Middle to Upper Miocene Geonettia clineae, an Opportunistic Coastal Embayment Dinoflagellate of the Homotryblium Complex

Abstract

Analysis of upper Miocene open bay fine sandy marls from the Windmill Point beds in Maryland yielded moderately diverse dinocyst assemblages dominated by a single quinqueform species. The new species, Geonettia clineae gen. et sp. nov., is characterized by a resting cyst having fields that all routinely dissociate, revealing a tabulation most similar to fossil Eocladopyxis peniculata Morgenroth 1966 and living Pyrodinium bahamense Plate 1906. Sufficient differences exist in the ventral epitabulation, style of excystment and the wall structure, to warrant separation at the generic level. Geonettia clineae has a known stratigraphic range from upper middle Miocene through upper upper Miocene in the western north Atlantic. Facies analysis and abundance patterns indicate that Geonettia clineae thrived in mesotrophic open embayments having low sediment influx. Species of Eocladopyxis Morgenroth 1966, Geonettia gen. nov., Homotryblium Davey and Williams 1966 and Pyrodinium Plate 1906, often occur as dominants in moderate to low diversity dinocyst assemblages, typically in unstable coastal environments. This trend suggests an opportunistic, bloom-forming, r-selected ecology for these pyrodinioid genera.