Eomaia

Eomaia scansoria ('climbing dawn mother') 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. The fossil is 10 centimeters (3.9 in) in length and virtually complete. An estimate of the body weight is between 20–25 grams (0.71–0.88 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 eutherian, a member of 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). 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.

However Eomaia is not a placental mammal, as it lacks some features that are specific to placentals:
 * 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 typical ancestral eutherian dental formula, 5.1.5.3/4.1.5.3 (incisors, canines, premolars, molars on each side of the top and bottom jaws respectively). Eomaia had five upper incisors, four lower incisors, and five premolars. Placental mammals, the only surviving eutherians, have three incisors on top and bottom and four premolars.
 * a wide opening at the bottom of the pelvis, 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 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 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 mammal clades and principal eutherian families of the Cretaceous, Eomaía 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 the docodont Castorocauda, discovered in rocks dated to about, also have traces of fur.