Climate changes in the Early to Middle Miocene of the Central Paratethys and the origin of its echinoderm fauna

Abstract

This study uses echinoderm diversity to assess climatic changes in the Early to Middle Miocene of the Central Paratethys. It is based mainly on a taxonomically standardised dataset of Central Paratethys echinoids, supplemented by data on asteroid, ophiuroid, crinoid, and holothuroid distributions. Potential constraints of the raw diversity data are explored and possible biases are identified. These include a heterogeneous rock record (in terms of outcrop area and facies of the preserved sedimentary rocks), stage duration and sampling bias. The latter was overcome by utilising rarefaction analysis to investigate species richness. These analyses show that the patterns observed in the raw data are robust and not severely biased by sampling intensity. The potential influence of a heterogeneous rock record was assessed by comparing surface outcrop areas of the various stages to standing diversity. A second bias involving a heterogeneous rock record, the non-preservation of distinct habitats, was overcome by carrying out within-habitat comparison of standing diversity and faunal composition.The observed patterns were therefore established as ‘true’ signals rather than artefacts produced by various biases. Consequently, echinoderm diversity is used to evaluate climate changes during the Neogene of the Central Paratethys, as well as to interpret minimum winter and summer sea surface temperatures in the Badenian (Middle Miocene). Climatic conditions during the deposition of the Badenian Leitha limestone were addressed along with the presence of N–S gradients within the Central Paratethys. The presently available data suggest warm temperate conditions for the Late Eggenburgian (Early Burdigalian) followed by a brief temperature drop in the Ottnangian (early Late Burdigalian). Temperatures increased again in the Karpatian and a climatic optimum was reached in the Early Badenian. A slight cooling during the Middle Badenian was reflected by the southwards shift of predominantly temperate genera from the northern fringes of the Central Paratethys in the Early Badenian to the central parts in the Late Badenian.The faunal gradients observed in the Early Badenian of the Central Paratethys suggest that the boundary between the warm temperate and tropical zone lay somewhere within the Paratethys at that time. Considering the more southerly position of the study area (about 34° palaeolatitude) in the early Middle Miocene, this implies that the tropical climate zone was much wider during the early Middle Miocene than today.Biogeographic investigations of the Central Paratethyan echinoid faunas show that the overwhelming part of the Central Paratethys echinoid species are immigrants from the Mediterranean area. Immigration occurred in three distinct waves during the Late Eggenburgian, the Karpatian, and Early Badenian. While no common echinoid species were identified between the Boreal region and the Central Paratethys, two species of asteroids from the northern-most fringes of the Central Paratethys also occur in the North Sea Basin. Endemism is low in most of the stages, except for the Ottnangian, when geodynamic processes and global cooling resulted in a large-scale re-organisation of Central Paratethyan faunas.