The evolution and palaeobiogeography of mesozoic planktonic foraminifera

Abstract

In the 1960s Oberhauser and Fuchs (palaeontologists working at the Geologische Bundesanstalt in Vienna) described a range of new taxa from the Triassic of Austria that were thought to be the earliest planktonic foraminifera. The first reactions of the palaeontological community were negative but in the subsequent forty years our knowledge of Jurassic planktonic foraminifera has expanded considerably. A thorough re-evaluation of the Oberhauser and Fuchs collections in Vienna has shown that these species are probably not planktonic and that the first planktonic taxa appeared in the Toarcian. This origination in the centre of Western Tethys was followed by a rapid expansion of planktonic foraminifera throughout Peri-Tethys. This expansion is dominated by the genera Conoglobigerina and Globuligerina and while some believe that their separation is straightforward (based on apertural characters) analysis of large assemblages shows that this differentiation is not reliable and requires further analysis not only of holotypes, paratypes and topotypes but of large assemblages. In Southern Poland, Middle Jurassic limestones in the Pieniny Klippen Belt are described as foraminiferal packstones and represent the first evidence of a foraminiferal ooze on the ocean floor. This indicates that, by the mid-Jurassic, there was an oceanic stratification of the Aragonite and Carbonate Compensation Depths and that the modem ocean system had developed, although the depths of these various layers may have been different to those of the present day. By the Oxfordian a relatively diverse planktonic fauna had expanded throughout Peri- Tethys and, probably, around the globe in the tropics. The fauna expanded further in the early Cretaceous as Gondwana fragmented but data across the important Jurassic to Cretaceous transition is extremely limited and requires further investigation.