What is Paleobotany?

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, from disjunct fossilized remains
To assign those fossil plants to particular taxonomic groups, and place them in an evolutionary hierarchy
To understand the (macro)evolution of extinct plants, including the origin of new plant structures,and ancestor-descendent relationships

What is a fossil?

Any evidence of previous life, either direct or indirect evidence, is a fossil
Fossils are the lithified remains of life, found 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

Conditions of Fossilization

The organic material (e.g., organism, leaf, bone, etc.) is removed from an oxygen-rich environment, preventing aerobic decay by bacteria and fungi
The organic material is buried and introduced to the sedimentary rock record
The organic material becomes "fixed", 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
Slowly over time, the living organic material is replaced by minerals created a fossil
- If the fossilizing minerals are fine-grained, like clay, then the fossil tends to retain fine details at the microscopic level
- If the fossilizing minerals are coarse-grained, like sand, then the fossil tends to be less detailed at the microscopic level

Direct Fossil Evidence

Organic material from past organisms, including leaves, stems, flowers, fruits, seeds, spores, etc. (e.g., fossil of a leaf)
The fossils are the materials in their original, or sometimes altered, form
- This could be from plants, animals, fungi, and even prokaryotes

Macrofossils

- Fossils that can be seen with the naked eye (e.g., leaves, wood, bones, etc.)

Microfossils

- Fossils that require the aid of a microscope (e.g., spores, pollen, fossilized plankton, etc.)
Direct evidence can provide information about…
- Morphology: the external form of an organism
- Anatomy: internal cellular structure of an organism
- Ultrastructure: subcellular structure of the organism's cells

Above: impression fossil of palm leaf

Below: trace fossil of Cambrian invertebrate

Rock Types

Sedimentary

Rocks formed by the accumulation or deposition of sediments at Earth's surface, followed by cementation. This is the rock in which fossils are found. (e.g., chert, shale, sandstone, etc.)

Igneous

Rocks formed through the cooling and solidification of magma or lava. (e.g., basalt, granite, quartz, etc.)

Metamorphic

Rocks subjected to increased temperatures and pressures, which physically contort and chemically change them. (e.g., gneiss, marble, schist, etc.)

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 (i.e., dung) 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

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 without 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

Three-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
Petrified wood is an example of a permineralized tree trunk

Above: Permineralized stump from Petrified Forest National Park

Molecular Fossils

Non-structural fossils, 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 breakdown products may also survive in sediment.
Genetic material was recovered from Cenozoic leaves, and the age of material from which DNA and RNA are recovered seems greater with every issue of Nature.

Subdisciplines of Paleobotany

Biostratigraphy

- Palynology

Development / Growth of ancient organisms

Form and function of plant structures

- Origin and evolution of major plant structures
- Biomechanics and allometry

Paleoecology

Interrelationships of organisms & environment

Origin and evolution of major plant groups

- Evolutionary patterns of taxa
- Macroevolution and speciation/extinction

Paleoclimate

- Dendrochronology

What are Morphotaxa?

The disarticulation of plant parts during preservation and 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 morphotaxa
- 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 morphotaxa names that may be assigned to a single organism

Example: Carboniferous Scale Tree (see 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

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

Molecular Clock Technique

From Ho (2008) The molecular clock and estimating species divergence. Nature Education 1(1):168
The molecular clock hypothesis states that DNA and protein sequences evolve at a relatively constant rate over time and among different organisms.
A direct consequence of this constancy is that the genetic difference between any two species is proportional to the time since these species last shared a common ancestor.
Therefore, if the molecular clock hypothesis holds, this hypothesis serves as an extremely useful method for estimating evolutionary timescales.
This is of particular value when studying organisms that have left few traces of their biological history in the fossil record...

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

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