A NUMERICALLY CALIBRATED REFERENCE LEVEL (MP28) FOR THE TERRESTRIAL MAMMAL-BASED BIOZONATION OF THE EUROPEAN UPPER OLIGOCENE

Abstract

The fauna of the Enspel (Westerwald) and the neighbouring Kärlich (Neuwied basin) fossil deposits correspond to the Upper Oligocene Mammal Paleogene (MP) reference level 28 and 28–30, respectively. Basaltic flows and a trachyte tuff terminating and predating the fossil deposit sedimentation allow to numerically calibrate the MP reference levels by radioisotope dating. Laser fusion 40Ar/39Ar step heating on volcanic feldspars yield a time interval of 24.9–24.5Ma for reference level MP28 at Enspel and a maximum age of 25.5Ma for the time interval MP28–MP30 at Kärlich. Interpolation between the time intervals determined for the Enspel reference level MP28 and the age of the global Oligocene/Miocene boundary of 24.0±0.1Ma taken from literature results in time intervals of 24.5–24.2Ma and 24.2–23.9Ma for the younger reference levels MP29 and MP30, respectively. These intervals of ≤0.4 m.y. for MP reference levels of the latest Oligocene are short relative to older Oligocene MP reference levels 21–27 between 34 and 25Ma. Since subdivision into MP reference levels essentially is based on assemblages of mammal taxa and on evolutionary changes in tooth morphology of mammals short MP time intervals during the latest Oligocene indicate a rapid evolutionary change relative to the early Oligocene.