Contributions Series Nr. 8

Description

Abstract

Samples from twenty-nine beds of the Middle Devonian Silica Formation yielded a diverse and well preserved organic-walled microphytoplankton flora. This assemblage consists of sixty-one species, eighteen of which are new. The new species include Cymatiosphaera acrochorda, Cymatiosphaera tryphera, Dictyotidium cohora, Diexallophasis simplex, Eisenackidium appendiculum, Estiastra rhytidoa, Exochoderma area, Exochoderma triangulata, Gorgonisphaeridium inflatum, Hapsidopalla chela, Hapsidopalla invenusta, Leiofusa pyrena, Lophosphaeridium ochthos, Muraticavea munificus, Polyedryxium ambitum, Oppilatala sparsa, and Uncinisphaera acantha. The genus Hapsidopalla is emended and two new combinations, Navifusa eisenacki and Navifusa minuta are made.

The distribution of organic-walled microphytoplankton within the section examined varies considerably. The lowest beds (1-6) of the Silica Formation, mainly massive bioclastic limestones had, quantitatively and qualitatively, the least diverse flora. The middle beds (7-18B), consisting mainly of calcareous shales and very argillaceous limestones, contained the most taxonomically diverse and quantitatively abundant assemblage. The flora from the upper beds (19-29), which are predominantly argillaceous limestones, was more diverse than that from the lower beds but less than that from the middle beds. The possible factors influencing the distribution of microphytoplankton are discussed.

The organic-walled microphytoplankton from the Silica Formation is most similar to those assemblages reported from the Middle Devonian (Givetian) of Canada, but common elements also occur in Africa, Europe, and South America. Close comparison of published data indicates some apparent endemism in Middle Devonian organic-walled microplankton floras.