New Docodontans from the Middle Jurassic of Siberia and reanalysis of Docodonta interrelationships
Two new docodontans, Simpsonodon sibiricus sp. nov. and Hutegotherium yaomingi gen. et sp. nov., and a new specimen of Itatodon tatarinovi Lopatin et Averianov, 2005 are described from the Middle Jurassic (Bathonian) Itat Formation of Krasnoyarsk Territory in West Siberia, Russia. A new phylogenetic analysis, incorporating 37 dental and mandibular characters and 18 docodontan and outgroup taxa demonstrated that Itatodon is the most basal known docodontan. Other docodontans can be classified in Docodontidae (Docodon and Haldanodon, with Borealestes recognized as a stem docodontid), Simpsonodontidae fam. nov. (Simpsonodon and Dsungarodon), Tegotheriidae (Krusatodon, Hutegotherium gen. nov., Tegotherium, and Sibirotherium), and Docodonta incertae sedis (Castorocauda, Tashkumyrodon, and Acuoduolodon). Simpsonodontidae fam. nov. is characterized, in particular, by a strongly developed additional groove dorsal to the Meckelian groove. The Middle Jurassic mammals of Laurasia are now known from three principal assemblages: Forest Marble in England, Balabansai in Kyrgyzstan, and Itat in West Siberia. All these three assemblages are very similar and comprise docodontans, allotherians, eutriconodontans, symmetrodontans, dryolestidans, and stem zatherians. The Middle Jurassic radiation of mammals has almost no roots in the known Early Jurassic mammal fauna and the evolutionary events which led to formation of the principal groups of Mesozoic mammals (Docodonta, Multituberculata, Eutriconodonta, Dryolestida, and stem Zatheria) should have occurred sometime between the Sinemurian and Bajocian in Laurasia, most probably in Asia (Siberia?).