Eomaia

Eomaia ('dawn mother') is an early fossil eutherian mammal. It is exceptionally well preserved for a 125 million-year-old specimen.

 Discovery 
The fossil was discovered in rocks of the Yixian Formation in China's Liaoning Province. These rock stratas date from the Late Jurassic period to the Cretaceous about 125 million years ago (mya).

 Description 
The fossil is  in length and virtually complete. It weighed between . Although the fossil's skull is squashed flat, its teeth, tiny foot bones, cartilages and even its fur are visible.

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 164 mya, also have traces of fur. See also the news item at 

 Type of mammal 
Eomaia is a eutherian. The eutherians are the placental mammals plus some extinct mammals which had not developed a placenta.

What distinguishes the Eutheria from metatherians, a group that includes modern marsupials, are various features of the feet, jaws and teeth.

The Eomaia fossil's discoverers sampled 268 characters from all major Mesozoic mammal clades and principal eutherian families of the Cretaceous period. Based on their results, they claimed Eomaia is at the root of the eutherian "family tree" with a few other fossils.

 Features 
Eomaia is not a true placental mammal as it lacks some features that are specific to placentals. Eomaia has:
variations in the shin and ankles.
an ancestral eutherian dental formula.
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.
Eomaia has epipubic bones extending forwards from the pelvis. These are not found in any placental mammal, but are found in all other mammals, 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.

 Related pages 
 Juramaia
 Sinodelphys

 References 

 Other websites 
 New Scientist article on original find.
 Nat. Geo. article

Category:Extinct mammals
Category:Fossils
Category:Cretaceous
Category:China