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.
- 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.
Its discoverers sampled 268 characters from all major Mesozoic mammal clades and principal eutherian families of the Cretaceous period. As a result, they claimed 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. (2002). "The earliest known eutherian mammal". Nature. 416 (6883): 816–822. doi:10.1038/416816a. PMID 11976675. Retrieved 2008-09-24.
- Novacek M. (Jun. 19, 1986). "The primitive eutherian dental formula". Journal of Vertebrate Paleontology. 6 (2): 191–196. doi:10.1080/02724634.1986.10011610. Retrieved 2009-08-26. Check date values in:
- Weil A. (April 2002). "Mammalian evolution: upwards and onwards". Nature. 416 (6883): 798–799. doi:10.1038/416798a. PMID 11976661. Retrieved 2008-09-24.
- Reilly S.M. and White T.D. (2003). "Hypaxial motor patterns and the function of epipubic bones in primitive mammals". Science. 299 (5605): 400–402. doi:10.1126/science.1074905. PMID 12532019. 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. doi:10.1038/39020. PMID 9333234. Retrieved 2008-09-24.
- Ji, Q. (2006). "A swimming Mammaliaform from the Middle Jurassic and ecomorphological diversification of early mammals". Science. 311 (5764): 1123. doi:10.1126/science.1123026. PMID 16497926. Unknown parameter
|author=suggested) (help) See also the news item at "Jurassic "Beaver" found; rewrites history of mammals".