Eutheria

Taxonomy

Eutheria (i.e. Placentalia sensu lato, Pan-Placentalia):

• incertae sedis:
  • ?Family †Holoclemensiidae (often considered a basal metatherian)
  • ?Genus †Hyotheridium
  • ?Genus †Endotherium
  • Genus †Durlstodon
  • Genus †Durlstotherium
  • Genus †Microtherulum
  • Genus †Sinodelphys
  • Genus †Cokotherium
  • Genus †Ambolestes
  • Genus †Montanalestes
  • Genus †Indoclemensia
  • ?Family †Horolodectidae (might belong somewhere within Placentalia sensu stricto)
  • ?Genus †Juramaia
  • Genus †Eomaia
• Genus †Acristatherium
• Clade †Tamirtheria ? (monophyly disputed)
  • 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
  • ?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

The divergence between the ancestors of eutherians and metatherians is likely to have taken place in Asia. The earliest unambiguous eutherians are known from the Early Cretaceous Yixian Formation of China, dating around 120 million years ago. 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. Another possible eutherian species Juramaia sinensis has been dated at 161 million years ago from the early Late Jurassic (Oxfordian) of China. However some authors have considered Juramaia as a stem therian instead, and some sources have doubted the dating of the specimen.

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. However, more recent studies have suggested these enigmatic taxa represent stem group eutherians, more basal to Placentalia. Many non-placental eutherians are thought to have been insectivores, as is the case with many primitive mammals. However, the zhelestids are thought to have been herbivorous. 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, corresponding with heightened diversification and evolutionary rates during this time period.

The weakly favoured cladogram favours Boreoeutheria as a basal eutherian clade as sister to the Atlantogenata.

Phylogeny after Wang & Wang, 2023.

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