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Metatheria
Sign in to saveMetatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as well as many extinct non-marsupial relatives. It is one of two groups placed in the clade Theria alongside Eutheria, which contains the placentals. Remains of metatherians have been found on all of Earth's continents.
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後獸下綱（学名：Metatheria）是一个包含有袋类及其已灭绝的旁系近亲的演化支，依其学名作者湯瑪斯·亨利·赫胥黎在1880年的定义，本下纲包含了所有比胎盘类更接近有袋类的物种，是真兽下纲的旁系群，而有袋类就是后兽下纲的冠群[2]。 早期分类法中将其称为后兽亚纲，直接置于哺乳纲之下，后来厘清演化关系，置于兽亚纲之下，称后兽下纲。 参考文献 ^ 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. The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals. Science. 8 February 2013, 339 (6120): 662–667 [9 February 2013]. PMID 23393258. doi:10.1126/science.1229237. ^ Wilson, Don E.; Reeder, DeeAnn M. (editors). Microtus (Mynomes) townsendii. Wilson and Reeder's Mammal Species of the World — A Taxonomic and Geographic Reference (Print) Third (Baltimore, Maryland: Johns Hopkins University Press/Bucknell University). 2005: 2,142 [October 21, 2014]. ISBN 978-0-8018-8221-0. 查 论 编 按亞綱分類的哺乳類現存動物 動物界 · 脊索動物門 · 脊椎動物亞門 · 羊膜動物 原獸亞綱Prototheria（泄殖腔類）單孔目 Monotremata （針鼴及鴨嘴獸） 兽亚纲Theria後獸下綱M
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Metatheria is a mammalian clade that includes all mammals more closely related to marsupials than to placentals. First proposed by Thomas Henry Huxley in 1880, it is a more inclusive group than the marsupials; it contains all marsupials as well as many extinct non-marsupial relatives. It is one of two groups placed in the clade Theria alongside Eutheria, which contains the placentals. Remains of metatherians have been found on all of Earth's continents.
== Description == Distinctive characteristics (synapomorphies) of Metatheria include a prehensile tail, the development of a capitular tail on the humerus, the loss of tooth replacement on the second and fifth premolars, lower canines that outwardly diverge from each other, an angular process on the dentary bone—which additionally bears a posterior shelf in its masseteric fossa in Metatheria—that is equal to or greater than the length of the ramus, and the lower fifth premolar with a "very trenchant" cristid obliqua/ectolophid. The permanent, retained deciduous fifth premolars are molar like and were historically identified as first molars, with the third premolar found in basal therians being lost, leaving four premolars in the halves of each jaw. Sinodelphys differs in its dentition and may represent a basal metatherian.