Page 219 - Introduction to Paleobiology and The Fossil Record
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206 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
tists (unicellular eukaryote organisms with a Ireland, the north of England and Sweden, the
variety of tests (external shells) and cysts Early Carboniferous Waulsortian mounds in
(enclosed resting stages)), microinvertebrates Ireland and elsewhere, together with the Early
(mainly the ostracodes, see p. 383), microver- Cretaceous mudmounds in the Urgonian lime-
tebrates (mainly the conodonts and various stones of the Alpine belt – were precipitated
other microscopic parts of fishes, see p. 441) by microbes. The infl uence of microorganisms
and spores and pollen (microscopic reproduc- may also be more subtle. Coccolith-producing
tive organs of plants, see p. 493). We devote organisms, for example Emiliania, can, during
this chapter, however, to the more advanced blooms, manufacture massive amounts of
microbes themselves, represented by the calcium carbonate; this material is much more
second group. The protists are most probably readily subducted than shelf carbonates and
derived from within the Archaea, splitting it is then recycled through volcanoes as carbon
from them between 4.2 to 3.5 Ga, but the dioxide (CO 2 ). The buildup of this greenhouse
group is almost certainly polyphyletic. The gas probably maintained warmer climates
prokaryotic Archaea and Bacteria are inti- during the last 200 million years.
mately tied to the origin of life and the limited The extraction and retrieval of microfossils
Precambrian fossil record (see pp. 191–4); from rocks and sediments requires a range of
this is all the evidence of life in rocks over 1 preparation techniques, some of which can
billion years old! only be attempted in purpose-built laborato-
The abundance and durability of many ries. For many groups, preparation consists
microfossil groups makes them invaluable essentially of disaggregation of the rock in
for biostratigraphic correlation (see p. 25). water or more potent solvents followed by
Sequences of samples can be collected from sieving to remove the clay fraction. The silt-
rock outcrops and even from the very small and sand-sized residue is then hand picked
samples available from drill cores and drilling under a microscope to collect microfossils
muds. Consequently they are very widely used such as foraminiferans and ostracods. For
in geological exploration by petroleum and other groups such as radiolarians, diatoms
mining companies. In addition, many micro- and conodonts, acetic or hydrochloric acid is
fossils are produced by planktonic organisms used to remove the carbonate fraction and
with very wide biogeographic distributions, concentrate the fossils. For palynomorphs,
making them invaluable for reliable long-dis- the silicate minerals are removed with hydro-
tance correlation. Microfossils in oceanic fluoric acid, an extremely dangerous chemical
sediments also provide a continuous record of that requires special facilities. Finally, micro-
environmental change and paleoclimate, and fossils may be concentrated by settling in
study of changing assemblages and the geo- heavy liquids or by electromagnetic separa-
chemistry of microfossil shells provide the tion. Many groups, such as algae and forami-
fundamental data for paleoceanographic niferans, may also be studied in thin section.
research. Moreover, consistent color changes
through thermal gradients have made micro-
fossils, particularly conodonts and palyno- PROTISTA: INTRODUCTION
morphs, invaluable for assessments of thermal The protists are predominantly single-celled
maturation and the prediction of hydrocar- organisms with nuclei and organelles, includ-
bon windows. ing both autotrophs, organisms that convert
Microorganisms have made a phenomenal inorganic matter such as CO 2 and water into
contribution to the evolution of the planet as food, and heterotrophs, organisms that eat
a whole. Many, such as the coccolithophores, organic debris or other organisms. The Pro-
diatoms, foraminiferans and radiolarians, are tista is a convenient grouping but it is not well
rock-forming organisms. The prokaryotic defined. Essentially it consists of all eukary-
cyanobacteria fundamentally changed the otes once the multicellular animals, fungi and
planet’s atmosphere from anoxic to aerobic vascular plants are removed. Consequently it
during the Precambrian, and probably contin- is a paraphyletic collection of rather disparate
ued to mediate atmospheric and hydrosphere organisms. Most are microscopic and unicel-
systems. For example, recent research sug- lular but multicellularity has evolved numer-
gests that carbonate mudmounds – such as ous times and the multicellular algae (seaweeds)
the Late Ordovician mudbanks in central are conventionally included in the Protista too