A preliminary study of the evolution of Chiasmolithus in the middle Eocene to Oligocene of Sites 647 and 748

Abstract

In this preliminary biometric study of the calcareous nannofossil species Chiasmolithus expansus, Chiasmolithus oamaruensis, and Chiasmolithus altus from the upper middle Eocene to Lower Oligocene of Sites 647 and 748, a complete gradation of forms among all three species is documented. The Chiasmolithus population at each site changes from C. expansus to C. oamaruensis and then to C. altus. This may not reflect a true evolutionary sequence because a major reversal in shape change of the central cross-bar structure accompanies this sequence, and because C. altus is morphologically closer to C. expansus than it is to C. oamaruensis. At Site 748, two fluctuations in morphology produce sample populations intermediate between all three species. Paleoenvironmental fluctuations at Site 748 may be the cause of the morphologic fluctuations in Chiasmolithus. -from Authors