MESOZOIC PLANKTONIC FORAMINIFERA 123almost to Oxfordian levels. The diversification
of the Cenozoic fauna following the extinction
event is, however, much more rapid than that
seen in the Mesozoic.SummaryThe origins of the planktonic foraminifera can
be traced back to the Toarcian, although it is
only in the Bathonian that meroplanktonic taxa
appear. The inflated O. quadrilobata and
Praegubkinella spp. present in Toarcian strata
are clearly not fully planktonic and may have
been benthonic throughout much of their life
cycle. The environmental disruption caused by
the early Toarcian sea-level rise and associated
'anoxic event' are potential triggers for this
change in lifestyle as may have been the massive
release of methane-derived carbon from sub-sea
gas hydrates in the falciferum Zone of the early
Toarcian (Hesselbo et al 2000).
These early faunas, including the Conoglo-
bigerina spp. assemblage in the Bathonian, are
all limited to the northern side of Tethys (Figs 3,
5 and 6). With such a limited fauna in the
restricted areas shown in these maps it is
unlikely that many diversification opportunities
were available, especially in the latest Jurassic
and earliest Cretaceous interval which was
characterized by reduced sea levels. Once ocean
floor production accelerated during the latest
Barremian and earliest Aptian and, as a result,
sea levels rose globally, the planktonic
foraminifera were able to use the newly formed
ocean basins (and expanded lengths of conti-
nental margin) to attain a near-global distri-
bution by the mid-Cretaceous (Fig. 7). The
continental fragmentation during the Aptian
and Albian allowed the hedbergellid faunas to
expand rapidly and as the various ocean basins
became connected, the stratification of the water
column generated the opportunity for the
evolution of a depth-stratified planktonic fauna
by the mid- to late Albian (see Hart 2000).The authors acknowledge the helpful discussions of
this topic with M. Simmons (Cambridge), M. Caron
(Fribourg), A. Gorog (Budapest), R. Wernli
(Geneva), J. Gregory (St Albans), A. Henderson
(London) and J. Whittaker (London). A. Gorog and J.
Svabo are thanked for their help in the field in
Hungary and J.-P. Berger is thanked for help in the
field in Switzerland. M.B.H. acknowledges a Research
Grant from the Royal Society while M.J.O. acknow-
ledges receipt of a Research Studentship funded by the
University of Plymouth and Amerada Hess (London).
J. Abraham is thanked for his patient work in prepar-
ing the figures.
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