Temporal range: Early Cretaceous1 – Holocene, 125–0 Ma
|Lycopsis longirostris, an extinct sparassodont, a relative of the marsupials|
Thomas Henry Huxley, 1880
Metatheria is a group of animals including all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a slightly more inclusive group than the marsupials; it contains all of the living mammals with abdominal pouches (most female marsupials) as well as their more primitive ancestors and relatives.
The relationships between the three extant divisions of mammals (monotremes, marsupials, and placental mammals) was long a matter of debate among taxonomists.2 Most morphological evidence comparing traits such as number and arrangement of teeth and structure of the reproductive and waste elimination systems favors a closer evolutionary relationship between marsupials and placental mammals than either with the monotremes, as does most genetic and molecular evidence.3
The ancestors of marsupials, part of a larger group called metatherians, probably split from those of placental mammals (eutherians) during the mid-Jurassic period, though no fossil evidence of metatherians themselves are known from this time.4 Fossil metatherians are distinguished from eutherians by the form of their teeth; metatherians possess four pairs of molar teeth in each jaw, whereas eutherian mammals (including true placentals) never have more than three pairs.5 Using this criterion, the earliest known metatherian is Sinodelphys szalayi, which lived in China around 125 million years ago (mya).6 This makes it a contemporary to some early eutherian species which have been found in the same area.7
The oldest metatherian fossils are found in present-day China.8 About 100 mya, the supercontinent Pangaea was in the process of splitting into the northern continent Laurasia and the southern continent Gondwana, with what would become China and Australia already separated by the Tethys Ocean. From there, metatherians spread westward into modern North America (still attached to Eurasia), where the earliest true marsupials are found. Marsupials are difficult to distinguish from fossils, as they are characterized by aspects of the reproductive system which do not normally fossilize (including pouches) and by subtle changes in the bone and tooth structure that show a metatherian is part of the marsupial crown group (the most exclusive group that contains all living marsupials). The earliest definite marsupial fossil belongs to the species Peradectes minor, from the Paleocene of Montana, dated to about 65 million years ago.1 From their point of origin in Laurasia, marsupials spread to South America, which was connected to North America until around 65 mya. Laurasian marsupials eventually died off, possibly due to competition from placental mammals for their ecological niches.
- O'Leary, Maureen A.; Bloch, Jonathan I.; Flynn, John J.; Gaudin, Timothy J.; Giallombardo, Andres; Giannini, Norberto P.; Goldberg, Suzann L.; Kraatz, Brian P.; Luo, Zhe-Xi; Meng, Jin; Ni, Michael J.; Novacek, Fernando A.; Perini, Zachary S.; Randall, Guillermo; Rougier, Eric J.; Sargis, Mary T.; Silcox, Nancy b.; Simmons, Micelle; Spaulding, Paul M.; Velazco, Marcelo; Weksler, John r.; Wible, Andrea L.; Cirranello, A. L. (8 February 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. Retrieved 9 February 2013.
- Moyal, Ann Mozley (2004). Platypus: The Extraordinary Story of How a Curious Creature Baffled the World. Baltimore: The Johns Hopkins University Press. ISBN 0-8018-8052-1.
- van Rheede, T.; Bastiaans, T.; Boone, D.; Hedges, S.; De Jong, 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): 587–597. doi:10.1093/molbev/msj064. PMID 16291999.
- Zhe-Xi Luo, Chong-Xi Yuan, Qing-Jin Meng and Qiang Ji (2011). "A Jurassic eutherian mammal and divergence of marsupials and placentals". Nature 476 (7361): 442–445. doi:10.1038/nature10291. PMID 21866158.
- Benton, Michael J. (1997). Vertebrate Palaeontology. London: Chapman & Hall. p. 306. ISBN 0-412-73810-4.
- Rincon, Paul (2003-12-12). "Oldest Marsupial Ancestor Found, BBC, Dec 2003". BBC News. Retrieved 2010-03-16.
- Hu, Yaoming last1=Hu; Meng, Jin Meng2 last2=Chuankui Li; Li, Yuanqing; Wang, Y. (2010). "New basal eutherian mammal from the Early Cretaceous Jehol biota, Liaoning, China". Proceedings of the Royal Society B 277 (1679): 229–236. doi:10.1098/rspb.2009.0203. PMC 2842663. PMID 19419990.
- Luo, Zhe-Xi; Ji, Qiang; Wible, John R.; Yuan, Chong-Xi (2003-12-12). "An early Cretaceous tribosphenic mammal and metatherian evolution". Science 302 (5652): 1934–1940. doi:10.1126/science.1090718. PMID 14671295. Retrieved 2010-12-27.
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