Blowinʹ in the wind… 100 Ma old multi-staged dinoflagellate with sexual fusion trapped in amber: Marine–freshwater transition

Here we report the unexpected discovery of multi-staged dinoflagellates with organic cellular contents from a paralic habitat trapped in 100 million year (100 Ma) old amber. Amber formed from resin, a fluid medium, that trapped, then remarkably embedded and preserved soft parts of organisms usually destroyed by fossilization processes. We assume that the marine-costal dinoflagellates reached the sticky resin carried away by the wind, inside spray droplets. We answer to a fundamental question dealing with the Peridiniaceae: the paratabulation of Cretaceous cysts reflects the tabulation of Cretaceous thecae. We provide the first life cycle of the fossil record with evidence of cellulosic thecae, sexual fusion and zygote. We highlight an ancestral behaviour for the sexual phase: naked gametes complete fusion outside of gametic thecae, a process known in rare extant Peridinium species. The new taxon, Succiniperidinium inopinatum gen. et sp. nov., belongs to the Peridiniaceae and shares characters with extant marine-brackish Scrippsiella and two freshwater Peridinium clades identified by morphological characters (tabulation, cingular plates, ecdysis, plasmogamy) and molecular phylogenies. Understanding the processes of marine–freshwater transition in microbial lineages is a central goal in evolutionary ecology. Marine dinoflagellates passed through the osmotic barrier and the studies of marine-costal Cretaceous species help to understand the adaptation and the diversification of these species. We discuss the migration according to biological, paleontological and molecular phylogenies data and suggest that the freshwater colonization was the result of Cretaceous species adapted to costal habitats; their lineage isolated in ponds following the Cenozoic global sea-level fall passed through the osmotic barrier. A Scrippsiella-like group (i.e. Subtilisphaera terrula, Palaeoperidinium cretaceum and Succiniperidinium inopinatum gen. et sp. nov.) is suggested as intermediate species in the Peridinium freshwater colonization line.