Symmetrodonta
Encyclopedia
Symmetrodonta is a basal group of Mesozoic
mammals characterized by the triangular aspect of the molar
s 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. 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 dryolestoid
s and theria
ns, 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.
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
mammals characterized by the triangular aspect of the molar
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
s when viewed from above and the absence of a well-developed talonid. The traditional group of symmetrodonts ranges in age from the latest Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
to the Late Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
. 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 dryolestoid
Dryolestoidea
Dryolestoidea is an extinct clade of Mesozoic mammals that only contains two orders. It has been suggested that this group contained the ancestors of modern therian mammals. They are mostly represented by teeth, fragmented dentaries and parts of the rostrum. The Jurassic forms retained a...
s and theria
Theria
Theria is a subclass of mammals that give birth to live young without using a shelled egg, including both eutherians and metatherians . The only omitted extant mammal group is the egg-laying monotremes....
ns, on the other. At least some genera of symmetrodonts may even be true therians and part of the (disputed) clade Trituberculata
Trituberculata
Trituberculata is an extinct group of animals existing in the fossil record from about 215 – 85 MYA. It contains the ancestors of Placentalia and Marsupialia; all modern mammals except Monotremata are descended from trituberculates. It is named for the three tubercles of the molar teeth...
.
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 KuehneotheriidaeKuehneotheriidaeKuehneotheriidae is a clade within Symmetrodonta and was created to embrace Kuehneotherium and Woutersia, which lived in Europe in the late Triassic and early Jurassic...
- Genus KuehneotheriumKuehneotheriumKuehneotherium was an early symmetrodont whose fossils have been found in Greenland, France and Luxembourg. It is known only from an upper molar as well as nine additional teeth and four dentary fragments.-Further reading:...
- Genus Delsatia
- Genus Kotatherium
- Genus Kuehneotherium
- Family Spalacolestidae
- Family Spalacotheriidae
- Genus Spalacotherium
- Genus Shalbaatar
- Family TinodontidaeTinodontidaeTinodontidae is an extinct family of actively mobile mammal, endemic to what would now be North America, Asia, Europe, and Africa during the Jurassic and Cretaceous periods.-Taxonomy:Tinodontidae...
- Genus TinodonTinodonTinodon is an extinct genus of Late Jurassic mammal from the Morrison Formation.Present in stratigraphic zone 5.-See also:* Prehistoric mammal** List of prehistoric mammals* Paleobiota of the Morrison Formation-References:...
- Genus Gobiotheriodon
- Genus Tinodon
- Family Woutersiidae
- Genus WoutersiaWoutersiaWoutersia was a triassic symmetrodont and the only representative of its family. It has been suggested that it may form the sister taxon to Docodonta. Its remains have been found in France.-Sources:*...
- Genus Woutersia
- Family Kuehneotheriidae