Page 220 - Introduction to Paleobiology and The Fossil Record
P. 220

PROTISTS 207




                                                                               Chromalveolates
                                Plantae                              Alveolates
                                                                 Colpoderllds  Aplcomplexa Dinoflageflates


                                                                       Oxyrrhis
                                                               Cillates  Perkinsus
                                                                       Brown Algas
                                                   Piants
                                             Charophytes
                                         Chlorophytes
                                                                      Diatoms  Chromists
                                         Red Algae
                                                                       Bicosoecide
                         Cercozoa +                                    Oomycetes
                                       Glaucophytes
                                                                        Opalinida
                                                                       Haptophytos
                                                                        Cryptomonads
                                        Apusozoa
                                       Hellozoa
                               Phytomyxids
                            Haplosporidia                                  Heteroloboses
                      Cercozoa
                                 Filosa                                     Euglonids  Discicristates
                           Foraminifera                                     Dipionemids
                               Radiolaria                               Carpediemonas
                                                                             Kinetoplastlds
                                                                         Core Jakobids
                                     Fungi
                                                                        Diplomonads
                                  Microsporidia Fungi  Lobosee  Archamoebae  Oxymonads  Retortamonads
                                                                      Trichomonads
                                           Animals Nuclcarids  Ichthyospores  Choanoflagelates  Amoebozoa
                                                                   Trimastix
                                                                      Hypermastigotes
                                                      Slime Moulds
                           Opisthokonts                                    Excavates
                                            Choanoroa
             Figure 9.1  Protist positions on the tree of life. In this tree, developed by Patrick Keeling, University of
             British Columbia, the protozoans (foraminiferans and radiolarians) lie within the Cercozoa far divorced
             from the chromists (diatoms and dinoflagellates) within the Chromalveolates. (From Keeling et al.

             2005.)
             (Fig. 9.1). Subdividing the diversity of protists   diversity but this is not the place to go into
             is equally problematic. The division into auto-  the rapidly changing details of this research.
             trophic protozoans and heterotrophic algae      Instead, we should simply note that groups
             (chromistans) is important ecologically, but    with microfossil records are widely scattered
             phylogenetically almost meaningless as both     across the diversity of protists. Here, follow-
             groups are polyphyletic. The fi rst  protists    ing Cavalier-Smith (2002) and others, the
             were almost certainly heterotrophs, but chlo-   protists are grouped into protozoans (forami-
             roplasts were acquired separately in at least   niferans, radiolarians, acritarchs, dinofl agel-
             six lineages, producing heterotrophs, and lost   lates and ciliophorans) and chromistans
             secondarily even more often: for example, the   (coccolithophores and diatoms); chitinozoans
             classic protozoan ciliates almost certainly     are difficult to classify in this scheme and are

             evolved from algae. Protists are also often     thus treated separately.
             subdivided according to their means of loco-
             motion, most simply into fl agellates  and
             amoebans. Again, however, these are poly-       EUKARYOTES ARRIVE CENTER STAGE
             phyletic groups. So simplisitic attempts at     So when did the eukaryotes fi rst appear? Uni-
             classifying protists do not really work and     cellular eukaryotes, with nuclei and organelles,
             they are perhaps better regarded as a loose     represented by acritarch cysts are known from
             grouping of 30 or 40 disparate phyla with       rocks dated at about 1.45 Ga. Spiral ribbons
             diverse combinations of trophic modes, mech-    of  Grypania, however, have been reported
             anisms of motility, cell coverings and life     from rocks as old as 1.85 Ga. One of the oldest
             cycles. Modern molecular genetic and cyto-      multicellular organisms is a bangiophyte red
             logic research is slowly making sense of this   alga (rhodophyte) preserved in silicifi ed car-
   215   216   217   218   219   220   221   222   223   224   225