Peridinium limbatum (Stokes) verglichen mit der tertiären Deflandrea phosphoritica eisenack

Abstract

Title: Peridinium limbatum (Stokes) compared with the Tertiary Deflandrea phosphoritica Eisenack. This study is based on material from post-glacial lake sediments in a small lake, today surrounded by black spruce bog, in northeastern-Minnesota, U. S. A. These lake sediments have been subject of a pollen-analytical study (Fries 1962). The dinoflagellate shown in Fig. 1 A—D occurs abundantly in the upper layer (brown organic mud, rich in humic substances, Swedish dy) of about 1 m in the center of the lake (at about 2 m water depth) and in the sediment below a floating bog at the shore. Najas seeds do not occur in the layers with Peridinium, but farther down, and Pediastrum coenobia are here much more scattered than in the lower layers (partly claygyttja). The stratigraphic occurrence is shown in the pollen diagrams in Fries 1962. It has not been possible to verify that this microorganism now lives in the lake. However, its abundance in the uppermost layer makes it most likely that this is the case. According to its morphology the dinoflagellate belongs to Peridinium limbatum (Stokes) Lemm., but by several subgradiate differences it is considered a new subspecies, P. limbatum subsp. minnesotense. This dinoflagellate shows a striking resemblance to the widespread Tertiary species Deflandrea phosphoritica, the description of which is briefly given on p. 246. A close relationship between these species might be supposed, especially because the numerous specimens studied of P. limbatum subsp. minnesotense show a spherical interior body like the members of the fossil genus Deflandrea. The difference in tabulation (D. phosphoritica without tabulation; P. limbatum well tabulated) might be the result of evolution, and the interior body in the subspecies not mentioned in the description of the recent P. limbatum might be a relic of the capsules which are obviously an integral part of the shell of about 30 genera of fossil dinoflagellates, probably comparable with the capsule in the recent genus Gymnaster. The problems raised by this similarity are discussed. Their solutions are to be expected in the future by the study of similar deposits and, if possible, of the life cycle of the recent Peridinium limbatum.