Dinoflagellates near the Cretaceous Jurassic Boundary

Abstract

MANY well-preserved dinoflagellate fossils occur in Jurassic and Cretaceous marine sedimentary sections of various lithologic characteristics in the western North Atlantic and the western side of the Sacramento Valley of California^1,2. We have discovered five species which are common to the two sections—las first noticed by W. R. Evitt—each occurring in the Cretaceous above the Cretaceous/Jurassic boundary, and not in the subjacent Jurassic (Fig. 1). These species are Biorbifera johnewingi Habib, Dingodinium cerviculum Cookson and Eisenack, Scriniodinium attadalense Cookson and Eisenack, and two as yet unnamed species, Polysphaeridium sp. and Pyxidiella sp. Two of the five species, B. johnewingi and Polysphaeridium sp., were found to range through most of the Berriasian (lowermost stage of the Cretaceous System) at its type locality in southern France. B. johnewingi satisfies the requirements of a stratigraphically useful guide fossil especially well; it is morphologically distinctive, geographically widespread, relatively abundant in the three areas of investigation, and of limited stratigraphic range (Berriasian-middle Valanginian). Furthermore, it first appears very near the Cretaceous/Jurassic boundary at the three localities. A sixth species, Occisucysta? sp., appears to be discordant, being confined to the uppermost Jurassic in California and the lowermost Cretaceous in the North Atlantic.