Page 219 - Introduction to Paleobiology and The Fossil Record
P. 219

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
   214   215   216   217   218   219   220   221   222   223   224