TITLE INTRODUCTION DETRITAL ROCKS CHEMICAL ROCKS
BIOCHEMICAL ROCKS ENVIRONMENTAL CLUES CREDITS
graded bedding cross bedding ripples mud cracks
raindrop impressions bioturbation tracks and trails
Fossils are the remains of once-living organisms that became preserved within sediment and sedimentary rock. Both plants and animals can form fossils, but usually only their hard, skeletal components are preserved. Scavengers and bacteria typically destroy softer tissues. Plants and animals that die on land are not readily preserved unless they are rapidly buried in sediment, perhaps during a flash flood or some other disasterous event. Plants and animals that live and die in the ocean are far more likely to be preserved because the ocean is an environment where sediment is constantly settling to the ocean floor, covering and preserving the remains of plankton, clams, fish and whales.
There are several modes of preservation, including preservation of the fossil in an unaltered condition, replacement or recrystallization of the original skeletal material, or dissolution of the original skeletal material which can lead to the formation of a mold or cast of the fossil. These different modes of preservation result in varying qualities of detail preservation of the fossil organism's surface shape and internal and external characteristics. The more details that are preserved, the more geologists can learn from the fossil about its behavior and habitat. With regard to the modes of preservation and the quality of detail preservation, unaltered fossils show the finest details, followed by replacement, recrystallization, and finally by typically crude preservation of molds and casts.
The following images show a variety of fossils in different modes of preservation.
LAND FOSSILS On the left is a dinosaur leg bone, Dinosaur National Monument, Colorado. The second image is a fossilized turtle shell. The third image is a fern leaf fossil cast. Image 4 shows a leaf fossil impression in the limestone wall rock of the Getty Museum, Los Angeles.
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row A The first image is fossiliferous limestone containing a variety of recrystallized fragments of brachiopods, trilobites and bryozoans. Image 2 shows a microscopic image of a similar limestone displaying closeup views of the fossil fragments. The third image is also a fossiliferous limestone, but the dominant fossils are brachiopod valves that are recrystallized. The last image is a fossiliferous limestone rich in recrystallized gastropod fossils.
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row B The first image is an urchin minus its spines, which tend to fall off soon after death. This specimen is partially recrystallized, and is filled with sedimentary rock forming an internal mold within. Image 2 shows a large recrystallized coral. This specimen was photographed in the desert of Morocco, a very different environment from the shallow ocean in which the coral colony once lived. The third image is of a trilobite whose exoskeleton has been replaced by a carbon film. The fourth image is of a recrystallized trilobite.
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row C The first image shows a cluster of ammonites whose original aragonite shells have been completely replaced by pyrite. The second image is of small gastropods that are partially replaced by pyrite. The third image contains an ammonite and a gastropod from the previous images. Note the metallic luster from the pyrite. (The fossils in this row were collected by Ms. Julie Draper.)
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row D These fossilized fish skeletons were collected from ancient lakebed deposits in Wyoming. Note the fine details preserved by the replacement of the original skeletal material with hematite.
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