia sensu 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.

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.^[32] However, more recent studies have suggested these enigmatic taxa represent stem group eutherians, more basal to Placentalia.^{[33] [34]}

The weakly favoured cladogram favours Boreoeutheria as a basal eutherian clade as sister to the Atlantogenata.^{[35] [36] [37]}

Phylogeny after Yang & Yang, 2023.^[38]

Below is a phylogeny from Gheerbrant & Teodori (2021):

Ecology

Many non-placental eutherians are thought to have been insectivores, as is the case with many primitive mammals.^[39] However, the zhelestids 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 the K-Pg extinction, predominantly among placentals.^[41]

Notes and References

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