Abstract
The morphology, stratigraphic ranges, and occasional intergradation of four dinoflagellate cyst species from the Barton Beds of the Hampshire Basin indicate that they represent an evolutionary series. The species are Areosphaeridium multicornutum Eaton, Areosphaeridium fenestratum n. sp., Cyclonephelium textum n. sp., and Cyclonephelium microfenestratum n. sp. It is proposed that A. multicornutum evolved into A. fenestratum by the development of distal process platforms, and that subsequent process branching on A. fenestratum produced cysts with process complexes, assigned to C. textum. Less conclusive evidence suggests that C. microfenestratum evolved from C. textum by increase in the process number and complexity of the distal process platforms. The proposed evolutionary series is a continuation of the lineage described by Eaton, which traced the development of Areosphaeridium diktyoplokus (Klumpp) Eaton, Areosphaeridium arcuatum Eaton, A. multicornutum, and Cyclonephelium intricatum Eaton from Membranilarnacia ursulae (Morgenroth) Gocht in the Bracklesham Beds of the Hampshire Basin.
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