Revision of the endemic dinoflagellate cyst genus <i>Pontiadinium</i> Stover &amp; Evitt, 1978 from Lake Pannon and the Paratethys realm (Late Miocene–Early Pliocene, Central Europe)

The biota of the brackish-water Lake Pannon in the Pannonian Basin is characterised by remarkable endemism due to the isolated evolution of the lake for 8 myr after the last Miocene marine connection ceased (∼11.6 Ma). A conspicuous feature of this endemism is the large, probably ecophenotypic variation in the morphology of brackish-water dinoflagellate cysts that challenges taxonomy and complicates biostratigraphical and ecological interpretations. We conclude that a widely debated Lake Pannon genus, <i>Pontiadinium</i>, includes several proximate dinoflagellate cyst species with prominent apical and antapical protuberances, and we show how the genus differs from the morphologically similar gonyaulacacean cyst genera <i>Impagidinium</i>, <i>Leptodinium</i>, <i>Cribroperidinium</i> and <i>Komewuia</i>. The generic description of <i>Pontiadinium</i> is emended together with the species descriptions of <i>Pontiadinium inequicornutum</i>, <i>Pontiadinium obesum</i> and <i>Pontiadinium pecsvaradanesis</i>. A new species is described as <i>Pontiadinium szentaiae</i> sp. nov. from Našice (northern Croatia) that is characterised by unique trabeculate sutural septa formed from a beaded tegillum supported by columellae or rod-like luxuriae. The dinoflagellate cyst assemblages of the long-lived brackish-water Lake Pannon clearly demonstrate that dinoflagellate cysts in low-salinity, isolated epicontinental seas display greater morphological plasticity than their normal-marine relatives. The development of an antapical horn appears to be a previously undocumented example of phenotypic morphological features that developed in response to subnormal salinities within at least two dinoflagellate cyst genera endemic to Lake Pannon and the Post-Paratethyan seas of the Ponto-Caspian realm. This ecophenotypic variation resulted in a higher level of morphological adaptation, leading to the evolutionary development of new dinoflagellate cyst species and genera.