Temporal range: Early Cretaceous, 125Ma
|Fossil specimen CAGS 01−IG−1|
Ji et al., 2002
Ji et al., 2002
Eomaia ("dawn mother") is a genus of extinct fossil mammals containing the single species Eomaia scansoria, 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 single fossil specimen of this species is 10 centimetres (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.
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 Castorocauda, discovered in rocks dated to about , also have traces of fur.
Eomaia scansoria possessed several features in common with placental mammals that distinguish them from metatherians, the group that includes modern marsupials. These include an enlarged malleolus ("little hammer") at the bottom of the tibia (the larger of the two shin bones) a joint between the first metatarsal bone and the entocuneiform bone in the foot which is offset further back than the joint between the second metatarsal and mesocuneiform bones (in metatherians these joints are level with each other), as well as various features of jaws and teeth.
However E. scansoria is not a true placental mammal as it lacks some features that are specific to placentals. These include 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, and an atypical ancestral eutherian dental formula, 188.8.131.52/184.108.40.206 (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.
Eomaia, like other early mammals and living marsupials, had a narrow pelvic outlet suggesting small undeveloped neonates requiring extensive nurturing. Epipubic bones extend forwards from the pelvis; these are not found in any placental, but are found in all other mammals, including non-placental eutherians, marsupials, monotremes and other Mesozoic mammals as well as 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.
The discoverers of Eomaia claimed that, on the basis of 268 characters sampled from all major Mesozoic mammal clades and principal eutherian families of the Cretaceous, Eomaia could be placed at the root of the eutherian "family tree" along with Murtoilestes and Prokennalestes. This initial classification scheme is summarized below.
In 2013, a much larger study of mammal relationships (including fossil species) was published by Maureen O'Leary and colleagues. The study, which examined 4541 anatomical characters of 86 mammal species (including Eomaia scansoria), found "100% jackknife support that Eomaia falls outside of Eutheria as a stem taxon to Theria", and so could not be considered a placental or a eutherian as previously believed. The results of this study are summarized in the cladogram below.
- Ji, Q., Luo, Z-X., Yuan, C-X.,Wible, J.R., Zhang, J-P.,and Georgi, J.A. (April 2002). "The earliest known eutherian mammal". Nature 416 (6883): 816–822. doi:10.1038/416816a. PMID 11976675. Retrieved 2008-09-24.
- Meng, J., Hu, Y., Wang, Y., Wang, X., Li, C. (Dec 2006). "A Mesozoic gliding mammal from northeastern China". Nature 444 (7121): 889–893. doi:10.1038/nature05234. PMID 17167478.
- 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): 1123–7. doi:10.1126/science.1123026. PMID 16497926.
- Novacek, M. (Jun 19, 1986). "The Primitie Eutherian Dental Formula". Journal of Vertebrate Paleontology 6 (2): 191–196. doi:10.1080/02724634.1986.10011610. JSTOR 4523087.
- 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. (January 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., Rougier, G.W, Wible, J.R., McKenna, M.C, Dashzeveg, D.,and Horovitz, I. (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.
- O'Leary, M. A.; Bloch, J. I.; Flynn, J. J.; Gaudin, T. J.; Giallombardo, A.; Giannini, N. P.; Goldberg, S. L.; Kraatz, B. P.; Luo, Z.-X.; Meng, J.; Ni, X.; Novacek, M. J.; Perini, F. A.; Randall, Z. S.; Rougier, G. W.; Sargis, E. J.; Silcox, M. T.; Simmons, N. B.; Spaulding, M.; Velazco, P. M.; Weksler, M.; Wible, J. R.; Cirranello, A. L. (2013). "The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals". Science 339 (6120): 662–667. doi:10.1126/science.1229237. PMID 23393258.
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- Hecht, Jeff (24 April 2002). "Stunning fossil is human's earliest mammal relative". New Scientist. Retrieved February 2012.
- Roach, John (25 January 2005). "Scientists Recreate Genome of Ancient Human Ancestor". National Geographic News. Retrieved February 2012.