Morphometrical variability of Helicosphaera ampliaperta (calcareous nannoplankton) in the Central Paratethys: biostratigraphic and paleogeographic applications

Abstract

Biometrical variability of the calcareous nannofossil species Helicosphaera ampliaperta was evaluated for 1021 helicoliths from the Early Miocene of the Central Paratethys. Morphometrical changes of helicoliths expressed by their length and width of central opening are gradual. Three morphogroups can be separated: (1) small helicoliths ( 10 μm) at the level of the first common occurrence of Helicosphaera ampliaperta in the later Eggenburgian. The new morphotypes migrated to the Central Paratethys from the Mediterranean via the Pre-Alpine passage in two steps. The first occurrence of Helicosphaera ampliaperta (event 1) may be connected with the transgression of Aq3/Bur1 global sea-level cycle and the first occurrence of large Helicosphaera ampliaperta (event 2) with the base of the Bur 2 cycle. Biometrical changes in the Ottnangian/ Karpatian boundary interval (late Burdigalian) were realized also in two steps: (3) disappearance of small morphotypes at the level of the FAD of the endemic bivalve Rzehakia; (4) appearance of morphotypes with a very wide central opening (> 5 μm) at the level of the FAD of the endemic benthic foraminifera Uvigerina graciliformis (base of the Karpatian). These events were recorded after the opening of a new connection between the Mediterranean and the Central Paratethys via the “Trans-Tethyan Trench Corridor”. The heterochronal transgression of the local sea-level cycle CPC 2 can be approximately correlated with the global sea-level cycle Bur 4. (5) The last occurrence of Helicosphaera ampliaperta in the Early Badenian (Langhian). The Helicosphaera ampliaperta biometrical events were initiated by palaeogeographic changes in the Central Paratethys and are therefore of regional significance.