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Temporal range: Lower Cretaceous, 125 mya
Ji et al., 2002
The fossil is 10 centimetres (3.9 in) in length and virtually complete. It weighed between 20–25 grams (0.71–0.88 oz). Although the fossil's skull is squashed flat, its teeth, tiny foot bones, cartilages and even its fur are visible.
However 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.
- 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.
Its discoverers sampled 268 characters from all major Mesozoic mammal clades and principal eutherian families of the Cretaceous period. As a result, they claim Eomaia is at the root of the eutherian "family tree" with a few other fossils.
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.
- Ji Q. et al. (April 2002). "The earliest known eutherian mammal". Nature 416 (6883): 816–822. . . http://www.nature.com/nature/journal/v416/n6883/full/416816a.html. Retrieved 2008-09-24.
- Novacek M. (Jun. 19, 1986). "The primitive eutherian dental formula". Journal of Vertebrate Paleontology 6 (2): 191–196. . http://www.jstor.org/pss/4523087. Retrieved 2009-08-26.
- Weil A. (April 2002). "Mammalian evolution: upwards and onwards". Nature 416 (6883): 798–799. . . http://www.nature.com/nature/journal/v416/n6883/full/416798a.html. Retrieved 2008-09-24.
- Reilly S.M. and White T.D. (January 2003). "Hypaxial motor patterns and the function of epipubic bones in primitive mammals". Science 299 (5605): 400–402. . . http://www.sciencemag.org/cgi/content/full/299/5605/400. Retrieved 2008-09-24.
- Novacek M.J. et al. (October 1997). "Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia". Nature 389 (6650): 483–486. . . http://www.nature.com/nature/journal/v389/n6650/full/389483a0.html. Retrieved 2008-09-24.
- Ji, Q.; Luo, Z-X, Yuan, C-X, and Tabrum, A.R. (February 2006). "A swimming Mammaliaform from the Middle Jurassic and ecomorphological diversification of early mammals". Science 311 (5764): 1123. . . http://www.sciencemag.org/cgi/content/abstract/311/5764/1123. See also the news item at "Jurassic "Beaver" found; rewrites history of mammals". http://news.nationalgeographic.com/news/2006/02/0223_060223_beaver.html.