Lake Pannon covered the area of the Pannonian Basin during the late Miocene. According
to the seismic profiles, prograding deltas from the NW and NE resulted in the S-SE
migration of the northern palaeoshoreline and the gradual aggradation of the lacustrine
basin. The molluscan fauna living in the lake underwent a very rapid evolution. For
the younger species, the possibilities of spreading became more and more restricted
due to the gradual shoaling of the lake. The bivalve species Congeria rhomboidea M.
HÖRNES occurs widely in the Upper Pannonian (Pontian sensu Stevanović) deposits of
Hungary and the neighbouring countries. Its evolution is relatively well understood.
According to magnetostratigraphic data this species appeared in the lake 8.5 mya.
According to the maximal geographical distributions of C. rhomboidea and its ancestor
Congeria praerhom - boidea STEVANOVIĆ occurring in sublittoral clay and silt along
with the representatives of Prosodacnomya coming from littoral and lagoon deposits
of the same age the estimated water coverage was around 75,000 km2 at the time of
first emergence of C. rhomboidea in the lake. In the north the distribution of C.
praerhomboidea is strictly restricted to the north of that of C. rhomboidea, its descendent,
implying a clear S-SE trend in the migration of the lake's northern palaeoshoreline.
Distributions of the littoral Prosodacnomyas in relation to the sublittoral C. rhomboidea
of the same age display a similar pattern. Meanwhile the western and southern palaeoshorelines
underwent only minor fluctuations.