What is Paleo-botany?

Paleobotany is a field of paleontology that studies plants throughout geologic history, and is primarily concerned with the fossil record and evolutionary history of plants.

What are the objectives of paleobotany?

  • The major aim is to reconstruct entire fossil plants

  • To assign extinct plants to particular taxonomic groups

  • To understand evolution of extinct (fossil) plants

What is a fossil?

  • Any evidence of previous life, either direct or indirect evidence

  • In sedimentary rock; usually in places where these rocks have been exposed

    • e.g. Eroded cliffs, Road cuts, Quarries, Mines, etc.

  • Fossil localities exist everywhere sedimentary rocks exist: from the Arctic, through the tropics, into Antarctica

Direct Fossil Evidence

  • Material from past organisms in its original, or sometimes altered form

  • Direct evidence can provide information about…

    • Morphology: the external form of an organism

    • Anatomy: internal cellular structure

    • Ultrastructure: subcellular structure

Above: impression fossil of palm leaf

Below: trace fossil of Cambrian invertebrate

Indirect Fossil Evidence

    • Indirect fossils are evidence left by an organism, but does not include the preserved remains of an organism

    • For example, imprints made by an organism are indirect fossil evidence, called trace fossils or ichnofossils

      • Impressions of plants or animals (e.g. leaf imprint, footprints)

      • Chemical fossils, or chemofossils, are chemicals found in the rocks that are organic signatures of past life

    • Some indirect fossils can demonstrate aspects of the organism's metabolism

      • Amber: resins produced by trees

      • Coprolites: waste products of animals

    • Overall, indirect evidence can provide information about…

      • Existence (e.g. footprints, but no direct fossils)

      • Behavior (e.g. community living)

      • Ecology (e.g. predation / defense)

      • Physiology (e.g. running speed)

Above: coprolite of dinosaur

Below: fossil amber with some inclusions

Conditions of Fossilization

  1. Removing the organic material (e.g. organism, leaf, etc.) from an oxygen-rich environment, preventing aerobic decay by bacteria and fungi

  2. Burial of the organic material and introduction to the sedimentary rock record

  3. "Fixing" the organic material , which prevents decay by anaerobic bacteria. The burial environment is usually rich in humic acids or clay minerals that can retard decay by blocking the chemical sites onto which decomposers fasten their degrading enzymes

Types of Fossilization

Compressions

    • Two-dimensional fossils (flattened into a single plane), with organic material

    • Physical deformation such that the three-dimensional structure is compressed to more-or-less two-dimensions

    • Compressions retain organic matter, usually more or less coalified

    • Peat, lignite, and coal are essentially compressions of thick accumulations of plant debris relatively free of encasing mineral sediment

Above: compression of fern leaf

Impressions

  • Two-dimensional imprints, devoid of organic matter

  • Impressions are essentially compressions sans organic material. If the sediment is very fine-grained, impressions may faithfully replicate remarkable details of original external form, regardless of subsequent consolidation of the sediment

  • Most commonly found in fine-grained sediment such as silt or clay

Above: impression of palm leaf

Casts and Molds

  • Three-dimensional fossils, may have a surface layer of organic material

  • A cast results when sediment is deposited into cavities left by the decay of plant parts

  • A mold is essentially a cavity left in the sediment by the decayed plant tissue. Molds are generally unfilled, or may be partially filled with sediment

  • Casts and molds commonly lack organic matter, but a resistant structure may be preserved as a compression on the outside of the cast or the inside of a mold

  • Casts and molds may be found together with the cast filling the mold

Above: Cast stump of Eospermatopteris

Permineralizations

  • 3-dimensional, tissue infiltrated by minerals allowing internal preservation

  • Permineralization occurs when the plant tissues are infiltrated with mineral-rich fluid

  • Minerals precipitate in cell lumens and intercellular spaces, thus preserving internal structures of plant parts in three dimensions

Above: Permineralized stump from Petrified Forest National Park

Molecular Fossils

  • non-structural, preserves organic compounds

  • Breakdown products of chlorophyll and lignin have been found in well-preserved fossil leaves.

  • Lipids and their derivatives have also been recovered from sediments.

  • Some carbohydrate break-down products may also survive in sediment.

  • Genetic material was recovered from Tertiary leaves, and the age of material from which DNA and RNA is recovered seems to be greater with every issue of Nature.

Subdisciplines of Paleobotany

Biostratigraphy

Development / Growth of ancient organisms

Form and function of plant structures

  • Interrelationships of organisms & environment

Origin and evolution of major plant groups

    • Evolutionary patterns of taxa

    • Macroevolution and speciation/extinction

Paleoclimate

What are Form Taxa?

  • Disarticulation of plant parts during fossilization creates problems for assigning taxonomy (e.g. which leaves are connected to which stems?)

  • The first task of paleobotany is to find evidence to connect plant parts to create a holistic picture of the plant

    • Connecting roots to stems to leaves

    • Connecting stems to reproductive structures (flowers, fruits, sporangia)

    • Connecting seeds and spores to fruits and sporangia, respectively

  • Since these parts may be found at different times, paleobotanists assign taxonomic names to each dis-articulated plant structure

    • These taxonomic names are called form taxa (e.g. form genus, form species, etc.)

    • Leaves, stems, roots, sporangia, spores, pollen, seeds, bark, etc. may all receive separate taxonomic names

  • The challenge for scientists and students of paleobotany is remembering all of these form taxa names that may be assigned to a single organism

Above: Lycopod scale tree reconstruction; note the different genus names for parts of the plant

  • Example: Carboniferous Scale Tree (see above image)

    • Lepidodendron: upright stem with bark

    • Lepidophylloides: leaves (microphylls)

    • Stigmaria: root-like rhizophore (modified rhizome-like stem that anchors and absorbs water/minerals)

    • Lepidostrobophyllum: sporophyll (leaf protecting sporangia)

    • Lepidocarpon: female cone (contains megaspores)

    • Cystosporites: megaspores, which will produce female gametophytes

    • Lepidostrobus: male cones (contains microspores)

    • Lycospora: microspore, which will produce male gametophytes

Additional Resources

  • Cladistics (from Botany 317): methodology to deciphering true evolutionary groupings

  • Paleontology is a historical science; therefore is not usually thought of as an experimental science

    • Modern computing had allowed for the introduction of experimental science to the field of paleontology