Paleontology -Sean Tvelia

Paleontology -Sean Tvelia- Introduction Paleontology is the area of geology that examines and interprets fossils. Fossils are an important key in identifying and dating past environments and can also be extremely useful for geologic mapping. In order to gain a better understanding of how fossils fit into geologic time, it is necessary to study their taxonomic differences so that we can better understand their evolutionary development and placement within the geologic record. In Latin the word fossilis means “dug up.” In the nineteenth century when geology was still a young science, the word fossil was used for virtually anything that was pulled out of the ground- including artifacts and even Egyptian mummies. However, today, geologists define fossils as the naturally preserved remains or traces of plants or animals. Since Egyptian mummies were preserved by humans and not through natural phenomena, they cannot be considered fossils. The preservation of life in the form of a fossil is not a simple or common process. Whether or not a plant or animal is preserved depends greatly on the environment in which it lives as well as the durability of the organism’s body. The potential for preservation may be greatly increased if the organism being preserved contains durable, or hard, internal or external body parts. The soft tissue of organisms is often destroyed by decomposition or by predators and is therefore seldom preserved. Also the rate at which an organism is buried will greatly affect the chances of preservation. Organisms that live in areas which experience little deposition will be exposed to the elements for greater periods of time and will therefore be more likely to decompose or scavenged by other organisms. Organisms that die in areas of high deposition, such as marine environments, will often be buried in a period of hours to days. Modes of Preservation After an organisms dies, and even after it is preserved, a number of processes can occur that may alter the original organism’s remains. In rare instances no alteration can take place and the original parts, including soft body tissue, can be preserved intact. This occurs most often in cold or arid climates where an organism can be frozen in ice or desiccated in dry desert air. Desiccation is the process by which all water is removed from the organism. Examples of these processes include frozen mammoths that have been retrieved from ice in Siberia and early Egyptian remains that have been dried in the dry desert sands. Although unaltered fossils can be found, these processes are extremely rare. Most fossils encountered in the field and in this course have been altered in some way from their original form. Some of the more common alterations that occur in fossils are permineralization, replacement, carbonization, recrystallization, and the production of molds and casts. Permineralization occurs when minerals that have been dissolved in groundwater begin to precipitate out of solution and fill the small pore spaces of hard remains. Most commonly these minerals are calcium carbonate, silica, pyrite, or dolomite. Petrified wood is an excellent example of permineralization. Under some conditions the original component of the hard body material (skeletal remains or shell) may dissolve, and new minerals will be deposited in the spaces left by the dissolved material. This process results in the complete replacement of the original material by a new mineral. Common replacement minerals include calcium carbonate, quartz, and pyrite. Carbonization is the complete change through chemical processes of the original plant or animal remains to a thin layer of carbon that outlines the original body shape. In some cases, such as in the Burgess Shale, this process allowed the preservation of details in soft body tissue. Recrystallization is the process by which less stable forms of a mineral structure convert into more stable forms. This is most often seen in the shells of clams and snails. These shells are typically made of a form of calcium carbonate known as aragonite; over time the aragonite structure will slowly turn into the more stable calcite. Many of the fossils that you will observe in the field are actually the mold or cast of the organism. A mold is formed when the hard body material of the buried organism is removed by dissolution. The void that is left behind will then retain the shape of previously existing body. This is similar to making a handprint in freshly poured concrete. Molds can express either the internal or external structure of the object. If, after the initial object has dissolved, the mold becomes filled with sediment or mineral deposits, the original shape of the object can be regained in the form of a cast. The fossil record is not a complete snapshot of life for a given time period. As stated earlier, fossilization depends greatly on environmental conditions. Whether or not an organism is preserved is more of a serendipitous coming together of natural conditions. However, environments that favor the process of fossilization, such as marine environments, do exist and because of this the fossil record is biased toward marine organisms. Furthermore, since it is much more likely that organisms containing hard body parts are preserved, very little is known about soft bodied species. When studying paleontology it is important that we recognize these biases so that we can more correctly interpret life history. Environmental Indicators Environmental factors have a great influence over the life forms that live in particular areas; for example, animals that live in cold environments tend to have more fur or thick layers of blubber to insulate the body than animals that live in warm tropic areas. This same concept can also be applied to organisms that live in different areas of marine environments; over time the bodies will evolve in such a way to best protect the organism in the given environment. This can best be seen in the high-energy shorelines and in calm, quiet water conditions. In the quiet water conditions, organisms do not have to protect themselves from wave bombardment or rapidly moving sediment, and so shells may develop quite differently than those of animals that live near the shoreline. One problem with using fossils as indicators of paleoenvironments is the fact that organisms are often transported and reburied after death. Whether transport is due to scavenging or some other geologic force, they typically will show signs of abrasion or wear so one must be careful to recognize these signs. To further prevent incorrectly identifying an environment, it is better to use more than one fossil from a given location. Typically geologists will collect an assemblage of fossils from a given formation. Using Fossil Assemblages to Determine Age One important reason for identifying fossils is the ability to use them to determine the ages of the strata the fossil was found in. The Law of Fossil Succession tells us that fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content. Unfortunately, since each fossil in a given assemblage may have lived through different time periods, it is impossible to use just one fossil to determine age. Therefore it is necessary to identify each fossil within the assemblage. In order to date a given strata, a number of assumptions must be made. The first assumption is that all fossils being identified were collected from the same formation and therefore represent one period of deposition. Secondly, it must be assumed that all fossils must have lived together and represent the life forms present during one time interval. Finally the complete geologic range of each fossil must be known. Once all fossils of a given assemblage have been correctly identified and the geologic range of each fossil has been established, the information can be plotted on a geologic time diagram as shown below. Even though each of the fossils present in the assemblage may have been present through multiple time periods, there is only one period in which all fossils were present together:the Pennsylvanian. Questions 1. How might the shell of an animal that lives within a wave-torn shoreline differ from that of one that lives in a quiet bay? 2. If most snails build their shells out of calcium carbonate, what type of fossilization has occurred in a fossil snail composed of silica? 3. Explain why more fossils are found of marine organisms than land organisms? 4. What assumptions can be made regarding a past environment which formed a fossiliferous stone composed of mostly broken shells. 5. Why are fossils rarely recovered from recrystallized or metamorphosed rocks? Use of Fossil Assemblages in Determining Age Use the description of the following fossil assemblages and the geologic time diagram to answer the following questions 1. The following fossils were recovered from a quarry in the Midwest, determine the age of the strata Fossil 1: Silurian-Permian Fossil 2: Miss-Permian Fossil 3: Devonian-Pennsylvanian Fossil 4: Devonian-Cretaceous Fossil 5: Mississippian-Permian Fossil 6: Devonian-Mississippian A. During what period was this rock layer deposited? B. Of the fossils listed which would make the best index fossil? Explain. 2. The following fossils were recovered from a limestone bed. Using the geologic time diagram and the charts on the following pages determine the time period in which the limestone was deposited. 1. Imitoceras – figure 8 2. Bellerophon- figure 6 3. Grammysia- figure 7 4. Hindia- figure 2 5. Streptelasma- figure 3 6. Sphaerospongia- figure 2 3. Use the figures at the end of this lab to determine the date of the fossils described on the following diagram. Then, using those dates correlate the layers in the two stratigraphic columns and determine the missing time interval represented by the unconformity (represented by the dark black line).

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