Trechnotheria

Trechnotheria is a group of mammals that includes therians and fossil mammals from the Mesozoic Era. It comprises both extinct symmetrodonts and living Cladotheria. The term was initially described as a "superlegion" but later redefined node-based, encompassing the last common ancestor of Zhangheotherium and living therian mammals, along with all its descendants. Another definition focuses on the apomorphy of true symmetrodontia, which includes the Tinodontids.

Key characteristics of Trechnotheria are based on dental features, such as the "hypertrophied postvallum/prevallid shearing mechanism," and skeletal traits in the shoulder blade, tibia, humerus, and ankle joint. The phylogeny is complex, with various species listed under Unassigned Trechnotheria, Cladotheria, Zatheria, and Boreosphenida. These include extinct genera like †Austrotriconodon, †Chaoyangodens, and †Kotatherium, among others.

The text discusses the evolutionary relationships within Trechnotheria, suggesting that zanjeotherids may be ancestral to spalacotherids and cladotherians. The group Alethinotheria (sensu Alexander O. Averianov) is noted for its close relationship between Spalacotheriidae and Meridiolestids. Overall, Trechnotheria represents a significant clade within Mesozoic mammals, with diverse dental and skeletal adaptations.