| Eomaia Temporal range: Early Cretaceous, 125 Ma |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Subclass: | Theria |
| Infraclass: | Eutheria |
| Genus: | Eomaia Ji et al., 2002 |
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Eomaia ('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 125 million years ago.[1] The fossil 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.[1]
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.[1]
- 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.[1]
- various features of jaws and teeth.[1]
However Eomaia is not a placental mammal as it lacks some features that are specific to placentals. Eomaia features:
- the presence of a malleolus at the bottom of the fibula, the smaller of the two shin bones.[1]
- 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.[1]
- an atypical 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 and four lower incisors (much more typical for metatherians) and five premolars to three molars.[1] Placental mammals, the only surviving eutherians, have only three incisors on each top and bottom and four premolars to three molars, but the premolar/molar proportion is similar to placentals.[2]
- 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.[3]
In addition Eomaia has epipubic bones extending forwards from the pelvis,[1] 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.[4] This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.[5]
Its discoverers claim that, on the basis of 268 characters sampled from all major Mesozoic mammal clades and principal eutherian families of the Cretaceous, Eomaia is placed at the root of the eutherian "family tree" along with Murtoilestes and Prokennalestes.[1] The Eomaia fossil shows clear traces of hair.[1] 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 million years ago, also have traces of fur.[6]
[edit] See also
[edit] References
- ^ a b c d e f g h i j k 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. http://www.nature.com/nature/journal/v416/n6883/full/416816a.html. Retrieved 2008-09-24.
- ^ Novacek, M. (Jun. 19, 1986). "The Primitie Eutherian Dental Formula". Journal of Vertebrate Paleontology 6 (2): 191–196. doi:10.1080/02724634.1986.10011610. http://www.jstor.org/pss/4523087. Retrieved 2009-08-26.
- ^ Weil, A. (April 2002). "Mammalian evolution: Upwards and onwards". Nature 416 (6883): 798–799. doi:10.1038/416798a. PMID 11976661. 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. doi:10.1126/science.1074905. PMID 12532019. http://www.sciencemag.org/cgi/content/full/299/5605/400. 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. 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. doi:10.1126/science.1123026. PMID 16497926. 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.