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Symmetrodonta
Symmetrodonta is a paraphyletic group of Mesozoic mammals and mammal-like synapsids characterized by the triangular aspect of the molars when viewed from above and the absence of a well-developed talonid. The traditional group of 'symmetrodonts' ranges in age from the latest Triassic to the Late Cretaceous. One species, ''Spalacotheridium noblei'', is notable for its small size. It is one of the smallest known mammals. Each individual molar is little more than 0.25 mm across. 'Symmetrodonts' are generally rare and poorly represented in the fossil record. Most research during the 21st century has concluded that they do not represent a discrete phylogenetic category. However, the name is still used informally by some researchers for convenience, or restricted to the spalacotheriids and zhangheotheriids.
Particular sub−groups of Symmetrodonta are better studied, e.g. Spalacotheriidae, which has acute−angled molariform teeth, strongly reduced talonids, and conspicuous anterior and posterior cingulids.
==See also==
* Evolution of mammals
* Spalacotheriidae (Full genus and species list on the Portuguese Wikipedia)
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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