The study of fossil plants, palaeobotany, it not only of interest in itself, but can be applied to solving a wide range of biological and geological problems. Palaeobotany is concerned with the study of macroscopic plants, but can also include palynology—the study of spores, pollen, and other microscopic plants remains.
The nature of fossil plants
Fossilization processes: taphonomy
Plants differ from animals in a number of important respects so far as fossilization is concerned: they tend to live in erosional rather than depositional sites; they are entirely organic (except for some microscopic algae such as diatoms and coccoliths); they can show an alternation of generations, one being more easily fossilized than the other; and they readily fall apart. The size of the plant may also play an important role in controlling the nature of fossilization: plants range in height from less than 1 cm to over 100 m.
Many plants have a number of distinct organs: roots, stems or trunks, leaves, fertile parts. On death these organs separate and will usually be found as individual fossils. In life also, plants may shed leaves, seeds, and other organs. In consequence, only isolated organs are commonly encountered as fossils. Palaeobotanists are thus rarely, if ever, faced with a whole plant. Even today, relatively few plants have been reconstructed for which all the organs are known.
Because the fossil record is fragmentary, different parts of the same plant are given different names. For example, the large tree-like Carboniferous plant known as Lepidodendron has a rhizome underground called Stigmaria, leaves known as Cyperites, cones known as Lepidostrobus, and so on.
To complicate matters even further, the chemistry of the various plant organs also controls their preservation. Wood, for example, is composed of cellulose and lignin, each of which decays at a different rate. Leaves similarly have cellulosic tissues that readily decay but have an outer cuticular layer that may be more resistant.
Plant fossils accumulate in a wide variety of environments and can be found in a wide range of rock-types. They may be buried in place by inundation of sediment or even by lavas or pyroclastic flows. Fossil forests are commonly found entombed by volcanic rocks (e.g. the early Carboniferous forests on Arran, at Weeklaw, Scotland; the Jurassic forest in Patagonia, Argentina; and the Tertiary forest of Yellowstone National Park, USA). In other instances they have simply been entombed by clastic sediment from river flooding (e.g. Joggins, Nova Scotia) (Fig. 1a). Wetland settings, such as bogs and swamps, provide areas where plant material can accumulate under reduced oxygen conditions so that the decay of the organic material is slowed. Production of plant material then exceeds decay, leading to an organic accumulation in the form of peat.
Waters charged with minerals may infiltrate buried plants and calcite, silica, or pyrite may be precipitated in the empty cells spaces, ‘entombing’ the plants. ‘Permineralization’ is the term used where the original organic cell walls of the plant remain (Fig. 1b). The organic cell walls may subsequently be replaced by minerals (Fig. 1g). A good example of this type of petrification is the Triassic fossil forest of Arizona.
Plants can be transported and buried in sites far from where they lived, to become fossils. A fossil leaf may retain some of its organic component in the form of a coaly layer. An impression of a leaf can also be preserved in the rock; it is possible for all the organic material to be removed at any stage after burial, leaving a simple impression (Fig. 1h). This form of preservation may look spectacular but it is seldom particularly informative to the palaeobotanist.
Some plants may have been subjected to wildfire, and incomplete combustion produces charcoal which is rela-tively inert and has an enhanced preservation potential (Fig. 1d). Fossil charcoal (fusain) characteristically shows excellent preservation of three dimensional anatomy (Fig. 1e, f).
Palaeobiology of plants
The reconstruction of plant fossils is important for the interpretation of the biology of the plants. Investigation of the morphology and anatomy of fertile structures is needed to interpret the reproductive biology of the plants, which may be important for the understanding of their ecology. For example, vegetative structures may yield data on the life habit of a plant: whether it was free-standing, a scrambling plant, or a climber; investigations on seeds may yield data on their mode of dispersal.
Palaeoecology and reconstruction of ecosystems
An important aspect of palaeobotanical research is to unravel the palaeoecology of plant fossils to use the results in reconstructing ecosystems. Palaeoecological data are gathered in the field, where the occurrence of plants, in situ and drifted, can be documented, together with aspects of their completeness, preservation, and abundance. Interpretation of the biology of the plants, their transport and burial history, together with data on their preservation, can all help in palaeoecological reconstructions. The ecology of plants in wetland mires (peat-forming systems) has been widely studied using both microfossils and mesofossils.
Plant biology can also give clues to ecology. Plants with numerous small seeds are usually colonizers, whereas those with larger seeds usually live in more mature communities. The evolution of trees in the late Devonian opened up many new ecological niches. Likewise the evolution of the seed enable plants to thrive in drier environments. It is not until the late Carboniferous that we have evidence of upland vegetation. Palaeoecological data on plants, together with evidence of plant–animal interactions, may be integrated with data from animals and sedimentary environments to reconstruct an ecosystem.
Palaeobotany is also concerned with the evolution of plants. Most studies now consider relationships between plants, and the use of cladistic analysis has become widespread. Traditionally palaeobotanists had sought ancestor–descendant relationships, for example, the evolution of the conifers from the cordaites (an extinct group found in rocks of Carboniferous and Permian age), but more recently cladistic analysis has shown that these two groups are sister groups and that the conifers may be a polyphyletic group.
The evolution of plant strategies or organs has been the main focus of some research. Other research has been concerned with the evolution of a plant family or genus: for example, the evolution of the flowering plant family the leguminoseae or the genus Ginkgo.
Plants form the base of most food chains. Plants may be eaten dead, as part of the decaying litter, as by many arthropods, or alive, as by leaf-feeding insects (especially the caterpillars of butterflies and moths). It is not only invertebrates that rely on a regular supply of plant food but also vertebrates such as grazing and browsing mammals.
Such plant–animal interactions may be of three types: feeding, dispersal, and shelter. These interactions can be recognized from the study of both plant and animal fossils. Interactions can be revealed by damage to plants (Fig. 1h, i); for example, chewed leaves, bored wood, or the occurrence of specially evolved features such as nectaries in flowers, glandular hairs, and thorns on stems and leaves. Plant–animal interactions have, however, rarely been studied using fossil material. Damage to plants by feeding arthropods is only rarely recorded, because many collectors throw away ‘damaged’ specimens. In particular, leaves showing evidence of feeding (chewed leaves, mines and galls) become increasingly common from the Carboniferous to the Recent; Tertiary fioras show a marked increase in numbers and diversity of specimens. A number of evolved features in flowers from the Cretaceous onwards have been linked to insect pollination.
The origin of life
Studies on the Precambrian fossil record have focused attention of the origin and early evolution of life. Simple, single-celled micro-organisms including bacteria, cyanobacteria, and green algae have a considerable Precambrian fossil record. Studies of the diverse fossil biota from the 1800 Ma Gunflint Chert in Canada alerted researchers to the possibility of Precambrian fossils. Rocks more than 3000 Ma old in Australia have since yielded fossils, pushing further back the origin of life. Early fossils are all simple-celled prokaryotes such as bacteria and cyanobacteria. The evolution of eukaryotic algae with a discrete nucleus made possible the evolution of sex and more rapid diversification; it also led to the evolution of multicellular organisms.
The uses of fossil plants
Fossil fuels: their origin, use, and exploitation
Plant fossils provide the basis for most fossil fuels (coal, oil, and gas), and palaeobotany helps in all aspects of our understanding of these fuels. In terrestrial wetlands plants may accumulate in waterlogged conditions and the anaerobic environment may prevent decay and promote the formation of peat. In aquatic settings, organic matter transported into the environment or living in the environment (e.g. algae, including planktonic algae in the sea) can accumulate in anaerobic settings. On burial and with increasing temperature, the organic materials alters and produces oil and gas. Peats will in situ change successively to lignite, to bituminous coal, and eventually to anthracite. Gas, and in some instances oil, will be generated from such deposits. In organic-rich sediments oil and gas are generated from hydrogen-rich organic particles (e.g. algae, spores or pollen, cuticles, etc).
Because much of the plant material remains in coals, it is possible to isolate chemically identifiable plant parts such as spores and cuticles. Palaeobotanical data can be used to interpret the ecology and environment of the original peat deposit and to determine sequential changes that have taken place during the development of the peat. These data may help in dating coal seams and in their correlation, and can even be used to identify coals sold on the international market.
Plant fossils can be extracted from oil and gas source rocks, again helping not only in their dating but also providing palaeoenvironmental data and information on the petroleum potential. The chemical or physical alteration of the organic matter may can help with maturity assessment (i.e., whether the oil or gas ‘window’ has been reached).
Pollen, spores and marine phytoplankton are widely used to date sediments. When oil migrates from its source to a trap it may during that journey pick up pollen and spores from the rocks through which is passes. A study of this material can aid studies of fluid migration.
The study of fossil plants has contributed to our understanding of the evolution of past atmosphere, and in particular of the changes in oxygen and carbon dioxide (CO2). Oxygen and carbon dioxide levels are controlled by the activity of plants. The evolution of plants that photosynthesize caused major changes in the biosphere–atmosphere cycle. Green plants utilize sunlight to extract carbon from the atmosphere and (through photosynthesis) to incorporate the carbon into their organic ‘skeletons’. Oxygen is released as a by-product of photosynthesis. Oxygen concentrations in the atmosphere began to rise significantly in Precambrian times. Shortly after the spread of plants on to the land during the Devonian period, oxygen concentrations reached their present levels. We know that oxygen levels must have been between 15 and 35 per cent through the period from the Devonian to the Recent because of evidence from the charcoal record of wildfire from the latest Devonian onwards.
Carbon dioxide is also controlled by the global carbon cycle. When they decay, plants release carbon dioxide (CO2) into the atmosphere. However, if sufficient carbon is held back in the lithosphere, in sediments or peats, then the overall CO2 levels will decline. This can become self-perpetuating, because low CO2 levels lead to global cooling. (CO2 is an important greenhouse gas.) This in turn can lead to the formation of ice caps, a lowering of sea level, and a spread of land vegetation, increasing carbon draw-down. There have been many fluctuations in CO2 levels during the Earth's history. The stomata (pores) on plant leaves vary with CO2 concentrations in the atmosphere, and changes in stomatal density can act as a proxy in interpreting ancient CO2 levels in the atmosphere.
Fossil plants have been widely used to interpret past climates. Five main approaches have been used.
Nearest living relative (NLR) In Tertiary plant assemblages the climatic ranges of the nearest living relative (either at generic or family level) can be used to make broad interpretations of past climates. The method has the disadvantage that it assumes that the present climate range of a particular taxon is the same as it was in the past.
Leaf physiognomy Several aspects of the shape and size of angiosperm leaves are related to climate. For example, leaves in the tropics are generally over 10 cm long, are tough and evergreen, and have entire margins and drip-tips. In contrast, temperate leaves are often less than 10 cm long, are deciduous, and have interrupted margins and no drip-tips. Calculations can be made using data from leaf assemblages to extract a climate record.
Growth rings In the tropics plants and trees grow continuously, their wood consequently shows no interruption of growth. Wildfires can cause fire scars, and in tropical fire-prone environments these may be regular. In temperate latitudes, because of the variation of light throughout the year, there are interruptions of growth which produce distinctive growth rings. These appear yearly and hence provide the basis for dendrochronology. The size of any growth ring and its regularity depend not only on latitude and hence the length of the growing season, but also on temperature; the largest rings form in warmer climates. Indices such as mean sensitivity are used to categorize growth rings in fossil plants for palaeoclimate analysis. Growth rings are known in fossil woods from the Devonian onwards, and trees from polar areas have been shown to have had strong growth rings in the Mesozoic.
Fossil charcoal Charcoal (as fusain) occurs widely in post-Devonian sediments and coals and forms as the result of wildfire (Fig. 1d). Wildfires can occur in many climatic zones, but they are most prevalent where there is a build-up of fuel and where there are periods of dryness and frequent lightning strikes.
Isotopes In fixing their carbon from carbon dioxide in the atmosphere, plants may use a variety of processes. Each of these incorporates different amounts of the stable carbon isotopes 12C and 13C. The most common photosynthetic cycle, known as C3, is common to most plants. Some plants that are found in drier environments and in highly stressed environments such as salt marshes use the C4 metabolic pathway. C4 plants characteristically produce carbon with a lighter δ13C isotopic signature. Studies of on the isotopic composition of soil carbon have been used to interpret climatic changes by means from C3 to C4 plants in the profile.
Biogeography and plate movements
Plant distribution is broadly controlled by climate. Climate is in turn controlled not only by latitude but also by height above sea level. Mountain ranges and oceans both provide barriers for the migration of plants. The movement of continental plates and the changing distribution of oceans and mountains, together with changing climates, have all contributed to the formation of distinct biogeographical regions on Earth today. The occurrence of distinctive palaeobiogeographical regions is recognized as a feature since the early evolution of life on land.
The study of such provinciality may provide useful data on the positions of former continents. In early Permian times the gymnosperm Glossopteris lived in the southern hemisphere temperate zone. Fossils of Glossopteris are now found widely in South America, South Africa, Antarctica, Australia, and India. This reinforces the idea that these continents were once joined together into a large southern continental landmass called Gondwana.
The development of plant palaeobiogeography through the late Palaeozoic has been widely studied, and increasing regionalization is seen as continents move and split up. Equally, when previously separate continental blocks with different fioras (e.g. the North and South China blocks) collide, their fioras intermix.
Plant fossils have been widely used for dating rocks. Macrofossil plants have traditionally been used for dating non-marine clastic sequences. Fossil plants zonations have been erected for the Upper Carboniferous and have been used to correlate coal measures sequences across Euramerica.
Microfossil plants have been applied to biostratigraphical problems since the 1970s. Spores and pollen occur widely and abundantly in a variety of sediments and environments. Several types of zonation scheme have been used, but the concurrent-range biozone defined by the simultaneous presence of two or more species) is the most common. Of particular use is the fact the spores and pollen can be obtained from a variety of facies (including clastic rocks and even volcanic ashes). Spores and pollen also have the advantage of being transported into marine environments. In some cases instances (e.g. in the late Devonian and Carboniferous), megaspores have been used successfully to date and correlate sequences. In marine sediments marine phytoplankton (floating plants) are widely used in biostratigraphy. Palaeozoic sequences yield abundant and diverse acritarchs (organic-welled microplankton of unknown affinity). Mesozoic and Tertiary sequences yield diverse dinoflagellates (a class of unicellular algae), which have been particularly useful in correlating the marine Jurassic rocks of the North Sea. Other plant microfossils, including diatoms and coccoliths (remains of unicellular algae) have been found useful for dating in Cretaceous and younger sediments.
Fossil soils are known as palaeosols. Plant roots leave distinctive traces in sediments: the activity of plants rooting on a rock substrate has the effect of altering the rock physically and chemically. Organic decay releases acids that chemically attack rock and produce a soil. Study of the texture and zonation of palaeosols can yield data on the temperature and humidity of the environment.
In some instances smaller plant fragments and organs may be reworked and redeposited into younger rocks. This commonly happens with palynomorphs (organic microfossils), including megaspores, with coal particles and with fossil charcoal (fusain). The erosion and transport history of the early Tertiary Thanet beds of southern England were deduced from the presence of megaspores of various ages, including some from the Carboniferous and Mesozoic.
Techniques in the study of fossil plants
Fossil plants can be found in a wide variety of terrestrial and marine rocks, not only sedimentary but also igneous. Methods of collection vary according to the nature of their preservation and whether qualitative or quantitative data are required. Most geologists are familiar with the occurrence of plant compression fossils in bedded sedimentary rocks. Both the part and the counterpart of the fossil need to be collected, since they will yield complementary data. Quantitative collection may require the counting of specimens per bed or the excavation of a uniform bedding area. Palynological samples may be just rock samples in which no macroscopic plants are visible.
Fossil wood is generally easy to find and collect. Permineralized plants (other than large pieces of wood) are often difficult to recognize in the field. Stems, leaves and, fructifications may not look well preserved in the field, and as they occur in cemented rocks that do not break along bedding planes they can easily be missed. Even volcanic ashes and some lavas may contain such material. For example, the Lower Carboniferous green volcanic ashes and agglomerates from Oxroad Bay in East Lothian, Scotland contain abundant anatomically preserved plants that were overlooked by those studying the geology. With such material it is necessary to collect large blocks of material for subsequent treatment in the laboratory.
Coals can be sampled in a variety of ways, either by taking a single-channel sample of the coal or by taking more detailed samples. The choice will depend on the thickness of the coal and the nature of the investigation. Some studies require continuous sampling of every lithology; others require regular spaced samples.
Isolated plant compressions are also commonly found, as are permineralized plants, in marine sediments, even in some instances associated with ammonoids. These are often overlooked.
Fusain (fossil charcoal) is rarely collected, for it is often mistakenly believed to be poorly preserved. Under the scanning electron microscope blocks containing charcoal commonly show exquisite preservation (Fig. 1e).
Serious study of plant fossils may require extensive preparation. Any parts of the specimen buried in the rock are carefully dug away with a fine mounted needle under a microscope using a technique known as dégaugement. Organic material can be removed and treated chemically for microscopic study. Permineralized fragments can be embedded in resin and sectioned or peeled. Bulk maceration (physical and chemical treatment) can be used to break down the rock matrix and release plant tissues. This is widely used for the study of plant mesofossils and microfossils. Plant mesofossils (in the size range 180 μm to 5 mm) include small seeds, megaspores (Fig. 1c), and plant cuticle. Such material has been referred to as ‘fossil tea leaves’.
Permineralizations and petrifications
Petrifications, where no organic material remains, are usually prepared as thin sections or polished and studied under reflected light. For permineralized plants, the peel technique is useful; this provides a section of the original plant material embedded in a cellulose acetate sheet (Fig. 1b).
Fossil plants are studied using a wide variety of microscopic techniques. For many initial investigations, a low-power binocular microscope may be all that is required. For more detailed investigations of macrofossil, mesofossil, or microfossil plants there are many techniques that can be employed, ranging from traditional high-power transmitted microscopy, using normal or UV light, to more specialist microscopes such as laser scanning microscopy, scanning acoustic microscopy, infrared microscopy, and cathodoluminescence microscopy. Some relatively simple techniques, such as the use of polarized light to study macroscopic and compression fossils or the immersion in liquids, are also useful.
More powerful electron microscope techniques are widely used. These include scanning electron microscopy (which is now possible for large specimens by using an environmental chamber) and transmission electron microscopy, in which details of the plant ultrastructure can be observed.
Plant fossils have been subjected to a number of chemical studies, on the organic material itself, on the permineralizing cements, or on the enclosing sediments.
Organic geochemical studies Organic compounds extracted from fossil plants can be studied using such techniques as gas chromotography–mass spectrometry. The compounds identified include biomarkers: compounds that can be related to identifiable biological chemical ‘species’. This technique is particularly useful for interpreting the source of oil. More resistant plant tissues containing macromolecules can be studied using a range of techniques, including spectroscopic methods such as solid-state 13C nuclear magnetic resonance and fourier transform micro-infrared spectroscopy and by destructive techniques such as flash pyrolysis–gas chromotography–mass spectrometry. Information has been obtained on the chemistry of sporopollenin, the walls of spores and pollen, on wood chemistry, on fossil cuticles, and on algal cell walls. Studies on both modern and fossil plants are helping to interpret changes that occur in the organic structure of the plant tissues during burial diagenesis. As indicated above, isotopic studies are also undertaken. Claims have been made for the occurrence of DNA in several fossil plant specimens. The records are sparse and none has yet been repeated by independent laboratories. There is thus still doubt as to whether such proteins can survive more than a few thousand years.
Inorganic geochemical studies Enclosing sediment and permineralizing cements can be subjected to geochemical analysis to help understand their origin. Destructive geochemical methods are commonly combined with petrographic methods (thin sections, stained sections, cathodoluminescence) to interpret the history of rock cementation. Chemical analysis may include the use of energy-dispersive X-ray analysis (EDAX) linked to a scanning electron microscope or such techniques as X-ray spectroscopy or inductively coupled plasma mass-spectroscopy (ICP), which yields data on element abundances. More recently, stable isotopes of oxygen and carbon have been used to interpret the origin and history of cementation of calcareous and siliceous plant permineralizations.
Geochemical analysis of the enclosing sediment has been used to ‘fingerprint’ plant horizons with the aim of identifying the origin of loose specimens, whether from the field or in old museum collections.
Useful data have been obtained from taphonomic experiments, which include studying the transport and burial of plants in modern environmental settings and in laboratory experiments using flume and settling tanks. The natural landscape has also been used for fire experiments. Charring experiments have been undertaken under controlled laboratory conditions. Plant decay has been subjected to controlled experiments both in the laboratory and under ‘natural’ conditions.
Diagenetic processes have also been studied experimentally from compression experiments using artificial and natural materials; in addition there have been attempts to permineralize and petrify plants.
The occurrence of fossil plants
Facies distribution of fossil plants
Fossil plants occur in a wide variety of terrestrial clastic rocks. They are also equally common in some limestones, volcanic rocks, and several marine facies. It is not only the spectacular plants that are worth collecting; less ‘promising’ fragments can yield abundant data of high quality.
In general, rocks that are grey, black, or green yield organic material, whereas brown and red rocks do not, the plant material being oxidized. Even in red rocks some plant impressions may be found. Also as a rule coarser clastic rocks contain fewer fossil plants then finer clastic rocks, but large logs and even small charcoal fragments can be found in such rocks. Likewise, while most lavas and ashes do not contain plant material, others do and they may be spectacularly preserved. Marine rocks commonly contain plant fossils but they are outnumbered by more common and spectacular shelly invertebrate fossils. Some rocks are made up entirely of plant fossils: for example, coals, diatomites, and even chalk, made up of the coccoliths, algae that secrete calcium carbonate.
Stratigraphical distribution of plants and plant groups
Microscopic algae are known from some of the oldest rocks on Earth. Indeed, many hundreds of Precambrian localities are now known to yield plant fossils. Macroscopic terrestrial plants are not known until the late Silurian, and it was in the early Devonian that vascular plants diversified. Early land plants were small, some only a few centimetres high, and did not grow more than a metre tall. All early plants reproduced by spores. These early plants belong to a group known as Psilophyta, which is probably a group of simple plants, some of which, but not all, were descendants of a common ancestor (paraphyletic). The Lycophyta (clubmosses such as the living Lycopodium and Selaginella) also evolved in the early Devonian. The Mid–Late Devonian saw the evolution of the tree habit, with the evolution of secondary growth.
The seed habit evolved among plants in the late Devonian in the gymnosperms. Seed-ferns or pteridosperms evolved rapidly in the Carboniferous; the first conifers are known from the Upper Carboniferous. The Carboniferous saw the spread and diversification of three groups of spore-bearing plants: the lycophytes, the sphenophytes (horsetails), and the ferns. The Lycophytes became particularly important in tropical peat-forming areas and were particularly well represented by the tree-like forms such as Lepidodendron. The sphenophytes were also represented by small herbaceous forms as well by as arborescent taxa such as Calamites. Ferns diversified in the Carboniferous, but much of the fern-like foliage found in the clastic rocks associated in the coals belongs to pteridosperms or seed-ferns such as Neuropteris and Alethopteris.
Major climate changes at the end of the Carboniferous caused the extinction of many of the spore-bearing groups. Seed plants evolved rapidly during the Permian and late Mesozoic. Conifers continued to expand both geographically and ecologically, and they also diversified. Cycads and the extinct bennettites (cycadeoids) became important, and in some areas dominant, elements of the vegetation. Seed-bearing glossopterids characterize the Permian of the southern continents. The Triassic saw the continued evolution of seed-plants with the first occurrence of Ginkgo (a living genus) and also the Peltasperms.
A major change in terrestrial vegetation took place in the Cretaceous with the evolution of the angiosperms, or flowering plants. Some of these plants are wind-pollinated; others are insect-pollinated, and the close interaction between the angiosperms and insects has often been cited for the success of both groups.
By the end of the Cretaceous, several groups of plants, such as the bennettites, peltasperms, and pteridosperms, had become extinct. The Tertiary saw the rapid diversification of the angiosperms. Grasses did not evolve until the mid-Tertiary, and grasslands were consequently not abundant until Mid–Late Tertiary times.
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