Three Species of Coscinodiscus Ehrenberg from North Pacific Sediments Examined in the Light and Scanning Electron Microscopes

Abstract

Specimens of Coscinodiscus marginatus, C. oculus-iridis, and C. radiatus from Plio-Pleistocene North Pacific sediments were examined in the light microscope and scanning electron microscope, then compared with descriptions and illustrations of various taxa in the literature. C. marginatus and C oculus-iridis belong to the group of species characterized by a ring of labiate processes on the mantle, with two larger processes broadened and curled. C. radiatus, in contrast, possesses labiate processes upon the valve face as well as in a mantle ring, and the two larger processes have the same shape as others in the ring. C. marginatus is distinguished from C. oculus-iridis by the possession of a broad hyaline band projecting inward from the valve edge, which looks like a striate margin in light microscopy. The three taxa are otherwise very similar. Based on descriptions and figures in the literature, it is suggested that specimens of C. marginatus have occasionally been misidentified as Endictya species; C. oculus-iridis may be a synonym of C. argus, and C. obscurus and C perforatus may be junior synonyms of C. radiatus. In modem North Pacific sediments, C. marginatus and C. oculus-iridis are characteristic of the subarctic zone, while C radiatus is restricted to the subtropical and transitional zones. Core records spanning the last 2.6 m.y. (Late Pliocene to Recent) indicate that C. marginatus was abundant prior to about 2.4 Ma, and decreased markedly in relative abundance at that time; in the Pleistocene it has only been common during peak interglacial intervals. C. radiatus attained its greatest relative abundance in subtropical waters during the late Quaternary (since 0.7 Ma). C. oculus-iridis has always been a minor component of the sediment flora.