Morphological variations among species of the fossil dinoflagellate genus Gonyaulacysta

Abstract

Species assigned to the widely known genus Gonyaulacysta are typically bicavate with an apical horn, parasutural crests, and a precingular archeopyle. Closely related genera are currently distinguished by variations in the degree of cavation, and shape and size of the parasutural crests. Based on details of morphology and paratabulation, three morphologic groups can be distinguished within species now assigned to Gonyaulacysta, and they are here assigned to three genera: Gonyaulacysta emend., distinguished by well‐developed apical horn and cavation, elliptical cingular cross‐section, and a paratabulation pattern including the contacts 1u/1i, A/1i, ai/II, and the presence of accessory K intercalary paraplates. Forms included in this genus as emended here have been described only from Jurassic rocks. Stanfordella gen. nov., distinguished by weakly developed apical horn and cavation, circular cingular cross‐section, and a paratabulation pattern including the contacts 1u/1i, 1u/6, au/Iu, and absence of accessory K intercalary paraplates. Forms included in this new genus have been described from Tithonian to Albian rocks. Wrevittia gen. nov., distinguished by well‐developed apical horn, weakly developed cavation, circular cingular cross‐section, and paratabulation pattern including the contacts A/ai, lu/ai, au/ lu, and variable presence or absence of accessory K intercalary paraplates. Forms included in this new genus have been described from Berriasian to Cenomanian rocks. The paratabulation pattern of this genus is commonly found in Tertiary forms, and in the living species Gonyaulax spinifera. The separation of these three genera points out the morphological similarities and differences between the species treated here, and should also help to biostratigraphically resolve Middle Jurassic to Cretaceous sedimentary sequences. New taxa include: Stanfordella granulosa gen. nov., sp. nov., S. exsanguia comb, nov., S. fastigiata comb, nov.; S. ordocava comb, nov., Wrevittia gen. nov., W. helicoidea comb, nov., W. cassidata comb, nov., W.? diutina comb, nov., and W. ? perforobtusa comb nov.