Cruciform dinoflagellate cysts from the Mid-Pleistocene Caspian Sea Bakunian Stage (MIS 21–11), and MIS 19–9 of the Black Sea and Gulf of Corinth: a species flock mechanism for morphological variation?

Dinoflagellate cysts (dinocysts) with wing-like ornament and/or cruciform endocysts are often used to characterise the salinity, timing and direction of Eurasian inter-basin biotic migrations. These dinocysts appear during the Late Miocene when the brackish-marine Paratethys extended from the Pannonian to Caspian basins, before basinal isolations during the Plio–Pleistocene. The oldest species, Galeacysta etrusca, is a Pannonian galeate taxon abundant during the Messinian Paratethys–Mediterranean connection, co-occurring with Spiniferites cruciformis. Both taxa occur sparsely in Caspian Basin Lower Pliocene sediments. Spiniferites cruciformis becomes common in the Caspian Sea and 15 other cruciform taxa appear after 5 Ma, reaching a Bakunian Stage acme. Studies of contemporary Black Sea dinocysts group diverse ‘Pontocaspian’ taxa into a morphogradational matrix or ecophenotypic series. Re-examination of types and new scanning electron microscopy images reveal these complexes include two or more gonyaulacalean subfamilies, suggesting mixed-species flocks. We emend the family Gonyaulacaceae to include dorsoventrally compressed cysts and show that this matrix includes gonyaulacoid and cribroperidinioid genera. Two new cruciform taxa, Pterocysta richardsii sp. nov. and Spiniferites bakunianus sp. nov., occur with Spiniferites cruciformis, Pterocysta cruciformis and cruciform Pyxidinopsis psilata during the Bakunian in the Caspian Sea. Eight formae of Spiniferites cruciformis are recognised, identities of Black Sea Dinocysts A–D and F of Hoyle (2019) are clarified, and we show that only Spiniferites cruciformis and cruciform Pyxidinopsis psilata occur in the Gulf of Corinth. We review evidence for salinity-driven ecophenotypic expression of cruciform shape, note its Eurasian endemism, and present a model accommodating a mixed-species-flock origin for cruciform cyst lineages.