Preface: The planktonic foraminifera of the Jurassic
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Preface: The planktonic foraminifera of the Jurassic Felix Gradstein1 • Ludmila Kopaevich2 • Michael Knappertsbusch3
Received: 22 March 2017 / Accepted: 6 June 2017 / Published online: 8 July 2017 Ó Akademie der Naturwissenschaften Schweiz (SCNAT) 2017
Abstract Planktonic foraminifera originated in late Early Jurassic and, for reasons poorly understood, only underwent proliferation of species and geographic spreading from mid-Cretaceous onwards. Their evolutionary development in the oceans and adjacent seaways since early mid-Cretaceous has spurned a large volume of increasingly sophisticated literature. Specimens of this diverse group are now routinely subject to a wide variety of taxonomic, biostratigraphic, geochemical and molecular research. This microfossil group now plays a major role in studies on Cretaceous and Cenozoic palaeoclimatology and palaeoceanography. This global attention is in striking contrast to the interest and research effort put in their Jurassic and earliest Cretaceous forerunners. Keywords Jurassic planktonic foraminifera Introduction Historical background Why is there not a large body of sophisticated palaeobiological and -geological literature on Jurassic and earliest Cretaceous planktonic foraminifera? There are several plausible answers. A quick and easy answer is that the aragonitic tests of these microfossils hamper fossilisation. Indeed, a majority of Editorial handling: D. Marty. & Felix Gradstein [email protected] 1
Natural History Museum, University of Oslo, 0318 Oslo, Norway
2
Geological Faculty, Lomonosov Moscow State University, Leninskie Gory, GSP-1, 119991 Moscow, Russian Federation
3
Naturhistorisches Museum Basel, Augustinergasse 2, 4001 Basel, Switzerland
samples in wells or outcrop sections may lack specimens of these taxa. On the other hand, some Jurassic marly shale sections, like in wells on the Grand Banks of Newfoundland, in outcrops in Portugal and in Dagestan may contain abundant (65–125 lm size, and some larger size) free specimens. Thin-sections of Jurassic limestones in Poland, the Balkan countries and N. Africa may also abound with tiny shells of these taxa, but do not lend itself to taxonomy. A problem is that we do not really understand why geographic and stratigraphic distribution is irregular and patchy. One answer may come from palaeobiological and evolutionary reasoning, i.e. that organisms at their closeness to origination from their ancestor have low frequencies and a restricted biogeographical distribution. Early planktonic foraminifers from the Jurassic are known to be usually of low frequency and only seldomly reach sporadic mass occurrences. In this context, it is not so much surprising that Jurassic foraminifers were only rarely encountered in samples. A second consideration is that Jurassic planktonic foraminifera are limited to low and mid latitude marine basins of Pangaea, adjacent to the true oceans. The Jurassic oceans themselves were empty of planktonic foraminifera. Worldwide sampling by the ocean drilling campaigns since
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