Trechnotheria is a group of mammals that includes the

theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

ns and some fossil mammals from the

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

Era. It includes both the extinct symmetrodonts and the living Cladotheria. Trechnotheria has been assigned various ranks, but was originally described as a "superlegion" by the naming authority. A later node-based definition for Trechnotheria was the clade comprising the last common ancestor of '' Zhangheotherium'' and living therian mammals, and all its descendants. Alternatively it can be defined as the group with true symmetrodontia, being defined as an apomorphy rather than being defined as a node, the redefinition includes the Tinodontids.

Characteristics

Like most Mesozoic mammal groups, early trechnotherians are known mainly from their teeth. Hence, one of the most prominent features of this group is the "hypertrophied postvallum/prevallid shearing mechanism", along with other dental characters. Features of the shoulder blade, tibia, humerus, and ankle joint also diagnose this clade.

Phylogeny

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

based on Luo, Cifelli & Kielan-Jaworowska, 2001, Luo, Kielan-Jaworowska & Cifelli, 2002 and, Kielan-Jaworowska, Cifelli & Luo, 2004. * Unassigned Trechnotheria: **†'' Austrotriconodon mckennai'' Bonaparte 1986 **†'' Chaoyangodens lii'' Hou & Meng 2014 *

Paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

Symmetrodonta Simpson 1925 **†'' Atlasodon monbaroni'' Sigogneau-Russell 1991 ryolestida, or Zatheria indet., milk tooth (Averianov, 2002)**†'' Kennetheredium leesi'' Sigogneau-Russell 2003a **†'' Kotatherium haldanei'' Datta 1981 **†'' Trishulotherium kotaensis'' Yadagiri 1985 * Unassigned Cladotheria: **†'' Afriquiamus nesovi'' Sigogneau-Russell 1999 **†'' Butlerigale'' Kühne 1968 **†'' Chunnelodon alopekodes'' Ensom & Sigogneau-Russell 1998 **†'' Guimarota freyi'' Kuhne 1968 * Unassigned Zatheria: **†'' Arguimus khoshajari'' Dashzeveg 1979 Arguitherium cromptoni'' Dashzeveg 1994">Arguitherium.html" ;"title="'Arguitherium">Arguitherium cromptoni'' Dashzeveg 1994 **†''Minimus richardfoxi'' Sigogneau-Russell 1999 **†''Magnimus, Magnimus ensomi'' Sigogneau-Russell 1999 **†''Mozomus, Mozomus shikamai'' Li et al. 2005 **†''Nanolestes'' Martin 2002 ***†'' Nanolestes drescherae, N. drescherae'' Martin 2002 ***†'' N. krusati'' Martin 2002 ***†'' N. mackennai'' Martin, Averianov & Pfretzschner 2010 * Unassigned Boreosphenida: **†'' Argaliatherium robustum'' Cifelli & Davis 2015 **†'' Carinalestes murensis'' Cifelli & Davis 2015 **†'' Hypomylos'' Sigogneau-Russell 1992 ***†'' H. phelizoni'' Sigogneau-Russell 1992 ***†'' H. micros'' Sigogneau-Russell 1994 **† Picopsidae Fox 1980 ***†'' Picopsis pattersoni'' Fox 1980 ***†'' Tirotherium aptum'' Montellano-Ballesteros & Fox 2015 Alternatively, the zanjeotherids would be ancestral to both the spalacotherids and the cladotherians ( driolestoids and prototribosphenids), with the tinodontids equally

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

. The group that contains the Spalacotheriidae and Cladotheria is called Alethinotheria (''sensu'' Alexander O Averianov. 2013), which was initially proposed to highlight the closeness between the spalacotherids and the meridiolestids.

Characteristics

Phylogeny

Sources

See also