Dinoflagellate cysts, Acritarchs and Tasmanitids from the uppermost Callovian of England and Scotland: With a reconsideration of the “xanthidium pilosum” problem

Abstract

Four assemblages of fossil organic-walled microphytoplankton from the Upper Callovian (Lamberti Zone), three from England and one from Scotland, are described. They comprise at least 50 species of dinoflagellate cysts, 18 species of acritarchs and 2 species of tasmanitids. After a review of the « Xanthidium pilosum problem, a revised diagnosis is proposed for Sentusidinium pilosum ( Ehrenberg) and three new species, Sentusidinium creberbarbatum, S. sparsibarbatum and S. erythrocomum are proposed. The new family name Scriniodiniaceae is substituted for the now invalid family name Endoscriniaceae. The new genus Escharisphaeridia is proposed to contain two existing species, E. pococki ( Sarjeant) and E. dictydia ( Sarjeant). The present content of the genus Compositosphaeridium is examined; the new combination C. polonicum ( Górka) is proposed and a new species, C. bulgaricum, is erected. One further new species of dinoflagellate cyst, Prolixosphaeridium anasillum, is proposed. The new term «peniplate is proposed for paraplates whose boundaries are penitabular in situation. One new species of acritarch, Solisphaeridium?rhachos, is described and the new combination Polygonium aster ( Sarjeant) is proposed. Evidence for reworking of some of the species of Micrhystridium present in the Hackness Rock assemblage is noted, though it is considered that the majority of species in that assemblage are indigenous. It is considered that all four assemblages are derived from a single phytoplankton province, the differences between them resulting from environmental factors such as water depth or distance from a shoreline.