Ebridians

The ebridians are unicellular, marine and planktonic with an endoskeleton of silica, but unlike that of the similar silicoflagellates, they are solid with a tetraxial or triaxial symmetry. Ebridians possess two flagella of unequal length and lack photosynthetic pigments, surviving instead by the ingestion of food (especially diatoms) with the aid of pseudopodia. Reproduction is mostly by asexual division.

Classification of ebridians is complicated by their uncertain biological status, resembling algal groups such as the silicoflagellates and dinoflagellates as much as animal groups like radiolarians. Generally regarded as algae they are placed by some in the division Chrysophyta and by others in the Pyrrhophyta as a distinct class, the Ebriophyceae.

Genera and species are distinguished on the basis of endoskeleton morphology (see Loeblich et al. 1968). For example, Ebria (Mioc.-Rec., Fig. 10.10c) has three

Two unequal flagella

Nucleus

Protoplasm Skeleton

Fig. 10.10 Ebridians. (a) A living cell and skeleton of Hermesinium. (b) Hermesinum skeleton, X500. (c) Ebria skeleton, X533. ((a) Based on Hovasse 1934.)

Nucleus

Protoplasm Skeleton

Two unequal flagella

or four radiating bars (actines) with the ends joined by curved hoops called hafts. Hermesinum (Palaeoc.-Rec., Fig. 10.10a,b) consists essentially of four actines resembling a sponge spicule in their tetraxial arrangement, the ends of which are joined by a series of sub-circular hoops.

Ebridians are known in rocks of Palaeocene age, the majority of genera thriving until the Pliocene when their diversity dropped sharply (Tappan & Loeblich 1972). The geological value of ebridians has been little exploited as yet, largely because they are neither abundant nor uniformly distributed and preserved. None the less, they have been used successfully with silicoflagellates in Cenozoic biozonal schemes, such as those in the north Pacific area (see Ling 1972, 1975). A comprehensive review can be found in Ernisse (in Lipps 1993, pp. 131-141).

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