Eutheria is a group of
mammalMammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s consisting of
placentaThe placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
l mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living
marsupialMarsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s (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
fossilFossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
and living mammals (
monotremeMonotremes are mammals that lay eggs instead of giving birth to live young like marsupials and placental mammals...
s and
marsupialMarsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s).
The oldest known eutherian species is
Juramaia sinensis, dated at from the
JurassicThe Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
in China. The previously earliest known fossil eutherian,
Eomaia scansoria, was also from China and is dated to the Early
CretaceousThe Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
period, about .
Eutheria was introduced by Thomas Henry Huxley in 1880, meant to be broader in definition than its precursor
Placentalia.
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
marsupialMarsupials are an infraclass of mammals, characterized by giving birth to relatively undeveloped young. Close to 70% of the 334 extant species occur in Australia, New Guinea, and nearby islands, with the remaining 100 found in the Americas, primarily in South America, but with thirteen in Central...
s (such as kangaroos).
There are no living nonplacental eutherians, and so knowledge of their
synapomorphiesIn cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
("defining features") is entirely based on a few
fossilFossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s, 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
metatheriaMetatheria 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.The earliest known...
ns, a group that includes modern marsupials, are:
- an enlarged malleolus ("little hammer") at the bottom of the 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, the smaller of the two shin bones.
- a complete mortise and tenon
The mortise and tenon joint has been used for thousands of years by woodworkers around the world to join pieces of wood, mainly when the adjoining pieces connect at an angle of 90°. In its basic form it is both simple and strong. Although there are many joint variations, the basic 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
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...
, 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
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
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