Eutriconodonta

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Eutriconodonts
Temporal range: Middle Jurassic - Late Cretaceous, 167–70Ma
Gobiconodon.jpg
Life restoration of a Gobiconodon
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Eutriconodonta
Kermack et al., 1973
Subgroups

Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack et al. in 1973[1] as a replacement name for the paraphyletic Triconodonta.[2]

Classification[edit]

Illustration of the lower jaw of Triconodon mordax, 1861

"Triconodonta" had long been used as the name for an order of early mammals which were close relatives of the ancestors of all present-day mammals, characterized by molar teeth with three main cusps on a crown that were arranged in a row.[3] The group originally included only the family Triconodontidae and taxa that were later assigned to the separate family Amphilestidae,[4] but was later expanded to include other taxa such as Morganucodon or Sinoconodon.[3] The phylogenetic analyses found that all these taxa did not form a natural group, and that some traditional "triconodonts" were more closely related to therian mammals than others. Some traditional "triconodonts" do seem to form a natural group (or "clade"), and this was given the name Eutriconodonta, or "true triconodonts).

Cladogram after Averianov & Lopatin, 2011:[2]

 Holotheria 
Gobiconodonta

Gobiconodontidae



Klameliidae




Amphilestidae

Amphilestes



Phascolotherium





Amphidontidae



Eutriconodonta

Bocaconodon




Jeholodentidae



Triconodontidae






Tinodon


Trechnotheria


Kiyatherium




Spalacotheriidae



Zhangheotheriidae





Cladotheria








Cladogram after Gao et al., 2010[5] and Meng, Wang & Li, 2011:[6]

 Holotheria 

Amphidontidae



Gobiconodontidae



Eutriconodonta

Jeholodens




Yanoconodon




Liaoconodon


Triconodontidae

Priacodon





Triconodon



Trioracodon





Arundeloconodon




Astroconodon




Alticonodon



Corvicondodon












Phascolotherium



Amphilestes




Tinodon


Trechnotheria


Spalacotheriidae



Zhangheotheriidae




Cladotheria







The cladograms above resulted from the analyses using only dental and mandibular characters.[2][5][6] Gao et al. (2010) conducted a second analysis as well, using a modified version of the matrix from the analysis of Luo et al. (2007);[7] this analysis involved a broader range of Mesozoic mammaliaforms and more characters, including postcranial ones. Both Luo et al. (2007) and the second analysis of Gao et al. (2010) recovered a more inclusive monophyletic Eutriconodonta that also contained gobiconodontids and Amphilestes;[5][7] in the second analysis of Gao et al. it also contained Juchilestes (recovered as amphidontid in their first analysis, the only amphidontid included in their second analysis).[5] However, Gao et al. (2010) stressed that jeholodentids and gobiconodontids are the only eutriconodonts with known postcranial skeletons; according to the authors, it remains uncertain whether the results of their second analysis represent true phylogeny or are merely "a by-product of long branch attraction of jeholodentids and gobiconodontids".[5] Phylogenetic studies conducted by Zheng et al. (2013), Zhou et al. (2013) and Yuan et al. (2013) recovered monophyletic Eutriconodonta containing triconodontids, gobiconodontids, Amphilestes, Jeholodens and Yanoconodon.[8][9][10]

The exact phylogenetic placement of eutriconodonts within Mammaliaformes is also uncertain. Zhe-Xi Luo, Zofia Kielan-Jaworowska and Richard Cifelli (2002) conducted an analysis that recovered eutriconodonts within the crown group of Mammalia, i.e. the least inclusive clade containing monotremes and therian mammals. The analysis found eutriconodonts to be more closely related to therian mammals than monotremes were, but more distantly than (paraphyletic) amphitheriids, dryolestids, spalacotheriid "symmetrodonts" and multituberculates were.[11] This result was mostly confirmed by Luo et al. (2007), the second analysis of Gao et al. (2010), Zheng et al. (2013), Zhou et al. (2013) and Yuan et al. (2013), although in the phylogenies of Luo et al. (2007) and Yuan et al. (2013) eutriconodonts were in unresolved polytomy with multituberculates and trechnotherians.[5][7][8][9][10] If confirmed this would make eutriconodonts one of the groups that can be classified as mammals by any definition. Several other extinct groups of Mesozoic animals that are traditionally considered to be mammals (such as Morganucodonta and Docodonta) are now placed just outside Mammalia by those who advocate a 'crown-group' definition of the word "mammal".[12] However, Luo, Kielan-Jaworowska and Cifelli (2002) tested alternative possible phylogenies as well, and found that recovering eutriconodonts outside the crown group of Mammalia required only five additional steps compared to the most parsimonious solution. The authors stated that such placement of eutriconodonts is less likely than their placement within the mammalian crown group, but it cannot be rejected on a statistical basis.[11]

References[edit]

  1. ^ Kermack, K.A.; Mussett, F.; Rigney, H.W. (1973). "The lower jaw of Morganucodon". Zoological Journal of the Linnean Society 53 (2): 87–175. doi:10.1111/j.1096-3642.1973.tb00786.x. OCLC 4650939832. 
  2. ^ a b c A. O. Averianov and A. V. Lopatin (2011). "Phylogeny of Triconodonts and Symmetrodonts and the Origin of Extant Mammals". Doklady Biological Sciences 436 (1): 32–35. doi:10.1134/s0012496611010042. 
  3. ^ a b Zofia Kielan-Jaworowska, Richard L. Cifelli, Zhe-Xi Luo (2004). "Chapter 7: Eutriconodontans". Mammals from the Age of Dinosaurs: origins, evolution, and structure. New York: Columbia University Press. pp. 216–248. ISBN 0-231-11918-6. 
  4. ^ George Gaylord Simpson (1929). "American Mesozoic Mammalia". Memoirs of the Peabody Museum of Yale University 3: 1–235. 
  5. ^ a b c d e f Chun-Ling Gao, Gregory P. Wilson, Zhe-Xi Luo, A. Murat Maga, Qingjin Meng and Xuri Wang (2010). "A new mammal skull from the Lower Cretaceous of China with implications for the evolution of obtuse-angled molars and ‘amphilestid’ eutriconodonts". Proceedings of the Royal Society B: Biological sciences 277 (1679): 237–246. doi:10.1098/rspb.2009.1014. PMC 2842676. PMID 19726475. 
  6. ^ a b Jin Meng, Yuanqing Wang and Chuankui Li (2011). "Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont". Nature 472 (7342): 181–185. doi:10.1038/nature09921. PMID 21490668. 
  7. ^ a b c Luo, Z.-X.; Chen, P.; Li, G. and Chen, M. (2007). "A new eutriconodont mammal and evolutionary development in early mammals.". Nature 446 (7133): 288–293. doi:10.1038/nature05627. PMID 17361176. 
  8. ^ a b Xiaoting Zheng, Shundong Bi, Xiaoli Wang and Jin Meng (2013). "A new arboreal haramiyid shows the diversity of crown mammals in the Jurassic period". Nature 500 (7461): 199–202. doi:10.1038/nature12353. 
  9. ^ a b Chang-Fu Zhou, Shaoyuan Wu, Thomas Martin and Zhe-Xi Luo (2013). "A Jurassic mammaliaform and the earliest mammalian evolutionary adaptations". Nature 500 (7461): 163–167. doi:10.1038/nature12429. 
  10. ^ a b Chong-Xi Yuan, Qiang Ji, Qing-Jin Meng, Alan R. Tabrum and Zhe-Xi Luo (2013). "Earliest Evolution of Multituberculate Mammals Revealed by a New Jurassic Fossil". Science 341 (6147): 779–783. doi:10.1126/science.1237970. 
  11. ^ a b Zhe-Xi Luo, Zofia Kielan-Jaworowska and Richard L. Cifelli (2002). "In quest for a phylogeny of Mesozoic mammals". Acta Palaeontologica Polonica 47 (1): 1–78. 
  12. ^ Traditionally, membership in Mammalia is diagnosed by the presence of a single dominant jaw joint, in which the dentary contacts the squamosal. However, taxonomists debate whether established names, such as Mammalia, should correspond to the clade which is closest to the traditional definition or, alternatively, should be restricted to the 'crown-group' (which includes only descendants of the most recent common ancestor shared by all living member species and excludes any fossil forms which diverged at an earlier stage, even if they meet the traditional criteria). Supporters of the crown-group approach refer to the broader grouping as the Mammaliformes or Mammaliaformes, whereas traditionalists describe the entire assemblage as "mammals". For a summary of the argument and issues, see Benton 2005: 289.