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Pontiadinium – an endemic brackish-water dinoflagellate cyst genus from Lake Pannon and the Paratethys realm (Central Europe, late Miocene - early Pliocene)

The biota of the brackish-water Lake Pannon in the Pannonian Basin (Central Europe) is characterized by remarkable endemism due to its isolated evolution in the Central Paratethys realm for 8 myr. Lake Pannon was a large endorheic brackish-water lake during the late Miocene–early Pliocene that evolved isolated from the world ocean and the Mediterranean Sea after the last Miocene marine connection ceased ca. 11.6 Ma. A conspicuous feature of the endemism is the large, probably ecophenotypic morphological variability of dinoflagellate cysts as a response to brackish-water that challenges taxonomy and complicates biostratigraphy and ecological interpretations. Comparable dinoflagellate cyst taxa with morphological similarity to Lake Pannon cysts (including Pontiadinium) have been recorded from Pliocene-Pleistocene deposits of the Ponto-Caspian realm, but their classification and ecology remains largely unclear due to large intrageneric and intraspecific morphological plasticity and lack of laboratory experiments on living brackish-water Ponto-Caspian cysts. The Lake Pannon Pontiadinium is a proximate cyst with biconical outline and prominent apical and antapical horn-like protuberances. This cyst type was first - although invalidly - described from Romanian Lake Pannon deposits as Diconodinium inequicornutum Balteş, 1971. The genus Pontiadinium Stover & Evitt, 1978 was erected later. The taxonomic relations of these Lake Pannon horned cysts are highly debated starting with the questionable assignment of its species, Pontiadinium inequicornutum to the Mesozoic nontabulate genus Komewuia. Pontiadinium obesum and Pontiadinium pecsvaradensis have been questionably assigned to Impagidinium. These taxonomical issues are rooted mainly in the lack of widely accessible, high quality illustrations. Our new material from northern Croatia and Hungary provides the basis for the taxonomical re-evaluation of Pontiadinium. The new Lake Pannon material clearly shows that a gonyaulacacean paratabulation is indicated by sutural features on large populations of cysts. Atrophied expression of sutures can be explained by low salinity conditions. A new species of Pontiadinium is described as Pontiadinium szentaiae n. sp. that is characterized by unique sutural septa formed from a beaded tegillum supported by columellae or rod-like luxuriae. While the morphological features of Pontiadinium such as the characteristic antapical horn might have developed as response to subnormal salinities or to help flotation and anti-predation, simple ecophenotypic variability cannot resolve the morphology of the Pontiadinium species and other Lake Pannon dinoflagellate cysts. The various Pontiadinium species can co-occur in the same samples thus indicating that their morphological variability is inherent. The Lake Pannon assemblages including Pontiadinium cysts show-case how morphological adaption can lead to the evolutionary development of new dinoflagellate cyst species and genera due to long-term isolation over an extended amount of time.

Pontiadinium, endemic dinoflagellate, Lake Pannon, Miocene

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