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Form and Variation of the Dinoflagellate Sirmiodinium grossi Alberti, from the Upper Jurassic and Lower Cretaceous of California

John S. Warren
Journal of Paleontology
Vol. 47, No. 1 (Jan., 1973), pp. 101-114
Stable URL: http://www.jstor.org/stable/1302871
Page Count: 17
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Form and Variation of the Dinoflagellate Sirmiodinium grossi Alberti, from the Upper Jurassic and Lower Cretaceous of California
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Abstract

Previously known only from rare occurrences of middle Lower Cretaceous (Hauterivian and Barremian) age, the dinoflagellate Sirmiodinium grossi Alberti, 1961, was found to be abundant and long-ranging (uppermost Jurassic to middle Lower Cretaceous) in California. Cysts of S. grossi consist of two walls, in contact on the flattened dorsal and ventral surfaces, separated around the margins of these surfaces. In the zone of separation, the periphragm is perforated by holes of various size, shape, and distribution. Tabulation is distinctly gonyaulacacean, with a plate formula of 4′, 6″, 6c, 6″′, 1p, 1″″, plus five sulcal plates; plate outlines are reflected by low sutural ridges. The archeopyle is a combination type, with a compound operculum consisting of two pieces; one corresponds to the apical plates, and the other corresponds to the third precingular plate. Usually, both opercular pieces are attached, the apical piece on the ventral side at the anterior edges of reflected plate 6″ and the reflected sulcus, and the precingular piece on the dorsal side at the reflected cingulum. Occasionally, the apical piece is free and the precingular piece is closed. Cyst shape is highly variable, completely intergrading from pentagonal to trilobate. Several reversals of trends occur between end-members of this series, indicating that the variations in shape do not represent evolutionary trends. The presence of reflected striate growth bands of varying width shows that differences in shape were produced through differential thecal growth, and were preserved because individuals could encyst after any amount of growth, or even before any growth had occurred. The amount of growth which preceded encystment was apparently influenced by non-seasonal, recurring fluctuations of environmental factors. The intraspecific variations in archeopyle structure and cyst shape suggest that perhaps other fossil dinoflagellate species are too narrowly construed.

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