Umbriadinium and Polarella: an example of selectivity in the dinoflagellate fossil record

Abstract

The extant Antarctic dinoflagellate genus Polarella and the southern European Early Jurassic dinoflagellate cyst Umbriadinium are extremely similar in morphology, particularly in their size, ornamentation and tabulation. Polarella is therefore placed in the subfamily Umbriadinioideae on this morphological evidence. The two genera, however, are maintained as separate entities for several reasons including minor differences in tabulation. This means that the stratigraphical distribution of the subfamily Umbriadinioideae is extended from the Early Jurassic (late Pliensbachian - early Toarcian) to Recent. The two species (Polarella glacialis and Umbriadinium mediterraneense) are separated by around 187 Ma. This large stratigraphical gap is an example of the selectivity of the dinoflagellate fossil record, produced by the loss of the capacity of Polarella/Umbriadinium to produce fossilisable cysts during the early Toarcian. The widely differing records of these genera attests to their longevity and wide geographical and ecological ranges.