Planktic Foraminifera are an extremely abundant, important and successful group of marine protists. They are particularly useful in reconstructing past environments and for biostratigraphic dating. Despite their importance, the origin of the group is uncertain. Previous work has suggested that they evolved from a benthic ancestor during the Triassic or, perhaps, the Mid-Jurassic (?Bajocian), but a reason for their origination has remained unclear. Here, we present evidence from the Toarcian (early Jurassic) of NW Europe that the origin of the planktic Foraminifera may have been one of the results of the early Toarcian oceanic anoxic event. This event appears to have been associated with a massive dissociation of gas hydrates and other, perhaps related, water chemistry changes.

Abstract. ‘Globigerina Ooze’, Foraminiferal Ooze or Carbonate Ooze as it is now known, is a widespread and highly characteristic sediment of the modern ocean system. Comparable sediments are much less common in the geological record although, as we describe here, a number of Middle Jurassic carbonate sediments with distinctive assemblages from Central Europe fulfil many of the criteria. One important component of these assemblages in the Middle Jurassic is ‘Globigerina bathoniana’ Pazdrowa, 1969, first described from the Bathonian sediments near Ogrodzieniec (Poland). The generic assignment of this species and other coeval Jurassic taxa is discussed. This species and many of the other early planktic foraminifera evolved in the Aragonite ll Ocean, together with the other two oceanic carbonate producers: the calcareous nannofossils and the calcareous dinoflagellates. The preservation of carbonate sediments with abundant planktic foraminifera on the sea floor indicates that, by the mid-Jurassic, the carbonate/aragonite compensation depths (and associated lysoclines) must have developed in the water column.

Over the last 50 years, our knowledge of early planktonic foraminifera has changed markedly. In 1958 Grigelis described “Globigerina oxfordiana” from the Upper Jurassic of Lithuania and this has, subsequently, become identified as one of the most geographically widespread of Jurassic planktonic taxa. There is a danger in that many authors identify any planktonic foraminiferid from the Jurassic as this taxon, rather than consider the alternative species described in the literature. In the period from 1967 to 1973, however, Dr W. Fuchs (Austria) identified a number of new taxa from the Triassic and Jurassic of Austria and the Jurassic of Poland and claimed that the history of planktonic foraminifera began in the Triassic. Following a long re-investigation of all his material in the Geologische Bundesanstalt in Vienna by the authors it appears that one of his new genera, Praegubkinella, was probably the ancestor of Conoglobigerina in the Toarcian (Early Jurassic). The first conoglobigerinids have been described from Toarcian – Aalenian – Bajocian strata in Central and Eastern Europe (including parts of the Former Soviet Union). The palaeobiogeography of Conoglobigerina and other related genera such as Globuligerina, Haeuslerina and Compactogerina are discussed and the problems yet to be resolved (e.g., records of “planktonic” taxa in the Triassic/Jurassic offshore N.W. Australia) are documented. The distribution (and evolution) of the Jurassic and Early Cretaceous planktonic foraminifera appear to be related to sea level highstands and continental fragmentation, especially in the earliest Cretaceous.

The planktonic foraminifera almost certainly evolved from benthonic ancestors in the early Jurassic. The meroplanktonic genus Conoglobigerina, known from south-central and eastern Europe, appears in the Bajocian and is probably derived from the even more geographically restricted Praegubkinella. This genus was represented by a single taxon in the earliest Toarcian but diversified after the Toarcian anoxic event. At the same level Oberhauserella quadrilobata Fuchs, 1967 became more inflated and there is some evidence to suggest that the ‘anoxic event’ was the environmental perturbation that began the transition to a planktonic mode of life. In the Callovian-Oxfordian interval, the planktonic foraminifera are still restricted to a relatively limited area bounded by the North Atlantic Ocean, NW Europe and Eastern Europe and this remained the case even in the earliest Cretaceous. It was only in the Aptian-Albian that the palaeogeographical distribution changed dramatically, probably as a response to the elevated sea levels caused by the increased rate of ocean crust production which began in the Early Aptian. The principal diversification events in the Jurassic (Toarcian, Bajocian, Callovian-Oxfordian) also appear to be related to sea level highstands.

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