The fate of most organic material produced by living systems is to be decomposed to carbon dioxide and water, and recycled into the biosphere. The circulation of elements through biogeochemical cycles indicates that decomposition is, indeed, efficient; however the presence of organic material in sedimentary rocks (e.g., coal, petroleum, dispersed organic matter, and fossils) shows that some organic matter - or its traces - escapes these cycles to be preserved in the rock record. The study of paleobotany relies on this preserved material - fossils - as evidence of past life. In the early history of modern paleontology, fossils were thought of mostly as static parts of the rock record ("sticks in mud"). This fostered description and classification as the main activities of scientific paleontologists. However, a shift in emphasis to thinking of fossils as "once-living organisms" gave paleontology a more biological flair and, more importantly, opened a new world of research questions. One tangible outcome of this philosophical shift is the movement of many academic paleontologists from Geology Departments to Biology Departments.
Plants become fossilized in a variety of ways. Each type of preservation carries different information about the once-living organism. Thus, an appreciation of plant fossils requires that one understand the processes of fossilization, and how each type of preservation may influence our view of once-living organisms.
The study of how organisms or their parts become fossils is called taphonomy. Taphonomy is literally everything that happens to an organism - or part of an organism, as is often the case with plants - from the moment that it dies until it is collected and curated for scientific study. Figure 3.1 illustrates some of the many taphonomic pathways a plant or plant part can take from it's living community to the museum drawer.
|Figure 3.1. Some of the many possible fates of plants or their parts as they enter the fossil record.
Plant fossil preservation can take place at a number of levels. Each level contains a different type of information.
Not all organic compounds are equally resistant to chemical degradation and decay. Plant cell walls (composed primarily of the polysaccharide polymer cellulose) are far more likely to escape decomposition than internal membranes and organelles, which are rich in protein, lipids and sugars. Preservation of cytological details has been reported in fossil plants, but occurrences are rare, and most reports of fossilized nuclei and organelles should be read with caution. Secondary compounds, such as those impregnating or covering cell walls, can also be resistant to decomposition; examples include lignin, waxes, cutin (which comprises plant cuticle), and sporopollenin, which forms the external shell of spores, pollen, and the resting cysts of some marine algae.
Decay-resistant materials are distributed differentially throughout the plant. Consequently, some tissues are more amenable to preservation in the fossil record than others. With respect to vascular tissue, xylem is often preserved, while phloem commonly is not. This is because the cell walls of xylem are impregnated with decay-resistant lignin, while phloem cell walls are cellulosic. Cuticle, composed of the resistant material cutin and various waxes, is more likely to be preserved than actual epidermal cells; however, the shape and distribution of epidermal cells, including guard cells, is faithfully preserved in cuticle (VG 1:2). Spores and pollen (VG 1:8), because of their resistant spore coats, are the most abundant and ubiquitous structural remains of vascular plants preserved in the rock record. Because they are easily preserved and found in great numbers, pollen and spores (palynomorphs) provide important quantitative data for vegetation reconstruction and a variety of paleoecological questions.
Plants break apart, both in life (organ senescence or dispersal) and after death; dispersed parts may be transported before settling into the sediment to be buried and become fossils. Assemblages of plant fossils that are preserved close to where their parent-plants originally grew are called autochthonous; assemblages that have been transported are referred to as allochthonous. Whether an assemblage is autochthonous or transported has obvious implications for what sorts of ecological interpretations we can make from it. Therefore, as you study each type of plant fossil preservation, think about whether fossils produced in these ways are autochthonous or not.
The differential hydrodynamic properties of different plant organs often results in segregation of different parts in the fossil record. Some deposits contain only palynomorphs (e.g., pollen, spores, and a few other do-dads), while others may contain only large chunks of wood, or scattered leaves. Reproductive organs, especially flowers, are relatively rare in the fossil record because they are delicate and full of energy-rich, easily broken-down compounds. Other reproductive structures are sometimes preserved, however, and constitute a valuable source of taxonomic and evolutionary information. Fruits and seeds often contain resistant tissues (e.g., sclerenchyma) that yield clues to the reproductive morphology and biology of ancient plants.
The differential dispersal or plant organs before they enter the fossil record creates an interesting problem for the paleobotanist: We have to reassemble our plants before we can think of them as unified organisms. Botanists studying living plants take for granted that they know which leaves, wood, and reproductive structures belong to a species. For the paleobotanist, many of our plants come as isolated parts that have to be reunited. Obviously, the strongest evidence that organs belong together is actual organic connection. For example, we find leaves and reproductive structures both attached to a single branch. Another example is isolating spores from sporangia that are preserved on their parent leaf. Somewhat weaker inference is possible when two organs found in isolation have a unique, derived feature. For example, particular glands on the epidermis of stem, petiole, foliage and seeds allowed F.W. Oliver and D.H. Scott (1904) to unite these separate organs into a single form: the Devonian seed plant Lyginopteris. The weakest inference for uniting organs into a single plant species concept is when a suite of organs always occur together. However, a variety of other processes could generate this pattern and so caution is advised.
Because we don't always know which leaves belong to which seeds when they are first discovered, we use the convention of form taxa. When organs are found isolated (not in organic connection), each type of leaf and seed is given it's own binomial name (genus and species name according to the International Code of Botanical Nomenclature), without making any assumption about what belongs to what. To use the example discussed by Oliver and Scott (1904), leaves were described as Lyginopteris (genus only for brevity), seeds as Lagenostoma, and stems as Lyginodendron. The similarity of the first syllable gives a hint that the describing paleobotanists (others besides Oliver and Scott) suspected some relationship, but were unable to make a strong inference link. The last syllable of each name gives a hint to the organ type: "dendron" = stem, "pteris" is often used for frond-like foliage, "stoma" = seed. However, after Oliver and Scott's recognition of the unique glands on Lagenostoma lomaxi and species in the other organ form genera, they were able to make the whole-plant link with greater confidence. The whole plant then takes the name of the organ first described, in this caseLyginopteris. When you are writing, take care to make clear whether you are talking about form taxa (organs) or whole-plants.
Form taxa are not unique to paleobotany. For example, the planktonic larvae of many marine invertebrates are commonly described as separate species when they are first discovered in the ocean. Only later when they can be reared in the laboratory can the link to their adult form be recognized. Similarly, the different life stages of many fungi are given different names because they have different physical forms and hosts. Only through detailed inoculation studies can mycologists work out which forms are members of the same life cycle. Since some fungi may have more than five discrete life cycle stages, this can be a long process. Similar problems exist for some marine algae and multiple-host parasitic organisms of many kinds. Even among well-studied vertebrates, some tropical birds have been described as separate species until they are observed to mate and rear young together.
Not all plants in a given community are equally likely to find their way into the fossil record. Processes of fossilization often favor large or woody plants with resistant tissues over small herbs. Likewise, wind dispersed pollen is much more common in the fossil record than pollen dispersed by animals. Also, plants growing in or near an area of preservation (e.g., riverbank or swamp) are more commonly preserved than their counterparts growing far from water or anoxic sedimentary environments.
Plant preservation depends on removing the organic material from the zone of aerobic decomposition. This is most easily accomplished by burying the plant. Consequently, swamps, deltas, lakes, lowland flood plains, and volcanic areas are good spots for fossilization (VG 1:9)(VG 1:12)(VG 1:13). Arid regions and mountainsides are not likely candidates for plant fossil preservation (with the exception of exquisitely preserved plant material from Pleistocene packrat middens in the southwestern United States.
All of these taphonomic factors influence the information that can be recovered from the plant fossil record. While taphonomic filtering does not preclude biological interpretation of fossil material, taphonomy can introduce substantial biases into the record and influence our interpretation of the fossils, and thus our reconstruction of the ancient plants. It is, therefore, important always to keep in mind the mode of preservation of fossils when making any interpretations from fossils.