Symmetrodonta

Taxobox
name = Symmetrodonts
fossil_range = Late Triassic - Cretaceous
regnum = Animalia
phylum = Chordata
subphylum = Vertebrata
unranked_classis = Amniota
classis = Synapsida
unranked_ordo = Mammaliaformes
ordo = Symmetrodonta
subdivision_ranks = Families
subdivision =
* Kuehneotheriidae
* Shuotheriidae
* Spalacolestidae
* Spalacotheriidae
* Tinodontidae

Symmetrodonta is a basal group of Mesozoic mammals 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 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. Symmetrodonta are generally rare and poorly represented in the fossil record. It remains entirely possible they do not represent a discrete phylogenetic category, but with a series of intermediates between triconodonts, on the one hand, and dryolestoids and therians, on the other. At least some genera of symmetrodonts may even be true therians and part of the (disputed) clade Trituberculata.

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.

Classification

* Order SYMMETRODONTA
** Family Kuehneotheriidae
*** Genus "Kuehneotherium"
*** Genus "Delsatia"
*** Genus "Kotatherium"
** Family Shuotheriidae
** Family Spalacolestidae
** Family Spalacotheriidae
*** Genus "Spalacotherium"
*** Genus "Shalbaatar"
** Family Tinodontidae
*** Genus "Tinodon"
*** Genus "Gobiotheriodon"
** Family "Woutersiidae"
*** Genus "Woutersia"

ee also

* Evolution of mammals

External links

* [http://www.carnegiemuseums.org/cmag/bk_issue/1998/marapr/feat3.htm Discovering a Missing Link]