Morphotypic characteristics of the second upper molar (M2) of the brown (<i>Ursus arctos</i>) and polar (<i>Ursus maritimus</i>) bears (carnivora, ursidae)

The structure of the second upper molar (M2) of brown (Ursus arctos) and white (U. maritimus) bears was studied, with 9 polymorphic characters identified, and their descriptions given. A west to east geographic pattern was discovered in the variability of the lingual cingulum M2 in continental U. arctos. In island populations of U. arctos, there is likewise a decrease in talon M2 observed from west to east. In general, M2 in brown and polar bears is a rather variable tooth. The brown bear can be reliably differentiated from the polar bear based on the structure of M2 based on 6 characters. Changes in the structure of M2 in U. maritimus is associated with the transition to myophagy. The tooth is reduced, first of all losing its secondary elements and reducing the pressing surface due to the talon. Utsus maritimus has the largest number of progressive features among ancient and modern members of the genus Ursus. In evolutionary terms, the variability of M2 in members of the genus appears to be quite high, reflecting the main phylogenetic directions.