Fossils and Their Preservation

Laboratory I Fossils and Their Preservation Objective: The objective of this lab is to understand the processes that preserve organisms in the fossil record, and to appreciate the biological information that can be garnered from fossils. You will also learn the major types of preservation. Since some information is always lost during preservation, you should think particularly about what each type of preservation tells us about the once−living organism. 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. Paleontology 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. 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. Today, well−trained paleontologists will have extensive backgrounds in both the geological and biological sciences. Organisms become fossilized in a variety of ways. Each type of preservation carries different information about the once−living organism. Thus, an appreciation of fossils requires that one understand the processes of fossilization, and how each type of preservation may influence our view of the organism that produced the fossil. 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−from the moment that it dies or is shed until it is collected and curated for scientific study. Figure 1.1 illustrates some of the many taphonomic pathways an organism may take from its living community to the museum drawer. Fossil preservation can take place at a number of levels. Each level contains a different type of information. Microstructural or Cellular Level Not all organic compounds are equally resistant to chemical degradation and decay. The mineral component of shells and bones easily resists bacterial degradation. (Remember that bone and shell are composite materials made up of both mineral and organic components. Each component may behave differently in a given preservational environment.) Plant cell walls (cellulose and lignin) are far more likely to escape decomposition than are internal membranes and organelles, which are rich in protein, lipids and sugars. Preservation of cytological details has been reported in fossil plants, vertebrate bone and some microorganisms, 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 cells, can also be resistant to decomposition; examples include waxes, cutin (which comprises plant cuticle), chitin in arthropods and fungi, and sporopollenin (which forms the external shell of spores, pollen, and the resting cysts of some marine algae). Geoscience 390 Paleontology Figure 1.1: Some of the many possible fates of organisms and their parts as they enter the fossil record. Tissue Level Decay−resistant materials are distributed differentially throughout the bodies of organisms. Consequently, some tissues are more amenable to preservation in the fossil record than others. The microscopic shells of pollen and spores are the most common of all fossils. Their abundance is due first to sporopollenin from which they are constructed, and second to the abundance with which plants produce them. The calcite or aragonite shells of invertebrates are the next most commonly preserved fossils. Teeth are the most commonly preserved tissues in vertebrates, with bone coming in a distant second. It is unusual, but not impossible, for soft tissue (muscle, connective tissue, skin etc.) to be preserved. However, when these tissues are preserved, they can lead to quantum leaps in our understanding of ancient organisms. Organ Level Some organisms tend to break apart, both in life (organ senescence or dispersal in the case of plants) and after death (vertebrate bones are particularly vulnerable to being scattered if not buried immediately; clam valves commonly separate after death); dispersed parts may be transported before settling into the sediment to be buried and become fossils. Assemblages of fossils that are preserved close to where the organism originally lived 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. Organism Level Not all organisms in a given community are equally likely to find their way into the fossil record. Processes of fossilization often favor large parts (the bones of large dinosaurs rather than those of a shrew; limb bones rather than ribs) or parts composed of resistant materials (wood rather than flowers; mineral rather than chitinous shells). Also, organisms that live and die in sites with conditions favorable for preservation are more commonly preserved than are their counterparts growing far from water, anoxic environments, or active sedimentation. The differential hydrodynamic properties of plant or animal parts can lead to segregation of different parts in the fossil record. For example, waves or currents may winnow small, light shells from a death assemblage, leaving behind only large, clunky shells 2 Geoscience 390 Paleontology Environmental Level Fossil preservation depends on removing the organism from the zone of aerobic decomposition or physical destruction. This is most easily accomplished by burial. Consequently, sediment−rich coastal areas, epicontinental seas, swamps, deltas, lakes, lowland flood plains, and volcanic areas are good spots for fossilization. Arid regions and mountainsides are not likely candidates for fossil preservation (with the exception of exquisitely preserved plant material from Pleistocene packrat middens and mummified animals remains in the southwestern United States). Because some environments are more amenable to fossil preservation, the fossil record gives paleontologists a selective at past environments. All of these taphonomic factors influence the information that can be recovered from the fossil record. While taphonomic filtering does not preclude biological interpretation of fossils, taphonomy can introduce substantial biases into the record and influence our interpretation of the fossils, and thus our reconstruction of the organisms. It is, therefore, important always to keep in mind the mode of preservation when drawing any interpretations from fossils1 . Conditions Required for Plant Fossil Preservation Three conditions are required for the preservation of plant fossils: 1) Removing the material from oxygen−rich environment of aerobic decay; 2) Introducing the fossil to the sedimentary rock record (a.k.a., burial); and 3) "Fixing" the organic material to retard anaerobic decay, oxidation or other physical or chemical agents of destruction. Consequently, fossils are generally preserved in environments very low in oxygen (e.g., anaerobic sediment) because most decomposers (e.g., fungi, most decomposing bacteria and invertebrates) require oxygen for metabolism. Such sediments are commonly gray, green or black rather than red, a sedimentary signal of oxygen−rich conditions. The "fixing" requirement means that organism must fall into an environment rich in humic acids or clay minerals, which can retard decay by blocking the chemical sites onto which decomposers fasten their degrading enzymes. Plant material or bone can also be "fixed" by removing degradable organic compounds during the process of charring by wildfire. This is a common and spectacular mode of preservation for flowers. For mineral shells of many marine invertebrates, "fixation" may be as simple as the natural inversion of aragonite to calcite. Thus, burial may be the only key to the preservation of mineral shells. Furthermore, acid environments may actually harm both bone and shell because natural acids tend to dissolve the mineral carbonates. This creates the interesting but frustrating observation that fossil plants (which require acid conditions for preservation) and fossil vertebrates (which require basic conditions) are almost never found together. Fossils can be incorporated into the rock record in areas where sediment is being deposited, which usually, but not always, requires the presence of water. Consequently, streams, flood plains, lakes, swamps, and the ocean are good candidates for fossil−forming systems. Blowing (eolian) sand may bury vertebrates allowing good preservation, but this medium tends not to lock out enough oxygen to preserve organic material well. As you look at the various modes of preservation in lab, note the characteristics of the rock matrix in which fossil is preserved. Note color, grain size (i.e., sand, silt, clay), mineral composition (quartz, clay, mica, organic−rich, organic−poor), and any other unusual features. If you aren’t familiar

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