Eomaia

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 125 million years ago.[1] 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.[1]

Eomaia
Temporal range: Early Cretaceous, 125 Ma
↓
Fossil specimen CAGS 01−IG−1
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Tribosphenida
Genus: †Eomaia
Ji et al., 2002
Type species
†Eomaia scansoria
Ji et al., 2002

Description
Restoration

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 Volaticotherium,[2] and the docodont Castorocauda, discovered in rocks dated to about 164 million years ago, also have traces of fur.[3]

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),[1] 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),[1] as well as various features of jaws and teeth.[1]

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,[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] and an atypical ancestral eutherian dental formula of 5.1.5.34.1.5.3 × 2 = 54. Eomaia had five upper and four lower incisors (much more typical for metatherians) and five premolars to three molars.[1] Placental mammals 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.[4]

Eomaia, like other early mammals and living marsupials, had a narrow pelvic outlet suggesting small undeveloped neonates requiring extensive nurturing.[5] Epipubic bones extend forwards from the pelvis;[1] 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.[6] This stiffening would be harmful in pregnant placentals, whose abdomens need to expand.[7]

Classification
Fossil cast

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.[1] This initial classification scheme is summarized below.

 Theria
 Metatheria
 

 Sinodelphys szalayi

 
 

 Cainozoic metatheria

 
 
 Eutheria
 

 Juramaia sinensis

 
 

 Montanalestes keeblerorum

 
 
 

 Murtoilestes abramovi

 
 

 Eomaia scansoria

 
 

 Prokennalestes trofimovi

 
 
 

 Cainozoic placentalia

 
 
 
Restoration of Eomaia feeding on the insect Cretophasmomima

In 2013, a much larger study of mammal relationships (including fossil species) was published by Maureen O'Leary and colleagues.[8] 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.[8] The results of this study are summarized in the cladogram below.

 Mammalia
 

 Monotremata

 
 
 
 

 Henkelotherium guimarotae

 
 

 Zhangheotherium quinquecuspidens

 
 
 
 

 Eomaia scansoria

 
 

 Theria

 
 
 
 

The 2013 study by O'Leary et al. is controversial.[9] [10] [11] The original classification of Eomaia as a eutherian mammal is still widely accepted.[12] [13] [14]

See also

References
  1. Ji, Q.; Luo, Z-X.; Yuan, C-X.; Wible, J.R.; Zhang, J-P. & Georgi, J.A. (April 2002). "The earliest known eutherian mammal" (PDF). Nature. 416 (6883): 816–822. doi:10.1038/416816a. PMID 11976675. Archived from the original (PDF) on 2010-11-03. Retrieved 2008-09-24.
  2. 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.
  3. Ji, Q.; Luo, Z-X; Yuan, C-X; 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.
  4. 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.
  5. Weil, A. (April 2002). "Mammalian evolution: Upwards and onwards". Nature. 416 (6883): 798–799. doi:10.1038/416798a. PMID 11976661.
  6. Reilly, S.M.; 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.
  7. Novacek, M.J.; Rougier, G.W; Wible, J.R.; McKenna, M.C; Dashzeveg, D. & 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.
  8. 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. S2CID 206544776.
  9. Springer, M. S.; Meredith, R. W.; Teeling, E. C.; Murphy, W. J. (2013). "Technical Comment on "The Placental Mammal Ancestor and the Post-K-Pg Radiation of Placentals"". Science. 341 (6146): 613. doi:10.1126/science.1238025. ISSN 0036-8075. PMID 23929967.
  10. dos Reis, M.; Donoghue, P. C. J.; Yang, Z. (2014). "Neither phylogenomic nor palaeontological data support a Palaeogene origin of placental mammals". Biology Letters. 10 (1): 20131003. doi:10.1098/rsbl.2013.1003. ISSN 1744-9561. PMC 3917342. PMID 24429684.
  11. Romiguier, J.; Ranwez, V.; Delsuc, F.; Galtier, N.; Douzery, E. J. P. (2013). "Less Is More in Mammalian Phylogenomics: AT-Rich Genes Minimize Tree Conflicts and Unravel the Root of Placental Mammals". Molecular Biology and Evolution. 30 (9): 2134–2144. doi:10.1093/molbev/mst116. ISSN 0737-4038. PMID 23813978.
  12. Meng, J. (2014). "Mesozoic mammals of China: implications for phylogeny and early evolution of mammals". National Science Review. 1 (4): 521–542. doi:10.1093/nsr/nwu070. ISSN 2095-5138.
  13. Viriot, Laurent; Gheerbrant, Emmanuel; Amaghzaz, Mbarek; Bouya, Baadi; Goussard, Florent; Letenneur, Charlène (2014). "Ocepeia (Middle Paleocene of Morocco): The Oldest Skull of an Afrotherian Mammal". PLOS ONE. 9 (2): e89739. doi:10.1371/journal.pone.0089739. ISSN 1932-6203. PMC 3935939. PMID 24587000.
  14. Solé, Floréal; Falconnet, Jocelyn; Yves, Laurent (2014). "New proviverrines (Hyaenodontida) from the early Eocene of Europe; phylogeny and ecological evolution of the Proviverrinae". Zoological Journal of the Linnean Society. 171 (4): 878–917. doi:10.1111/zoj.12155. ISSN 0024-4082.
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