Page 239 - Introduction to Paleobiology and The Fossil Record

226 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
Calpionella
Calpionellites
Tintinnopsella
Deflandronella lack obvious shared characters with cocco-
Coxlielina Salpingellina
Figure 9.17 Morphology of some tintinnids in
cross-section from limestones (×100–200).
group of extinct, cup-shaped, calcareous
microfossils that were abundant in Late Juras- plankton first appeared during the Late Trias-
sic and Early Cretaceous pelagic sediments,
especially in the Tethyan realm. As an extinct
group with no complex characters, no defi ni-
tive evidence of their affi nities has been found;
however, they are strikingly similar in shape
and size to an important group of ciliates, the
tintinnids.
Tintinnids are part of the zooplankton,
grazing on phytoplankton and providing a
food source for larger members of the plank-
ton. The cell is enclosed within a cup-shaped
test or lorica, often 10 times larger than the
cell itself. Modern tintinnids have an organic
lorica with, in some cases agglutinated mineral
grains or coccoliths, but without biomineral-
ization, whereas the fossil calpionellids had a
primary calcareous test (Fig. 9.17).
Two families of fossil tintinnid have been
recorded, together ranging in age from the
Tithonian (Upper Jurassic) to the Albian
(Middle Cretaceous).
CHROMISTA
The chromistans are probably a paraphyletic
group of eukaryotes that usually contains
chloroplasts with chlorophyll c, which is
absent from all known plant groups. The
group includes various algae, the coccolitho-
phores and the diatoms and the majority are
primary producers, functioning as part of the
phytoplankton.
Coccolithophores
Nannoplankton, are defined as plankton less
than 63 μm across, the smallest standard
mesh size for sieves. Although the nanno-
plankton includes organic-walled and sili-
ceous forms, the calcareous groups are most
prominent in living floras and dominate the
fossil record. Coccolithophores are the domi-
nant members of the fossil calcareous nanno-
plankton, and the calcareous plates they
produce, coccoliths, dominate nannofossil
assemblages. Many calcareous nannofossils
liths and so are excluded from the coccolitho-
phores and instead are termed nannoliths.
These nannoliths may be related to coccolith-
bearing organisms, but in view of their diver-
sity in form, the group may contain calcareous
structures produced by quite unrelated
microbes. As a whole, calcareous nanno-
sic, increased in abundance and diversity
through the Jurassic and Cretaceous, reaching
an acme of diversity in the Late Cretaceous.
They were severely affected by the KT mass
extinction, but subsequently radiated in the
Early Paleogene and remained a major com-
ponent of the calcifying plankton throughout
the Cenozoic. They are extremely abundant
in the surface waters of modern oceans.
Morphology and classifi cation
Coccolithophores are unicellular algae, pre-
dominantly autotrophic in dietary mode,
usually ranging in size from 5 to 50 μm, and
globular, fusiform or pyriform in shape. The
group constitutes the Phylum Haptophyta,
within the Kingdom Chromista, together with
various closely related non-calcifying algae;
they have golden-brown photosynthetic pig-
ments and, in motile phases, two smooth
flagella together with a third fl agellum-like
structure, the haptonema. Coccolithophores
are almost exclusively marine (there is just
one, rather rare, freshwater species), usually
open marine, occupying the photic zone where
they photosynthesize. The group today is
most diverse and has its highest relative abun-
dances in the tropics although coccolitho-
phores occur at all latitudes. The shell is
composed of distinctive calcitic platelets or
coccoliths. These are produced intracellularly;