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Eutheria ("true beasts") are a group of mammal
Mammal

Mammals are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, with which they feed their young....
s consisting of placental mammals plus all extinct mammal
Mammal

Mammals are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, with which they feed their young....
s that are more closely related to living placentals (such as humans) than to living marsupial
Marsupial

Marsupials are an infraclass of mammals, characterized by a distinctive Pouch , in which females carry their young through early infancy....
s (such as kangaroos). They are distinguished from non-eutherians by various features of the feet, ankles, jaws and teeth. One of the major differences between placental and non-placental eutherians is that placentals lack the epipubic bones
Epipubic bones

Epipubic bones are a pair of bones which project forward from the pelvis of modern marsupials and of some fossil mammals - multituberculates, monotremes, and even eutherians ....
 which are present in all other fossil
Fossil

Fossils are the preserved remains or trace fossil of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous Rock formations and sedimentary rock layers is known as the fossil record....
 and living mammals.

The earliest known fossil eutherian, Eomaia
Eomaia

Eomaia scansoria is an extinct fossil mammal, discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower Cretaceous about ....
 was found in Asia and is dated to the Early Cretaceous
Cretaceous

The Cretaceous , usually abbreviated K for its German translation Kreide, is a geologic period from circa to million years ago . In the geologic timescale, the Cretaceous follows on the Jurassic period and is followed by the Paleogene period....
 period, about .

erians are a group of mammals consisting of placental mammals plus all extinct mammal
Mammal

Mammals are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, with which they feed their young....
s that are more closely related to living placentals (such as humans) than to living marsupial
Marsupial

Marsupials are an infraclass of mammals, characterized by a distinctive Pouch , in which females carry their young through early infancy....
s (such as kangaroos).

There are no living non-placental eutherians, and so knowledge of their synapomorphies
Synapomorphy

In evolutionary biology, a synapomorphy is a derived character state shared by two or more terminal groups and inherited from their most recent common ancestor....
 ("defining features") is entirely based on a few fossil
Fossil

Fossils are the preserved remains or trace fossil of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous Rock formations and sedimentary rock layers is known as the fossil record....
s – which means the reproductive features that distinguish modern placentals from other mammals cannot be used in defining the eutheria.






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Eutheria ("true beasts") are a group of mammal
Mammal

Mammals are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, with which they feed their young....
s consisting of placental mammals plus all extinct mammal
Mammal

Mammals are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, with which they feed their young....
s that are more closely related to living placentals (such as humans) than to living marsupial
Marsupial

Marsupials are an infraclass of mammals, characterized by a distinctive Pouch , in which females carry their young through early infancy....
s (such as kangaroos). They are distinguished from non-eutherians by various features of the feet, ankles, jaws and teeth. One of the major differences between placental and non-placental eutherians is that placentals lack the epipubic bones
Epipubic bones

Epipubic bones are a pair of bones which project forward from the pelvis of modern marsupials and of some fossil mammals - multituberculates, monotremes, and even eutherians ....
 which are present in all other fossil
Fossil

Fossils are the preserved remains or trace fossil of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous Rock formations and sedimentary rock layers is known as the fossil record....
 and living mammals.

The earliest known fossil eutherian, Eomaia
Eomaia

Eomaia scansoria is an extinct fossil mammal, discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower Cretaceous about ....
 was found in Asia and is dated to the Early Cretaceous
Cretaceous

The Cretaceous , usually abbreviated K for its German translation Kreide, is a geologic period from circa to million years ago . In the geologic timescale, the Cretaceous follows on the Jurassic period and is followed by the Paleogene period....
 period, about .

Definition

Eutherians are a group of mammals consisting of placental mammals plus all extinct mammal
Mammal

Mammals are a class of vertebrate animals whose name is derived from their distinctive feature, mammary glands, with which they feed their young....
s that are more closely related to living placentals (such as humans) than to living marsupial
Marsupial

Marsupials are an infraclass of mammals, characterized by a distinctive Pouch , in which females carry their young through early infancy....
s (such as kangaroos).

There are no living non-placental eutherians, and so knowledge of their synapomorphies
Synapomorphy

In evolutionary biology, a synapomorphy is a derived character state shared by two or more terminal groups and inherited from their most recent common ancestor....
 ("defining features") is entirely based on a few fossil
Fossil

Fossils are the preserved remains or trace fossil of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous Rock formations and sedimentary rock layers is known as the fossil record....
s – 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 metatheria
Metatheria

Metatheria is a grouping within the animal class Mammalia. First proposed by Thomas Henry Huxley in 1880, it is nearly synonymous with the earlier taxon Marsupialia though it is slightly wider since it also contains the nearest fossil relatives of marsupial mammals....
ns, a group that includes modern marsupials, are:
  • an enlarged malleolus ("little hammer") at the bottom of the tibia
    Tibia

    The tibia, shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates and connects the knee with the ankle bones....
    , 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
    Fibula

    The fibula or calf bone is a bone located on the lateral side of the tibia, with which it is connected above and below. It is the smaller of the two bones, and, in proportion to its length, the most slender of all the long bones....
    , the smaller of the two shin bones.
  • a complete mortise and tenon
    Mortise and tenon

    Simple and strong, the mortise and tenon Woodworking joints has been used for millennia by woodworkers around the world to join pieces of wood, usually when the pieces are at an angle close to 90?....
     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
    Pelvis

    The pelvis or pelvic girdle is the irregular bone structure located at the base of the spine . In the adult human, it is formed by the sacrum and the coccyx, the caudal part of the axial skeleton, and a pair of hip bones, part of the appendicular skeleton or human leg....
    , which allows the birth of large, well-developed offspring. Marsupials have and non-placental eutherians had a narrower opening that allows only small, immature offspring to pass through.
  • the absence of epipubic bones
    Epipubic bones

    Epipubic bones are a pair of bones which project forward from the pelvis of modern marsupials and of some fossil mammals - multituberculates, monotremes, and even eutherians ....
     extending forwards from the pelvis, which are not found in any placental, but are found in all other mammals - non-placental eutherians, marsupials, monotremes and mammaliformes – and even in the cynodont
    Cynodont

    Cynodonts, or 'dog teeth', are a taxon of Therapsids which includes modern mammals and their extinct close relatives. They were one of the most diverse groups of therapsids....
     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.


Evolutionary history

The earliest known eutherian species is the extinct non-placental Eomaia scansoria
Eomaia

Eomaia scansoria is an extinct fossil mammal, discovered in rocks that were found in the Yixian Formation, Liaoning Province, China, and dated to the Barremian Age of the Lower Cretaceous about ....
 from the Lower Cretaceous
Cretaceous

The Cretaceous , usually abbreviated K for its German translation Kreide, is a geologic period from circa to million years ago . In the geologic timescale, the Cretaceous follows on the Jurassic period and is followed by the Paleogene period....
 of China
China

China is a Culture of China, an ancient civilization, and, depending on perspective, a national or multinational entity extending over a large area in East Asia....
, dated to about . Some of its fossil
Fossil

Fossils are the preserved remains or trace fossil of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous Rock formations and sedimentary rock layers is known as the fossil record....
s show thick fur
Fur

Fur is a Hair of any non-human mammal, also known as the pelage. It may consist of short ground hair, long guard hair, and, in some cases, medium awn hair....
. Montanalestes was found in North America, while all other non-placental eutherian fossils have been found in Asia. The earliest known placental fossils have also been found in Asia.