Eomaia is an extinct fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

Mammals 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...

, discovered in rocks that were found in the Yixian Formation
Yixian Formation
The Yixian Formation is a geological formation in Jinzhou, Liaoning, People's Republic of China, that spans 11 million years during the early Cretaceous period...

, Liaoning
' is a province of the People's Republic of China, located in the northeast of the country. Its one-character abbreviation is "辽" , a name taken from the Liao River that flows through the province. "Níng" means "peace"...

 Province, China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...

, and dated to the Barremian
The Barremian is an age in the geologic timescale between 130.0 ± 1.5 Ma and 125.0 ± 1.0 Ma). It is a subdivision of the Early Cretaceous epoch...

 Age of the Lower Cretaceous about . The fossil is 10 centimetres (3.9 in) in length and virtually complete. An estimate of the body weight is between 20–25 g (0.705479242102239–0.881849052627799 oz). It is exceptionally well-preserved for a 125-million-year-old specimen. Although the fossil's skull is squashed flat, its teeth, tiny foot bones, cartilages and even its fur are visible.

It is a eutheria
Eutheria is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals than to living marsupials . They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth...

n, a member of a group of mammals consisting of placental mammals plus all extinct mammal
Mammals 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 that are more closely related to living placentals (such as humans) than to living marsupial
Marsupials 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). The features of eutheria that distinguish them from 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.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.

However Eomaia is not a true placental mammal as it lacks some features that are specific to placentals.
Eomaia features:
  • the presence of a malleolus at the bottom of the fibula, the smaller of the two shin bones.
  • a complete mortise and tenon
    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.
  • an atypical ancestral eutherian dental formula, (incisors, canines, premolars, molars on each side of the top and bottom jaws respectively). Eomaia had five upper and four lower incisors (much more typical for metatherians) and five premolars to three molars. Placental mammals, the only surviving eutherians, have only up to three incisors on each top and bottom and four premolars to three molars, but the premolar/molar proportion is similar to placentals.

  • 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 non-placental eutherians had a narrower opening that allows only small, immature offspring to pass through.

In addition Eomaia has epipubic bones
Epipubic bones
Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials and of some fossil mammals: multituberculates, monotremes, and even basal 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
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.
Its discoverers claim that, on the basis of 268 characters sampled from all major Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

 mammal clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

s and principal eutherian families of the Cretaceous, Eomaia is placed at the root of the eutherian "family tree" along with Murtoilestes and Prokennalestes.
The Eomaia fossil shows clear traces of hair. However this is not the earliest clear evidence of hair in the mammalian lineage, as fossils of Volaticotherium, and the docodont
Docodonta is an order of extinct proto-mammals that lived during the mid- to late-Mesozoic era. Their most distinguishing physical features were their relatively sophisticated set of molars, from which the order gets its name. In the fossil record, Docodonta is represented primarily by isolated...

 Castorocauda, discovered in rocks dated to about , also have traces of fur.

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