The Miocene Climatic Optimum: evidence from ectothermic vertebrates of Central Europe

Data sets of Central European temporal distributions of thermophilic ectothermic vertebrates (Channidae, Varanidae, Chamaeleonidae, Cordylidae, Tomistomidae, Alligatoridae, giant turtles) and of North Alpine Foreland Basin (NAFB) distributions of ectothermic vertebrates adapted to dryer habitats (Albanerpeton inexpectatum, Salamandra sansaniensis, Bufo cf. viridis, Chamaeleo caroliquarti, Gekkonidae, Varanus hofmanni, Bransateryx sp.) are analysed. Two main migration events of thermophilic ectotherms at 20 Ma and 18 Ma in the Lower Miocene are discerned. They indicate the beginning of the Miocene Climatic Optimum in Central Europe (42–45°N palaeolatitude) with a lower limit of the mean annual temperature (MAT) of 17.4°C derived from the minimal MAT of their extant relatives. Furthermore, additional palaeobotanical data and records of bauxite point to a MAT of 22°C. This warm and humid optimum peaked at 18–16.5 Ma (Ottnangian, Karpatian), and is confirmed by the coexistence of all investigated thermophilic taxa. The following period (Early Badenian) is characterised by probably unchanged temperatures but a seasonality in precipitation with dry periods up to six months. Two major seasonal phases between 16.3 and ∼15.7 Ma (earliest Early Badenian) and between 14.7 and ∼14.5 Ma (Early/Middle Badenian transition) are indicated by an immigration of dry adapted taxa from the surrounding karst plateau to the NAFB. It is presumed that the tectonical reorganisation of the Central Paratethys realm had considerable influence on this regional humidity pattern. The warm period ended abruptly between 14.0 and 13.5 Ma (Middle/Late Badenian transition) with major regional extinction events of most of the thermophilic groups in Central Europe and a drop of the MAT of probably more than 7°C to temperatures around 14.8–15.7°C. This drop can be attributed predominantly to a decrease of more than 11°C of the minimum cold months temperature. This temperature decrease marked the beginning of a climatic zonation of the European continent and is also evidenced by a progressively southward disappearance of the crocodile Diplocynodon from 38–45°N palaeolatitude to 30–37°N during the Middle and earliest Late Miocene. The results correlate well with palaeobotanical data from the mid-latitudes of Europe and North America, and the deep-sea temperature curve generated from oxygen isotope ratios.