Paradigm changes in Paleogene larger foraminiferal biozonation

A. Oppel’s seminal work in the 19th century contributed to the rise and development of biostratigraphy by means of what have now become known as the Oppel zones. This powerful and flexible tool, first used for Jurassic-Cretaceous ammonoids, was later employed for establishing biozonal schemes by using different fossil groups. The historical development of Paleogene larger foraminiferal biozonations is a striking example of change of zonation paradigms in time. A historical overview of these changes and their rationale, starting from the 1850’s, shows the changing role of early biozonations, Oppel zones, phylozones, chronozones, and datums. A rigorous definition of Paleocene-Eocene Oppel zones based on alveolinids and nummulitids for the western Neo-Tethyan domain was mainly due to the influential work of L. Hottinger and H. Schaub in the 1960’s. In their taxonomic approach, species and subspecies were typologically defined. Their Oppel zones are based on stratigraphically ordered key localities and their assemblages; the resulting zonation is discontinuous, being separated by intervals of unknown extent and not by sharp boundaries, as in continuous zonations. The choice of these key localities from the Mediterranean Neo-Tethyan domain was influenced firstly by the presence of larger foraminiferal assemblages and their historical significance, inasmuch many are type localities for species group taxa. Secondly, their vertical ordination reflected basic criteria of geological superposition and correlation with calcareous planktonic and nannofossil zonations. Thirdly, the sedimentary settings of these key localities were mainly neritic (in part, also deeper flysch deposits with their allochthonous assemblages were included) and thus their fossil record was intrinsically discontinuous, i.e. linked to unconformity- bounded units. A different biometrical tradition in larger foraminiferal studies, which originated in the 1930’s especially in radial (orbitoidiform) larger foraminifera and Oligo-Miocene nummulitids, led to the construction of zonal schemes based on phylozones based by chrono(sub)species represented by arbitrary segments of evolutionary lineages. In contrast to Oppel zones, phylozones based on this approach produce continuous zonations. This research program achieved its most complete formulation with C.W. Drooger and coworkers, and later studies in orthophragmines by G. Less. A change in paradigm took place in the late 1990’s, when Serra-Kiel and coworkers attempted to reconcile in a modern research program Paleogene larger foraminiferal Oppel zones and phylozones and supersede the previous approach with chronozones, mainly based on the calibration of larger foraminiferal zones with magnetostratigraphy from the Iberian peninsula. The refinement of this research program is still ongoing, also drawing upon new correlations with the plankton stratigraphy, magnetostratigraphy, and isotopic data.