Abstract
Amphibetulimus krasnolutskii is known from the Middle Jurassic (Bathonian) Itat Formation of Krasnoyarsk Territory, West Siberia, Russia, by several edentulous and three dentigerous dental fragments, preserving p1, antepenultimate, and ultimate lower molars, and by an upper molar. It is unique among stem therians by widely open trigonids on the posterior lower molars, paraconids that are higher than the metaconids and have keeled mesiolingual vertical crests, pronounced unilateral hypsodonty of the lower molars and correlated unequal alveolar borders of the dentary ramus, and a linear Meckelian groove that is not connected to the mandibular foramen and extends along the pterygoid ridge. Amphibetulimus differs from more derived stem therians by a simple unicuspid talonid without an incipient talonid basin and a distinct labial cingulum on the upper molars. The lack of an ectotympanic facet and the long linear Meckelian groove extending onto the pterygoid ridge suggest that Amphibetulimus had a derived state of the transitional mammalian middle ear, where the ear ossicles were connected to the dentary not by a thick “ossified” Meckelian cartilage, but by a thin Meckelian cartilage, as in prenatal and early postnatal stages of some modern therians.
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