| Abstract: | Cretaceous coccolithophores (and associated
calcareous nannofossils) reached levels of
evolutionary diversity and, arguably, carbonate
production that had not been achieved before and have
not been seen since, yet the period was characterised
by relatively stable evolutionary trends (dominantly
diversity increase) and was bracketed by far more
dramatic events - catastrophic extinctions at the
Cretaceous/Tertiary boundary and considerable
taxonomic turnover across the Jurassic/Cretaceous
boundary. Having said this, many authors have alluded
to possible relationships between nannofossil
evolution and major Cretaceous global environmental
change and, specifically, the oceanic anoxic events of
the mid-Cretaceous.
New nannoplankton diversity data reveal that the
Cretaceous saw relatively rapid and continuous
diversity increase to an "all-time" maximum in the late
Campanian. Three main intervals of increase - the
Berriasian-Hauterivian, Aptian-Albian and
Turonian-Campanian - were interrupted by diversity
minima in the Barremian and Cenomanian. The data
reveal no clear relationship between long-term diversity
trends and currently-recognised major global
environmental change events in the Cretaceous.
Oceanic Anoxic Events 1 (early Aptian) and 2 (late
Cenomanian) occurred well within nannoplankton
diversity minima and were followed in both cases by
periods of protracted diversity increase. However, the
most rapid evolutionary increases broadly correlate
with cooler climate intervals, and enhanced species
diversity in the Campanian was arguably related to the
onset of late Mesozoic climatic cooling with much of the
diversification the result of greater
palaeobiogeographic differentiation, and in particular
the evolution of high-latitude-restricted floras. The
Berriasian to Hauterivian increases were also
accompanied by increased endemism, but included a
greater number of low-latitude restricted groups,
including the enigmatic nannoconids.
In contrast, Cenozoic nannoplankton diversity patterns
are markedly more variable than those of the
Cretaceous, and there is good correlation between
diversity and climate trends. Notably, diversity maxima
are associated with warm intervals and minima
correlated with cooler intervals. This suggests that
climate-driven changes in ocean productivity and gyre
habitat-space may have exerted an important control
over the global diversity of the group in the Cenozoic, an
explantion that is currently difficult to apply to their
Cretaceous counterparts. |