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

Belongs within: Mammalia.
Contains: Meridiolestida, Prototribosphenida.

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).

<==Trechnotheria [Spalacotheriinae, Spalacotheroidea]
    |  |    |--Lactodens sheniMH20
    |  |    `-+--+--Zhangheotherium Hu et al. 1997MH20, CM99
    |  |      |  |  `--Z. quinquecuspidens Hu et al. 1997A02
    |  |      |  `--MaotheriumMH20
    |  |      |      |--M. asiaticusMH20
    |  |      |      `--M. sinensisMH20
    |  |      `--+--Origolestes Mao, Hu et al. 2020MH20
    |  |         |    `--*O. lii Mao, Hu et al. 2020MH20
    |  |         |--Anebodon luoiMH20
    |  |         `--KiyatheriumMH20
    |  `--Cladotheria [Dryolestidae, Dryolestoidea, Eupantotheria, Pantotheria]LK-JC02
    |       |  i. s.: Thereuodon Sigogneau-Russell 1989A02
    |       |         Atlasodon Sigogneau-Russell 1991A02
    |       |--+--PaurodontidaeRAG11
    |       |  |    |  i. s.: Brancatherulum tendagurense Dietrich 1927C78
    |       |  |    |         Archaeotrigon Simpson 1927SM93
    |       |  |    |         Tathiodon Simpson 1927SM93
    |       |  |    |         Pelicopsis Simpson 1927SM93
    |       |  |    |         Araeodon Simpson 1937SM93
    |       |  |    |--Foxraptor Bakker 1990RW12, SM93
    |       |  |    `--+--Paurodon Marsh 1887RW12, SM93
    |       |  |       `--DrescheratheriumRW12
    |       |  `--MeridiolestidaRW12
    |       `--+--DryolestidaRAG11
    |          |    |  i. s.: Casamiquelia Bonaparte 1990A02
    |          |    |         Bondesius Bonaparte 1990 [Bondesiidae]A02
    |          |    |--Henkelotherium Krebs 1991RW12, CM99
    |          |    |    `--H. guimarotaeOB13
    |          |    `--+--DryolestesKH14
    |          |       `--+--ComotheriumRW12
    |          |          `--+--Amblotherium Owen 1871RW12, CM99
    |          |             `--+--Laolestes Simpson 1927RW12, CM99
    |          |                `--Groebertherium Bonaparte 1986RW12, SM93
    |          `--PrototribosphenidaLK-JC02
         |  i. s.: Shalbaatar Nessov 1997A02
         |           `--S. bakht Nessov 1997A02
         |--Spalacotherium Owen 1854MH20, CM99 [incl. Peralestes Owen 1871A02]
         |    `--S. tricuspedensOB13
              |  i. s.: Akidolestes cifelliiMH20
              |--Spalacotheroides Patterson 1955CM99
              |    `--S. bridwelli Patterson 1955CM99
              `--+--Spalacotheridium Cifelli 1990CM99
                 |    |--*S. mckennai Cifelli 1990CM99
                 |    `--S. noblei Cifelli & Madsen 1999CM99
                 `--+--Spalacolestes Cifelli & Madsen 1999CM99
                    |    |--*S. cretulablatta Cifelli & Madsen 1999CM99
                    |    `--S. inconcinnus Cifelli & Madsen 1999CM99
                    `--Symmetrodontoides Fox 1976CM99 [incl. Mictodon Fox 1984A02]
                         |--*S. canadensis Fox 1976CM99
                         |--S. foxi Cifelli & Madsen 1986CM99
                         |--S. oligodontos Cifelli 1990CM99
                         `--‘Mictodon’ symmetrodontoides Fox 1984A02
Trechnotheria incertae sedis:
     |  |--HakusanodonMWL11
     |  `--Juchilestes liaoningensisMWL11, MH20
     `--Amphidontidae [Amphidontoidea]SM93
          |--Manchurodon Yabe & Shikama 1938SM93
          |--‘Gobiodon’ Trofimov 1980 non Bleeker 1856SM93
          `--Amphidon Simpson 1925MWL11, A02
               `--A. superstes Simpson 1925A02
  Heishanlestes changiMH06, MH20

*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.

[C78] Cooke, H. B. S. 1978. Africa: the physical setting. In: Maglio, V. J., & H. B. S. Cooke (eds) Evolution of African Mammals pp. 17–45. Harvard University Press: Cambridge (Massachusetts).

[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.

[MH20] Mao, F., Y. Hu, C. Li, Y. Wang, M. H. Chase, A. K. Smith & J. Meng. 2020. Integrated hearing and chewing modules decoupled in a Cretaceous stem therian mammal. Science 367: 305–308.

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

[MWL11] Meng, J., Y. Wang & C. Li. 2011. Transitional mammalian middle ear from a new Cretaceous Jehol eutriconodont. Nature 472: 181–185.

[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.

[RAG11] Rougier, G. W., S. Apesteguía & L. C. Gaetano. 2011. Highly specialized mammalian skulls from the Late Cretaceous of South America. Nature 479: 98–102.

[RW12] Rougier, G. W., J. R. Wible, R. M. D. Beck & S. Apesteguía. 2012. The Miocene mammal Necrolestes demonstrates the survival of Mesozoic nontherian lineage into the late Cenozoic of South America. Proceedings of the National Academy of Sciences of the USA 109 (49): 20053–20058.

[SM93] Stucky, R. K., & M. C. McKenna. 1993. Mammalia. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 739–771. Chapman & Hall: London.

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