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Marsupionta
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| Marsupionta | |
|---|---|
Scientific classification 
| |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Mammaliaformes |
| Class: | Mammalia |
| Subclass: | Marsupionta Gregory, 1947 |
| Subgroups | |
Marsupionta is a hypothetical clade of mammals containing marsupials and monotremes, but not the placentals. This hypothesis is contrary to the conventional view that marsupials and placentals form a clade (Theria) that excludes monotremes. Marsupionta was proposed in 1947 by the American zoologist William King Gregory[1] and has since been the subject of multiple studies.[2][3][4]
Evidence
[edit]Morphological evidence
[edit]The majority of researchers prefer the Theria hypothesis, in which marsupials and placentals form a clade to the exclusion of monotremes. Theria is characterised by a number of common derived characteristics (synapomorphies), which include among others, viviparity (the birth of live young), the presence of teats, and several features in the skull and shoulder girdle structures.
Potential synapomorphies of Marsupionta exist only in the epipubic (pouch) bones. The two epipubic bones that protrude from the pelvis bone, are present in both monotremes and marsupials, but are missing in placentals. However, some primitive mammals, as well as fossil ancestors of the Cretaceous higher mammals also exhibit these bones. It can therefore be assumed that the epipubic bones were an ancestral trait of mammals that has been reduced in today's placentals, and that no morphological evidence exists for the Marsupionta hypothesis.
Molecular evidence
[edit]Genetic findings regarding the correct classification scheme for marsupials and monotremes are contradictory. Comparisons of mitochondrial DNA support the Marsupionta hypothesis,[5] while genome sequencing[6] speaks for the Theria hypothesis. Other studies do not come to a clear conclusion.
References
[edit]- ^ Phillips, Matthew J.; Penny, David (2003). "The root of the mammalian tree inferred from whole mitochondrial genomes". Molecular Phylogenetics and Evolution. 28 (2): 171–185. Bibcode:2003MolPE..28..171P. CiteSeerX 10.1.1.518.7974. doi:10.1016/S1055-7903(03)00057-5. PMID 12878457.
- ^ Kullberg, Morgan; Hallström, Björn M; Arnason, Ulfur; Janke, Axel (2008). "Phylogenetic analysis of 1.5 Mbp and platypus EST data refute the Marsupionta hypothesis and unequivocally support Monotremata as sister group to Marsupialia/Placentalia". Zoologica Scripta. 37 (2): 115–127. doi:10.1111/j.1463-6409.2007.00319.x. S2CID 85196487.
- ^ Killian, J. K; Buckley, T. R; Stewart, N; Munday, B. L; Jirtle, R. L (2001). "Marsupials and Eutherians reunited: genetic evidence for the Theria hypothesis of mammalian evolution". Mammalian Genome. 12 (7): 513–7. doi:10.1007/s003350020026. PMID 11420613. S2CID 16509358.
- ^ Van Rheede, T; Bastiaans, T; Boone, D. N; Hedges, S. B; De Jong, W. W; Madsen, O (2006). "The platypus is in its place: nuclear genes and indels confirm the sister group relation of monotremes and Therians". Molecular Biology and Evolution. 23 (3): 587–97. doi:10.1093/molbev/msj064. PMID 16291999.
- ^ Janke, Axel; Magnell, Ola; et al. (2002). "Phylogenetic Analysis of 18S rRNA and the Mitochondrial Genomes of the Wombat, Vombatus ursinus, and the Spiny Anteater, Tachyglossus aculeatus; Increased Support for the Marsupionta Hypothesis". J. Mol. Evol. 54 (1): 71–80. Bibcode:2002JMolE..54...71J. doi:10.1007/s00239-001-0019-8. PMID 11734900. S2CID 17410715 – via ResearchGate.
- ^ Grützner, Frank; Graves, Jeniffer A. Marshall (2004). "A platypus' eye view of the mammalian genome" (PDF). Curr. Opin. Genet. Dev. 14 (6): 642–649. doi:10.1016/j.gde.2004.09.006. PMID 15531159. Archived from the original (PDF) on 2016-03-11. Retrieved 2018-08-18 – via University of Antwerp.
