Evolutionary trends in the fourth lower premolar of Ursus rossicus Borissiak, 1930: Molarisation of p4 in spelaeoid bears
Abstract
A detailed study was conducted of the fourth lower premolar (p4) of Ursus rossicus from several localities, representing different temporal stages in the evolution of the Eurasian Pleistocene biota. Distinctive features were identified of p4 molarisation at various stages of the evolution of small cave bears. The lower fourth premolar U. rossicus nordostensis from the Pleistocene of Arctic Siberia shares with U. rossicus s. l. the derived morphology of the lingual trigonid cusps, but retains the basal deningeroid trait, such as the narrowed distal contour of the crown of p4. By the end of the Middle Pleistocene, p4 of U. rossicus exhibited a maximum molarisation, displaying a multicuspidate pattern in both the trigonid and talonid parts, as observed in the type series from the Krasnodar locality and additional material from the Irgiz 1 locality. At the same time, the Late Pleistocene set of p4 from the Imanay locality, while having most of the U. rossicus characteristics, displayed a wide spectrum of morphotypes, ranging from simple to a multicuspidate one. Additionally, it was revealed that molarisation of p4 in cave bears followed a different evolutionary trajectory. In U. rossicus, there was a complication on both trigonid and talonid. In contrast, in large cave bears, mostly the trigonid part of p4 became complicated, while the talonid remained relatively short, lacking most additional cusps.