Eutheria (Greek: "true/good beasts") is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos). They 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 mammals and living mammals (monotremes and marsupials).

The earliest known fossil eutherian, Eomaia, was found in Asia, and is dated to the Early Cretaceous period, about 125 million years ago.

Contents 1 Definition 2 Subgroups 3 Evolutionary history 4 References 5 Further reading

[edit] Definition

Eutherians are a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos).^[2]

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 the 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.^[2]
• 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.^[2]
• various features of jaws and teeth.^[2]

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.^[2]
• 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.^[2]
• 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.^[3]
• 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.^[4] This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.^[5]

[edit] Subgroups

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
• Atlantogenata, e.g. sloths, hyraxes, aardvarks, tenrecs, armadillos, anteaters
  • 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).^[10]

[edit] Evolutionary history

The earliest known eutherian species is the extinct 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.[2]

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[2]

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.[2] † = extinct

[edit] References

[edit] 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

v · d · eExtant mammal orders by infraclass Kingdom Animalia · Phylum Chordata · Subphylum Vertebrata · (unranked) Amniota Australosphenida Monotremata (Platypus and echidnas) Metatheria (Marsupial inclusive) Ameridelphia Paucituberculata (Shrew opossums) · Didelphimorphia (Opossums) Australidelphia Microbiotheria (Monito del Monte) · Notoryctemorphia (Marsupial moles) · Dasyuromorphia (Quolls and dunnarts) · Peramelemorphia (Bilbies and bandicoots) · Diprotodontia (Kangaroos and relatives) Eutheria (Placental inclusive) Xenarthra Cingulata (Armadillos) · Pilosa (Anteaters and sloths) Afrotheria Afrosoricida (Tenrecs and golden moles) · Macroscelidea (Elephant shrews) · Tubulidentata (Aardvark) · Hyracoidea (Hyraxes) · Proboscidea (Elephants) · Sirenia (Dugongs and manatees) Laurasiatheria Soricomorpha (Shrews and moles) · Erinaceomorpha (Hedgehogs and relatives) · Chiroptera (Bats) · Pholidota (Pangolins) · Carnivora · Perissodactyla (Odd-toed ungulates) · Artiodactyla (Even-toed ungulates) · Cetacea (Whales and dolphins) Euarchontoglires Rodentia (Rodents) · Lagomorpha (Rabbits and relatives) · Scandentia (Treeshrews) · Dermoptera (Colugos) · Primates