Left upper molars of Tinodon (left one) and Symmetrodontoides canadensis (right two) from Palaeos.

Belongs within: Mammalia.
Contains: Cladotheria.

The Trechnotheria are a clade of mammals defined as the most exclusive clade including Zhangheotherium and crown Theria, with the earliest members known from the late Jurassic. Trechnotherians are characterised by the development of a strong postvallum/prevallid shear between the upper and lower molars (Luo et al. 2002). Basal members of the clade, the Spalacotheroidea and Tinodontidae, have been classified in the past as members of the 'symmetrodonts' but this group of early mammals with triangular molars and reduced talonids is now generally regarded as non-monophyletic.

Synapomorphies (from Luo et al. 2002): Hypertrophied postvallum/prevallid shearing mechanism present, with acute triangulation of molar cusps; petrosal with large post-tympanic recess and caudal tympanic process; epitympanic recess bounded by lateral wall formed by squamosal. Scapula with posterior part of supraspinous fossa relatively expanded, far wider than glenoid region, fossa fan-shaped or triangular in outline; supraspinous fossa wider than infraspinous fossa. Proximo-lateral process of tibia vestigial or absent. Humerus with posterior aspect of distal articulation for radius cylindrical. Calcaneus with sustentacular facet oblique to the horizontal plane (uncertain in Spalacotherioidea).

    |--Spalacotheroidea [Spalacotheriinae] A02
    |    |  i. s.: Heishanlestes MH06
    |    |--Zhangheotherium Hu et al. 1997 KH14, CM99
    |    |    `--Z. quinquecuspidens Hu et al. 1997 A02
    |    `--+--Akidolestes KH14
    |       `--+--Maotherium KH14
    |          `--Spalacotheriidae A02
    |               |  i. s.: Shalbaatar Nessov 1997 A02
    |               |           `--S. bakht Nessov 1997 A02
    |               |--Spalacotherium Owen 1854 KH14, CM99 [incl. Peralestes Owen 1871 A02]
    |               |    `--S. tricuspedens OB13
    |               `--Spalacolestinae CM99
    |                    |--Spalacotheroides Patterson 1955 CM99
    |                    |    `--S. bridwelli Patterson 1955 CM99
    |                    `--+--Spalacotheridium Cifelli 1990 CM99
    |                       |    |--*S. mckennai Cifelli 1990 CM99
    |                       |    `--S. noblei Cifelli & Madsen 1999 CM99
    |                       `--+--Spalacolestes Cifelli & Madsen 1999 CM99
    |                          |    |--*S. cretulablatta Cifelli & Madsen 1999 CM99
    |                          |    `--S. inconcinnus Cifelli & Madsen 1999 CM99
    |                          `--Symmetrodontoides Fox 1976 CM99 [incl. Mictodon Fox 1984 A02]
    |                               |--*S. canadensis Fox 1976 CM99
    |                               |--S. foxi Cifelli & Madsen 1986 CM99
    |                               |--S. oligodontos Cifelli 1990 CM99
    |                               `--‘Mictodon’ symmetrodontoides Fox 1984 A02
    `--+--Tinodontidae A02
       |    |--Gobiotheriodon Trofimov 1997 A02
       |    |    `--G. infinitus (Trofimov 1980) [=Gobiodon infinitus, Gobiondon (l. c.) infinitus] A02
       |    `--Tinodon Marsh 1879 KH14, A02 [incl. Eurylambda Simpson 1929 A02, Menacodon Marsh 1887 A02]
       |         |--T. bellus Marsh 1879 [incl. T. lepidus Marsh 1879] A02
       |         `--T. micron Ensom & Sigogneau-Russell 2000 A02
       `--Cladotheria LK-JC02

*Type species of generic name indicated


[A02] Averianov, A. O. 2002. Early Cretaceous “symmetrodont” mammal Gobiotheriodon from Mongolia and the classification of “Symmetrodonta”. Acta Palaeontologica Polonica 47 (4): 705–716.

[CM99] Cifelli, R. L., & S. K. Madsen. 1999. Spalacotheriid symmetrodonts (Mammalia) from the medial Cretaceous (upper Albian or lower Cenomanian) Mussentuchit local fauna, Cedar Mountain Formation, Utah, USA. Geodiversitas 21 (2): 167–214.

[KH14] Krause, D. W., S. Hoffmann, J. R. Wible, E. C. Kirk, J. A. Schultz, W. von Koenigswald, J. R. Groenke, J. B. Rossie, P. M. O'Connor, E. R. Seiffert, E. R. Dumont, W. L. Holloway, R. R. Rogers, L. J. Rahantarisoa, A. D. Kemp & H. Andriamialison. 2014. First cranial remains of a gondwanatherian mammal reveal remarkable mosaicism. Nature 515: 512–517.

[LK-JC02] Luo, Z.-X., Z. Kielan-Jaworowska & R. L. Cifelli. 2002. In quest for a phylogeny of Mesozoic mammals. Acta Palaeontologica Polonica 47: 1–78.

[MH06] Meng, J., Y. Hu, Y. Wang, X. Wang & C. Li. 2006. A Mesozoic gliding mammal from northeast China. Nature 444 (7121): 889–893.

[OB13] O'Leary, M. A., J. I. Bloch, J. J. Flynn, T. J. Gaudin, A. Giallombardo, N. P. Giannini, S. L. Goldberg, B. P. Kraatz, Z.-X. Luo, J. Meng, X. Ni, M. J. Novacek, F. A. Perini, Z. S. Randall, G. W. Rougier, E. J. Sargis, M. T. Silcox, N. B. Simmons, M. Spaulding, P. M. Velazco, M. Weksler, J. R. Wible & A. L. Cirranello. 2013. The placental mammal ancestor and the post-K–Pg radiation of placentals. Science 339: 662–667.

Last updated: 22 August 2018.

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