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Often grouped with the tintinnids (pelagic ciliate protozoans of the subclass Spirotheca), although modern tintinnids are organic-walled and calpionellids had calcareous walls. Thus, calpionellids are grouped by other workers as Protozoa incertae sedis.

Calpionellids — Late Jurassic (Tithonian) to Early Cretaceous (Valanginian; possibly into Albian)

Tintinnids — Jurassic-Recent (but with possible record extending into the Paleozoic, perhaps even to the Cambrian)

These open marine organisms are significant contributors to pelagic limestones and chalks in the Late Jurassic. Their distribution is largely restricted to the warm-water Tethyan region, within about 30-35° of the paleoequator.

All calpionellids apparently were composed of low-magnesium calcite; thus, generally well preserved. The TEM studies conducted by Fischer et al. (1967; cited at end of book’s introduction) showed that some calpionellids built two-layered tests in which the main layer incorporated carbonate detritus (including coccoliths) and was lined by an inner, secreted layer.

Small size (typically 45 to 150 μm in length and 30 to 90 μm in width), spherical to elongate, U- or V-shaped grains with a large opening rimmed, in some cases, by a narrowed, slightly thickened collar.

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