Characteristic assemblages and morphologies of benthic foraminifera from anoxic, organic-rich deposits; Jurassic through Holocene

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Abstract
Benthic foraminiferal assemblages from selected Early Jurassic through Holocene organic-rich deposits were studied, emphasizing external morphologic characteristics. Oxygenated conditions yield assemblages predominated by spherical, biumbilicate or lenticular forms. Morphologies of anoxic assemblages are predominantly flattened forms, which may predominate because of high surface area-to-volume ratios. These morphologies may enhance oxygen acquisition or buoyancy. Mesozoic shale assemblages are predominantly small, flattened, planispiral forms, and Cenozoic laminated diatomite assemblages are predominantly elongate, flattened forms of small and medium size. These morphologic differences may be related to sedimentologic differences or evolutionary trends. Test size may be indirectly related to the extent of anoxia. I infer that localized anoxia produces specimens of varied size and widespread anoxia generally yields assemblages of small specimens.

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