Multituberculate mammals from the Middle Jurassic of Western Siberia, Russia, and the origin of Multituberculata

Tashtykia primaeva gen. et sp. nov. and Tagaria antiqua gen. et sp. nov.
(Multituberculata incertae sedis) are described based on isolated teeth
from the Middle Jurassic (Bathonian) Itat Formation from the Berezovsk
coal mine in Krasnoyarsk Territory, Western Siberia, Russia. Tashtykia
primaeva gen. et sp. nov. is characterized by a P5 with three rows of
cusps (cusp formula 2B:4M:6L) and a high sectorial p4 with two triangular
lobes, five serrations associated with labial and lingual ridges, and a
unique distal cusp. Tagaria antiqua gen. et sp. nov. differs from all
other multituberculates by a very long P5 with flat crown and many cusps
arranged in three rows (4B:6M:7L). Morphologically the teeth of
Kermackodon and Megaconus are transitional between Euharamiyida and Middle
Jurassic Multituberculata. Kermackodon shares several characteristics with
Euharamiyida, such as pointed cusps on the molars connected by
longitudinal ridges, a distolabial cusp on the M2 that is higher than the
other cusps, and a single large sectorial premolar (p4) with a distal
basin. The euharamiyidan characters of Megaconus are an ultimate upper
premolar with a shorter lingual side, a p4 with a distal basin, and one
cusp on the molars being larger than the other cusps. In addition,
Megaconus shares an ultimate upper premolar with three rows of cusps,
horizontal wear on the molars, and pyramidal cusps on the molars separated
by transverse grooves with multituberculates. The multituberculate
characters of Kermackodon include horizontal rather than basined wear on
the surface of the molars and serrations on the p4.