Morphometric analyses of Aptian Assipetra infracretacea and Rucinolithus terebrodentarius nannoliths: implications for taxonomy, biostratigraphy and paleoceanography

Morphometric analyses of Aptian calcareous nannofossils, revealed the occurrence of differently sized specimens of Assipetra infracretacea and Rucinolithus terebrodentarius. The study was conducted on the Cismon core in the Tethys and DSDP Site 463 in the Pacific Ocean with good bio-, magneto- and chemostratigraphic control. Investigation was extended to selected samples from other low-latitude sections/cores to constrain the spatial and temporal distribution of the differently sized morphotypes. Normal- and large-sized specimens display the same ultrastructure, but are clearly separated when maximum diameter is measured. No transitional forms were observed. The large-sized specimens of Assipetra infracretacea and Rucinolithus terebrodentarius are here defined as new subspecies A. infracretacea larsonii and R. terebrodentarius youngii, respectively. Both taxa have FO close to the Barremian/Aptian boundary and LO close to the Aptian/Albian boundary in all investigated sections. They increase in abundance during the Early Aptian with maximum values during OAE la and then gradually decrease during the late Aptian. The distribution and abundance of A. infracretacea larsonii and R. terebrodentarius youngii are coeval with major Aptian igneous, geological, geochemical and biotic changes. These nannoliths might, therefore, result from nutrification events and/or physico-chemical reorganization of the mid-Cretaceous ocean. (C) 2002 Elsevier Science B.V. All rights reserved.