Trithyrodinium evittii Drugg 1967 and T. fragile Davey 1969 an artificially split of one dinoflagellate cyst species–Stratigraphic and palaeoenvironmental importance

Based on morphological observations carried out on the same material, but submitted to different processing, Trithyrodinium fragile Davey 1969 is herein demonstrated to be a junior synonym of Trithyrodinium evittii Drugg 1967. From both original descriptions, the only way that the two species can be separated is by the presence or the absence of a thick brown organic layer on the surface of the endocyst. The present study proves that this is an artificial separation caused by different processing techniques. As a result, T. evittii is emended in order to encompass specimens, wholly or partially exhibiting this brown organic layer and previously attributed to T. fragile.

These new observations also have stratigraphic and palaeoenvironmental implications. Combination of both synonym lists reveals that T. evittii, which is regarded as a warm water species (58°N to 52°S) in the latest Cretaceous, migrated to higher latitudes (80°N to 75°S) in the earliest Paleocene. Consequently, an increase in oceanic circulation and/or temperature increase of the surface water is suggested during this epoch. In support of this contention, several peak occurrences of T. evittii are recorded just above the K/T boundary in high latitudes.