Page 241 - Introduction to Paleobiology and The Fossil Record
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228 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
liths, but these are mostly in taxa with a
limited fossil record. More interesting prob-
lems are posed by the alternation of holococ-
colith-bearing and heterococcolith-bearing
phases in the life cycle of a single species. In
modern coccolithophores the taxonomy is
being adjusted to reflect this as data become
available.
Together with diatoms, dinofl agellates and
picoplankton (tiny, single-celled plankton
(a) (b) 0.2–2.0 μm in size), coccolithophores are the
1 2 most abundant phytoplankton in modern
oceans. The greatest diversity is developed in
the tropics. Dependence on sunlight for pho-
tosynthesis restricts the group to the photic
zone, with a depth range of 0 m to about
150 m. Within wave-mixed surface waters
2 μm there is normally only a slight vertical strati-
5 μm
fication of assemblages, but a quite different
assemblage is often developed beneath the
3 4
thermocline.
Evolution and geological history
Rare coccoliths first appeared in the Late Tri-
assic and increased in numbers during the
5 μm 2 μm Jurassic and Cretaceous; the group peaked in
the Late Cretaceous, and chalk from that
5 6
interval is almost entirely composed of these
nannofossils. Only a few species survived the
end-Cretaceous extinction event but they
radiated again during the Cenozoic, recover-
ing their numbers and abundance. However,
in the last 4–5 myr there has been a marked
5 μm 2 μm decline in the abundance of larger coccoliths
and, as a result, they have become less abun-
(c)
dant in oceanic sediments, typically forming
Figure 9.18 Some coccolith morphotypes: only 10–30% of modern calcareous oozes.
(a) coccospheres of the living Emiliana huxleyi, Biostratigraphic zonal schemes using cocco-
currently the most common coccolithophore lithophores have been established from the
(×6500), and (b) Late Jurassic coccolith Jurassic to the present day, and these are
limestone (×2000). (c) Coccolith plate styles: widely applied because they are reliable and
1 and 2, Coccolithus pelagus; 4 and 5, Oolithus operate over great distances. Moreover, basic
fragilis; 5 and 6, Helicosphera carteri. In biostratigraphic analyses of coccolithophore
C. pelagus and H. carteri growth was upwards samples can be carried out rapidly, typically
and outwards with the addition of layer upon requiring less than an hour per sample. This
layer of calcite; in O. fragilis growth was is because nannofossils are abundant enough
different with curved elements, in non-parallel to to be studied in simple strew mount prepara-
crystal cleavage directions. (a, b, courtesy of tions and can be reliably identified in cross-
Jeremy Young; c, courtesy of Karen Henriksen.) polarized light. Nannofossils only occur in
low-energy marine sediments and are easily
ing modern and fossil taxonomies. Some destroyed by diagenesis, but when they are
modern coccolithophores are polymorphic, present they provide an ideal means of rapidly
producing several different types of cocco- dating sediments.
