Eutheria

 

"Placental" redirects here. For the organ interfacing between a mammalian mother and a fetus, see Placenta.

Further information: Evolution of mammals

Eutheria Temporal range: Late Jurassic – Recent, 160–0 Ma PreЄ Є O S D C P T J K Pg N
Fossil specimen of Eomaia scansoria
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Synapsida
Class:	Mammalia
Supercohort:	Theria
Branch:	Eutheria Thomas Henry Huxley, 1880
Subgroups
†Bobolestes †Eomaia †Juramaia †Montanalestes †Murtoilestes †Prokennalestes Placentalia Owen, 1837 Xenarthra Epitheria

Eutheria (/juːˈθɪəriə/; from Ancient Greek ευθήριον, euthērion, meaning "true/good beasts") is the clade consisting of rodents, primates, and all other mammals—in many orders—that are more closely related to them than they are to marsupials. Placentalia is the clade originating with the last common ancestor of extant eutherians. By definition, all nonplacental eutherians are extinct; the picture to the right shows a fossil of one such animal, Eomaia.

Eutherians are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth. One of the major differences between placental and nonplacental eutherians is that placentals lack epipubic bones, which are present in all other fossil and living mammals (monotremes and marsupials).

The oldest known eutherian species is Juramaia sinensis, dated at 160 million years ago from the Jurassic in China. The previously earliest known fossil eutherian, Eomaia scansoria, was also from China and is dated to the Early Cretaceous period, about 125 million years ago.

"Eutheria" was introduced by Thomas Henry Huxley in 1880, meant to be broader in definition than "Placentalia", the term previously in use.

Characteristics

There are no living nonplacental eutherians, and so knowledge of their synapomorphies ("defining features") is entirely based on a few fossils, which means the reproductive features that distinguish modern placentals from other mammals cannot be used in defining Eutheria. The features of Eutheria that distinguish them from metatherians, a group that includes modern marsupials, are:

• an enlarged malleolus ("little hammer") at the bottom of the tibia, the larger of the two shin bones.
• the joint between the first metatarsal bone and the entocuneiform bone in the foot is offset further back than the joint between the second metatarsal and mesocuneiform bones – in metatherians these joints are level with each other.
• various features of jaws and teeth.

Reproductive features are also of no use in identifying fossil placental mammals, which are distinguished from other eutherians by:

• the presence of a malleolus at the bottom of the fibula, the smaller of the two shin bones.
• a complete mortise and tenon upper ankle joint, where the rearmost bones of the foot fit into a socket formed by the ends of the tibia and fibula.
• a wide opening at the bottom of the pelvis, which allows the birth of large, well-developed offspring. Marsupials have and nonplacental eutherians had a narrower opening that allows only small, immature offspring to pass through.
• the absence of epipubic bones extending forward from the pelvis, which are not found in any placental, but are found in all other mammals – nonplacental eutherians, marsupials, monotremes and mammaliformes – and even in the cynodont therapsids that are closest to mammals. Their function is to stiffen the body during locomotion. This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.

Subgroups

Placentalia	Xenarthra   Epitheria   Afrotheria Boreoeutheria     Laurasiatheria     Euarchontoglires

These are the subgroups of extant members of Eutheria:

• Boreoeutheria, e.g. badgers, rabbits, guinea pigs, dogs, dolphins
  • Euarchontoglires, e.g. humans, monkeys, rats, hares
  • Laurasiatheria, e.g. cattle, whales, bats, cats
• Xenarthra, e.g. armadillos, anteaters
• Afrotheria, e.g. elephants, manatees

These groups together make up the crown group Placentalia (placental mammals). Eutheria also includes now extinct lineages that lie outside of Placentalia (see below).

Analysis of transposable element insertions around the time of divergence of Boreoeutheria, Afrotheria, and Xenarthra strongly support a near-concomitant origin (trifurcation) of these three superorders, making further subdivision impractical and meaningless. These observations eliminate the need to choose between the previously proposed groupings of Boreoeutheria and Xenarthra (Exafroplacentalia), Afrotheria and Xenarthra (Atlantogenata), Afrotheria and Boreoeutheria (Epitheria).

Evolutionary history

The fossil eutherian species believed to be the oldest known is Juramaia sinensis, said to be about 160 million years ago. Another early eutherian mammal is the nonplacental Eomaia scansoria from the Lower Cretaceous of China, dated to about 125 million years ago. Some of its fossils show thick fur. Montanalestes was found in North America, while all other nonplacental eutherian fossils have been found in Asia. The earliest known placental fossils have also been found in Asia.

Millions of years ago — = Placentals     — = Other eutheria Origin of eutheria     = Asian fossils         = N American fossils     = Period when placental classes diverged according to molecular phylogenetics estimates Murtoilestes Prokennalestes Eomaia Montanalestes Ukhaatherium Asioryctes Kennalestes Zalambdalestes Daulestes Aspanestes Eoungulatum Protungulatum Gypsonictops Cimolestes Fossil record of Cretaceous eutheria

Cynodonts † Other Cynodonts † Tritylodontids Mammaliformes † Other mammaliformes † Hadrocodium Mammals † Other mammals Australosphenids † Other Australosphenids Monotremes Theria † Other Theria Metatheria † Other Metatheria Marsupials Eutheria † Other Eutheria Placentals Simplified, non-systematic, outline of evolution of eutheria from cynodont therapsids. † = extinct

Further reading

• Goloboff, P.A.; Catalano, S.A.; Mirande, J.M.; Szumik, C.A.; Arias, J.S.; Källersjö, M & Farris, J.S. 2009. Phylogenetic analysis of 73 060 taxa corroborates major eukaryotic groups. Cladistics 25 (3): 211–230