Thanetian and early ypresian orthophragmines (foraminifera: discocyclinidae and orbitoclypeidae) from the central western tethys (turkey, italy and bulgaria) and their revised taxonomy and biostratigraphy

Abstract

The rich orthophragminid assemblages from the upper Thanetian and lower Ypresian of Turkey are discussed together with the coeval faunas from Spilecco (N Italy) and Beloslav (Bulgaria). Their taxonomy, evolution and biozonation in the Western Tethys are revised. Our biometric study is based mainly on a large number of equatorial sections of megalospheric individuals. We present the emended description of Discocyclina seunesi, D. tenuis, Orbitoclypeus multiplicatus, O. bayani and Asterocyclina taramellii. A new species, Nemkovella stockari is introduced. The evolutionary lineages of Discocyclina seunesi, Orbitoclypeus multiplicatus and O. bayani are restored for the first time by using the consistent size-increase of the megalospheric embryon that also allowed introducing some new subspecies (Discocyclina seunesi beloslavensis, D. s. karabuekensis, Orbitoclypeus multiplicatus kastamonuensis, O. bayani kurucasileensis and O. munieri ponticus). By owing the most complete record of Thanetian and early Ypresian orthophragmines from the Western Tethys (using also data from SW France and the Crimean Peninsula) we could reconstruct their early evolution. The chronostratigraphical position of some localities was ascertained from planktic and larger benthic foraminifera, as well as calcareous nannoplankton. In the updated orthophragminid zonation (zones are marked by OZ), OZ 1a corresponds to the early Thanetian, OZ 1b to the middle Thanetian. They are distinguished on the base of the evolution of Discocyclina seunesi. In these zones, only two unribbed species of Discocyclina and Orbitoclypeus each are present. Ribbed Orbitoclypeus, genus Asterocyclina and Nemkovella appeared in the redefined OZ 2 zone belonging to the late Thanetian. Discocyclina archiaci and D. dispansa substituted D. seunesi at about the Paleocene/Eocene boundary. The early Ypresian can be subdivided into the OZ 3 and 4 zones that can be distinguished from each other by the different developmental stages of the simultaneously running evolutionary lineages such as Discocyclina archiaci, Orbitoclypeus schopeni and O. multiplicatus.Rivista italiana di Paleontologia e Stratigrafia, Vol 113, No 3