|
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 as a replacement name for the paraphyletic Triconodonta. ==Classification== "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. The group originally included only the family Triconodontidae and taxa that were later assigned to the separate family Amphilestidae, but was later expanded to include other taxa such as ''Morganucodon'' or ''Sinoconodon''.〔 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= |2=Cladotheria }} }} }} }} }} }} }} Cladogram after Gao ''et al.'', 2010 and Meng, Wang & Li, 2011: }} }} }} }} }} }} |2= }} }} }} }} }} The cladograms above resulted from the analyses using only dental and mandibular characters.〔〔〔 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); 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'';〔〔 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).〔 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".〔 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''. 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. 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.〔〔〔〔〔 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".〔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.〕 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Eutriconodonta」の詳細全文を読む スポンサード リンク
|