Virtual Fossil Museum Name: ______Fossilization - How Fossils Form

Virtual Fossil Museum Name: __________________________ Fossilization - How Fossils Form Fossils The term “fossil” is used for any trace of past life. Fossils are not only the actual remains of organisms, such as teeth, bones, shell, and leaves (body fossils), but also the results of their activity, such as burrows and foot prints (trace fossils), and organic compounds they produce by biochemical processes (chemical fossils). Occasionally, inorganically produced structures may be confused with traces of life, such as dendrites. These are called pseudo-fossils. The definitions below explain the types of fossils found in the context of fossilization processes. You will find there is some overlaps in the terminology commonly used in paleontology and geology. Body Fossils The processes of fossilization are complex with many stages from burial to discovery as a fossil. Organisms with hard parts such as a mineralized shell, like a trilobite or ammonite, are much more likely to become fossilized than animals with only soft parts such as a jellyfish or worms. Body fossils of plants and animals almost always consist only of the skeletonized or toughened parts because soft tissues are destroyed by decay or by scavengers. Even hard parts can be destroyed by natural processes such as wave action or can be eaten or destroyed by other organisms like fungi and algae. Many species of plant and animal fossils are known only from their fragments. The remains of an organism that survive natural biological and physical processes must then become quickly buried by sediments. The probability for an organism to become fossilized increases if it already lives in the sediment. Those on the sea floor are more readily fossilized than those floating or swimming above it. Catastrophic burial with a rapid influx of sediment is necessary to preserve delicate complete animals such as crinoids or starfish. This explains why most crinoids, for example, are found only as stem pieces. Since crinoids were not usually buried quickly, their hard stem parts are far more frequently found as fossils. Observations of rare living crinoids have shown that they will rapidly disarticulate within a few days of death. Rapid burial, in contrast, prevents this disintegration, and thus explains a few localities where beds of delicate crinoids, starfish and brittle stars are preserved in their entirety. Thus many factors affect the chances for fossilization. The common processes occurring after burial include chemical alteration or replacement and compaction. Most marine invertebrates have calcareous skeletons containing calcium carbonate (CaCo3) that occurs in one on two crystal forms, calcite or aragonite. Aragonite is comparatively unstable and will convert to calcite or dissolve over time. As a result, aragonite becomes progressively rarer in older rocks. If the calcite or aragonite is dissolved away the result is a fossil being preserved as a mold or cast. In contrast, the original calcite or aragonite might be replaced with other minerals such as silica or pyrite or a similar iron-containing mineral called hematite. Calcium phosphate is another important, but less common, skeletal material occurring in some arthropods, inarticulate brachiopods and conodonts. Apatite, a calcium phosphate mineral, is also found in bones and teeth of vertebrates. These are the most common replacement minerals other than calcite. Soft Tissue Fossilization It is possible to infer a certain amount about the missing soft parts of fossils by comparing them to living relatives. Information can be so deduced from traces such as muscle scars left on a skeleton for example. The preservation of soft parts is rare but scattered examples are found throughout the world at various localities. Examples of soft part fossil preservation include the frozen Siberian mammoths and ground sloth fur and feces. Preservation in this case is dependent on local climatic conditions and such fossils are unlikely to survive any significant amount of geological time since climate changes. Older soft bodied preservation due to protection from decay and scavenging under anaerobic conditions (without oxygen) especially at low temperatures rarely occurs. Decay is slowed allowing more time for soft parts to be buried and preserved. Despite the rarity, there are hundreds of fossil sites worldwide where soft tissue parts are preserved. Such a fossil site is known as a Lagerstätte. Examples include the early Cambrian Burgess Shale of Canada and Maotianshan Shales of China, the Carboniferous Mazon Creek Formation, and the Eocene Messel Pit of Germany. Simple Burial Limy shells and plant remains often lie in the ground without much change. Cones, stems, stumps, and fern roots in peat bogs have been known to exist up to 40 million years with little change, except for some discoloration and slight decay. The remarkable preservation in these peat bogs is due to the high concentration http://www.fossilmuseum.net/fossilrecord/fossilization/fossilization.htm#bodyfossils Virtual Fossil Museum Name: __________________________ of tannic acid. Mollusk shells, sand dollars and sea urchins with ages ranging from a few thousand years to 75 million years have been known to survive with little change, except the loss of color. Occasionally, ammonite fossils show the original iridescence present when they were alive. The clam fossil of the Spanish Point Formation of California is a good example of shells that have undergone little change. Imprints Imprints are simply the external molds of very thin organisms, such as leaves and trilobites. They are often found in rocks such as sandstone, shale and volcanic ash. Trilobites of the Marjum Formation in Utah are often found as impressions. Trace Fossils or Ichno-fossils Trace fossils, also called ichno-fossils are structures preserved in sedimentary rocks that record biological activity. Though trace fossils are often less interesting to view, they are very important because they represent both the anatomy of the maker in some way as well as its behavior. Sedimentary structures made by empty shells rolling along the sea floor are thus not trace fossils because they do not represent the anatomy of their maker. Trace fossils include footprints, tracks and trail marks, burrows, borings, feeding marks, and coprolites (fossilized droppings). The conditions under which animal remains are found differ from those favoring the survival of trace fossils they produce. The two are rarely found together. It is often difficult to determine what animals made a trace fossil with confidence. Traces made by wildly different animals can be very similar in appearance. Therefore trace fossils are classified according to the activity producing them rather than the animal that made them: resting, crawling, feeding, dwelling, etc. The majority of these trace fossils were made by infaunal (living in sediment) animals, especially deposit feeders like worms. Worm trails in Cambrian sediments are common. Bird tracks at some locations in the Green River Shales of Wyoming and Utah are also common. Trails, Tracks and Burrows Tracks, trails and burrows are a particular form of trace fossil. These traces range from the worm trails to dinosaur tracks and even the footprints of Stone Age people. The tracks of worms, amphibians, reptiles and birds are common at some localities. A great variety of invertebrate’s tracks have been found. Trilobite and even insects tracks are found commonly at some localities. Burrows of worms, snails and crabs are known as well as their petrified remains. Worm trails are often found in the Cambrian Wheeler Shale of Utah. Bird tracks are common in the Green River Formation of Utah in some locations. Freezing Freezing is a type of preservation in which an animal falls into a crevasse or pit and remains frozen. Such ideal remains are rare and almost always never very old. Animals have been restricted to ice age rhinoceros and hairy mammoth. These remains have preserved bone, skin, muscle, hair and even internal organs. Drying or Desiccation Remains of animals that have been found thoroughly dried include camel, ground sloth and even marsupial wolf. These remains were found in caves in arid and semi-arid areas of the Southwestern United States, South America, New Zealand and Australia. The dried dung of cave dwelling giant ground sloths have also been found in caves. Petrification Petrification is a geology term denoting the processes by which organic material is converted into stone or a similar substance. It is approximately synonymous with fossilization. Petrified wood is the most well-known result of this process. Petrification takes place in two related ways, replacement and Perimineralization, described below. Replacement Replacement takes place when water dissolves the original hard parts and replaces them with mineral matter. This chemical action may take place slowly, reproducing the microscopic structures of the original organism. Bone, shells and wood are often well preserved in this manner. The most common replacement minerals are calcite, silica, pyrite and hematite. The snails of the Green River Formation in Wyoming are often replaced by silica, a variation of quartz. The ammonites and goniatites of Europe and North Africa are commonly replaced by hematite, which is an iron mineral similar to, but more stable than pyrite. When the original hard parts are http://www.fossilmuseum.net/fossilrecord/fossilization/fossilization.htm#bodyfossils Virtual Fossil Museum Name: __________________________

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