Symmetrodonts (Order Symmetrodonta)
Symmetrodonta is a traditionally paraphyletic group of Mesozoic mammals and mammal-like synapsids, ranging from the latest Triassic to Late Cretaceous. Symmetrodonts belonged to the Superlegion Trechnotheria. They were placed in the Infraclass Pantotheria and are considered to be distant relatives of more derived therian mammals, including the extinct eupantotheres, such as dryolestids, and the living placental and marsupial mammals. These small insectivorous or carnivorous animals were the size of a shrew or mouse. The molar cusps have a symmetrical triangular aspect when viewed from above and the re is no well-developed talonid. The central high cusp is flanked by two smaller cusps and several low minor cusps. The triangular upper and lower cheek teeth fill in the gaps between the adjacent teeth of the opposite tooth row and are specialized to crush insects or slice worms. Symmetrodonts lack a basinlike heel in the lower teeth that helps grind ingested food more derived living therian mammals and their kin. They also differ from more derived therians by lacking the angle on the mandible.
'Symmetrodonts' probably do not represent a discrete phylogenetic category, but the name is still used informally for convenience, or is restricted to Spalacotheriids (Family Spalacotheriidae) and Zhangheotheriids (Family Zhangheotheriidae) . Other authors include 3 families: Kuehneotheriidae (considered to be close to the ancestry of all other therian mammals); Amphidontidae; and Spalacotheriidae, which have highly acute triangular cheek teeth.
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Fossil specimen of Zhangheotherium quinquecuspidens
Symmetrodonta is a traditionally 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 1/100th of an inch across. 'Symmetrodonts' are generally rare and poorly represented in the fossil record. Most reearch 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.
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