Photosynthetic Life in the Fossil Record

  1. Introduction
    1. Formation of the Earth
    2. Heavy bombardment period
    3. Formation of the oceans
    4. The early fossil record
    5. Liquid water and life were present on Earth by about 3.8 ba ("billion years ago")
  2. Geobiological evidence
    1. Iron depositing bacteria
      1. Several mechanisms
    2. Elemental sulfur deposits
      1. Nonbiological deposition of sulfur from mantle would require high temperatures (>700C), but sulfur deposits can be identified that show no sign of having been subjected to these high temperatures
    3. Petroleum
    4. The shift from a reducing to an oxidizing atmosphere
      1. A consequence of oxygenic photosynthesis
      2. In extant organisms, oxygenic photosynthesis is known only among cyanobacteria
      3. This implies that cyanobacteria must be at least old enough to have
      4. Red Beds
    5. Carbon burial and atmospheric composition
    6. The carbonate silicate cycle apparently regulates global temperature
  3. Structural evidence
  4. Prokaryotes
    1. Stromatolites & crust forming bacteria
      1. Stromatolites are gelogical formations that result in tidal areas from sediment entrapment in bacterial mats
      2. Most commonly associated with cyanobacteria, but other organisms can produce similar structures
        1. The filamentous bacteria glide up into the light, and leave a layer of sediment behind.
        2. Next tidal cycle, more sediment is deposited, and another layer forms
        3. Tidal cycles, and annual patterns in solar elevation can be identified in stromatolite laminae
      3. Supratidal stromatolites can form as a result of carbonate deposition
      4. Stromatolites are found in all geological strata
        1. In modern stromatolites, eukaryotes (including relative newcomers such as diatoms) are also found in stromatolites.
        2. A spectacular array of modern stromatolites are present in Shark Bay, Australia
      5. Hot spring stromatolites
        1. Filamentous bacteria in hot springs, probably green nonsulfur bacteria, can produce stromatolites
        2. Consequently the presence of a stromatolite does not necessarily indicate the presence of cyanobacteria
        3. However, the bacteria involved in formation of hot spring stromatolites are less than 1 micrometer in diameter
    2. Organismal preservation
      1. Precambrian fossils of bacteria
        1. Eobacterium isolatum
        2. Bacilliform dent in rocks radiometrically dated to 3.2 ba
  5. Multicellular Eukaryotes
    1. Rhodophyta
      1. The earliest reds in the fossil record are only tenuously assigned
        1. Eosphaera tyleri -- 1.9 ba -- more or less spherical with buds in a mucilaginous sheath . Thought to resemble Porphyridium purpureum
        2. Eotetrahedrion princeps -- 1.5 ba -- shows tetrahedral 'spores' with triradiate markings. Very likely a eukaryote, generally assigned to reds, but few red algal spore fossils are known.
      2. Later precambrian fossils of reds are more convincing, and penetrating Conchocelis filaments are known from from the early paleozoic.
      3. Coralline reds fossilize readily, and are quite distinctive
      4. Rhodolith (=rhodolite or rhodoid) is a geological term for spherical accumulations of crustose reds (and foraminifera) that roll around on offshore banks. They grow slowly; a 30 cm rhodolith is probably at least 500 years old, and perhaps much older.
    2. Chlorophyta
      1. An extensive and old fossil record
        1. Again, precambrian fossils are difficult to interpret, but greens are thought to extend back almost 2 ba
        2. Caryosphaeroides pristina is a precambrian sphere interpreted as a chlorella-like green alga
      2. Ordovician-Silurian (ca 500 - 400 ma) fossils that resemble extant taxa begin to appear
      3. As with reds, the best preservation is with calcified forms
      4. Marine siphonaceous greens have a spectacular fossil record
      5. Charales (primarily freshwater)
        1. Zygotes are covered with sporopollenin, often settle into lake sediments and fossilize. These are called gyrogonites.
  6. Phytoplankton
    1. Haptophyta (= Coccolithophorads or prymnesiophytes)
      1. Distinctive calcareous scales, called coccoliths, cover the haptophyte cell
      2. These are key stratigraphic markers
      3. The oil industry used to be a major employer of phycologists
      4. Unfortunately, there doesn't seem to be much oil left to find
    2. Diatoms
      1. Silica frustules fossilize incredibly well
      2. Easy preparation -- boil in acid until all organics & carbonates are gone.
      3. Diatomaceous earth is a mineral deposit consisting entirely of diatom frustules
      4. Also diatomaceous oozes, etc.
      5. However, diatoms are a geologically recent group, first appearing in the fossil record in the mid Cretaceous (ca 100-125 ma).
      6. Note that contamination is a potential problem with any diatom sample
    3. Dinophyta
      1. Diversification in the Permian
      2. Cysts preserve well, as do some armored species
    4. Ebridians
    5. Acritarchs (literally, "of unknown origin")
      1. "Small microfossils of unknown and probably varied biological affinities consisting of a central cavity encolsed by a wall of single or multiple layers and of chiefly organic composition..." (Tappan, 1980, quoting Evitt, 1963)
      2. Some acritarchs have been subsequently identified (most notibly as dinoflagellate prasinophycean cysts), and consequently removed from the acritarchs.

Supplementary Reading:

Tappan, H. 1980. The paleobiology of plant protists. W.H. Freeman and Co. San Francisco

Brasier, M. D., O. R. Green, A. P. Jephcoat, A. K. Kleppe, M. J. Van Kranendonk, J. F. Lindsay, A. Steele, and N. V. Grassineau. 2002. Questioning the evidence for Earth's oldest fossils. Nature 416: 76-81.

Schopf, J. W., A. B. Kudryavtsev, D. G. Agresti, T. J. Wdowiak, and A. D. Czaja. 2002. Laser-Raman imagery of Earth's earliest fossils. Nature 416: 73-76.