The history of the Paratethys and its inhabiting organisms was profoundly
influenced by its connections with the Global Ocean. As the sea-straits
linking Paratethys with the ocean expanded, they integrated the Paratethys
with the global ocean, causing its water level to mirror the global eustatic
fluctuations. These intervals of increased ocean connectivity existed during
most part of the Oligocene-Early Miocene, in the Early Badenian-Tarkhanian,
and in the Late Badenian-Konkian, when also fostered favourable environments
for marine life to flourish in the Paratethys. When the marine connections
became unstable and the sea-straits closed (e.g., in the second half of
Ruppelian-Solenovian time, late Ottnangian-Kotzakhurian, middle
Badenian-Karaganian and from the onset of the Sarmatian up to the Pliocene),
ransformed into vast lacustrine waterbodies where water levels and
salinities were controlled by the balance between river runoff and
precipitation versus evaporation. Excess of water led to the basin expansion
and brackish conditions, while deficits caused contraction and
fragmentation. This resulted in hypersaline basins and lagoons co-existing
with freshwater lakes, creating diverse environments and faunas that
hindered effective biostratigraphic correlations. These fluctuations caused
the decline of the marine biota, leaving behind only euribiontic organisms
capable of enduring drastic shifts in salinity, ion composition, and oxygen
levels. Over time, these survivors diversified and gave rise to endemic
faunal communities adapted to the brackish environments. The history of the
faunistic and phytoplankton composition of the Carpathian part of the basin
(Central Paratethys) became noticeably different from the Euxine-Caspian
ones from the beginning of the Neogene, which led to different stratigraphic
schemes of these parts of the Paratethys. In recent decades, our focus has
centred on refining our understanding of the Eastern Paratethys
paleogeography. While the periods of high base levels in the Eastern
Paratethys are well-documented, the significant base-level drops during
isolation phases have often been overlooked. This was primarily because,
until recently, there were no effective methods for reconstructing the
paleogeography and geochronology of Paratethys during these episodes of
base-level drops. However, with the advancements in seismic stratigraphy and
the utilization of integrated magneto-bio-stratigraphy, we now possess the
means to explore the scale and spatial distribution of paleogeographic
changes, particularly during the substantial basin reductions in partial
desiccation periods. Large regressions (during second part of the Ruppelian,
terminal Tarkhanian and Sarmatian in the Eastern Paratethys and during early
Pliocene in the Caspian basin) erode parts of sedimentary layers, forming
surfaces of inconsistency between geological bodies accumulated before and
after the sea-level drop, which can be traced on seismic profiles as erosion
boundaries. By deciphering such surfaces and the depth of river incisions
flowing into the basin, it is now possible to reconstruct the relief picture
resulting from these erosions and quantify the depth of baselevel drops in
the Paratethys during the largest regressions.