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The word fossil is derived from the Latin fossilis, which means "dug up". Fossils preserve remains or activities of ancient organisms older than 10,000 years (Garcia & Miller, 1998, p. 14; Schopf, 1975, p. 27). Two major types of fossils are recognized. Body fossils reveal the structure of an organism, while trace fossils reveal the activities of organisms. There are many reasons to study fossils. The fossil record indicates that different life forms have existed at different times revealing the evolution of life on Earth. Fossils and rock types serve as clues to determining ancient environments. Finally, fossils are the most practical way of telling time in geology (Prothero, 2004, p. vii). How do fossils form and how are they classified into different preservation types?
Taphonomy or
How Fossils Form
A
Fossil represents evidence of past life that is found in
the Earth’s
crust. Taphonomy ("laws of burial") is the term used to
describe the process that results in the formation of a fossil.
Taphonomy
or
the transition
of an organism or part of an organism from the biosphere to the
lithosphere
is accomplished in basically two steps. The process of burial
or entombment is referred to as biostratinomy. After burial or
entombment, diagenesis begins; the conversion of sediments or other
deposits
to rock. Biostratinomic and diagenetic processes destroy most
traces of organisms. The extent of preservation depends upon what
happens
during biostratinomy and diagensis. The biostratinomy
and diagensis associated with a fossil can reveal much
about the
environment
in
which the
organism lived. In other words, how fossils form can often provide
clues to past environments.
Lagerstatten
Fossilization often occurs as a result of rapid burial, usually by water-borne sediment, followed by chemical alteration. Rapid burial and specific chemical environments help to reduce decomposition from bacteria and fungi. Decomposition, erosion, deposition and rock formation are processes that often destroy soft tissue, so it is the hard parts of organisms such as shells, bones and teeth, which are most often preserved. Occasionally conditions exist that allow for preservation of organisms in environments that rarely produce fossils. Exceptional conditions may also help to preserve soft tissue or impression of soft body parts.
Fossil deposits with soft-bodied organisms well preserved or with terrestrial animals, such as dinosaurs in the Morrison formation are termed Lagerstatten. Lagerstatten is a German word used in mining that denotes a particularly rich seam and has been adopted by paleontologist to signify these rare fossil deposits because they give us a window into past environments seldom preserved in the fossil record (Selden & Nudds, 2004, p. 7). Two types of fossil lagerstatten are recognized. Deposits that contain vast numbers of fossils represent Concentration Lagerstatten. The preservation may not be exceptional, but the great numbers can be very informative. Conservation Lagerstatten contain fossils with soft body preservation, impressions of soft tissue or fossils of well-articulated skeletons without soft tissue preservation. Conservation Lagerstatten are particularly important because they provide knowledge of soft-bodied organisms, allow paleontologists to reconstruct paleoecosystems, and give insights into the morphology and phylogentic relationships of organisms (Nudds & Selden, 2008, pp. 8-9).
Fossiliferous
Rocks
The majority of fossils are found in sedimentary rocks. Organisms
become trapped within sediment layers due to the action
of water, wind or gravity.
Fossils can sometimes
be found in metamorphic rocks formed from fossiliferous
sedimentary rocks altered by heat and pressure. Fossils can even
be found in
igneous rock created from lahars or pyroclastic flows that
entomb trees or other organisms. Click
on the pictures to explore the major types of preservation.
Preservation Types
 Molds & Casts |
 Imprints & Impressions |
 Permineralization |
 Compressions & Carbonization |
 Amber |
 Chemical Preservation |
 Recrystallization |
 Freezing |
|
|
|
 Pseudofossils |
Bibliography |
Garcia, F.A. & Miller, D.S. (1998). Discovering Fossils: How to Find and Identify Remains of the Prehistoric Past. Pennsylvania: Stackpole Books. Nudds, J.R. & Selden P.A. (2008). Fossil Ecosystems of North America: A Guide to the Sites and Their Extraordinary Biotas. Chicago: University of Chicago Press. Prothero, D.R. (2004). Bringing Fossils to Life: An Introduction to Paleobiology [2nd edition]. New York: McGraw-Hill. Schopf, J.M. (1975). Modes of Fossil Preservation. Review of Palaeobotany and Palynology, vol 20: pp. 27-53. Selden P. & Nudds, J. (2004). Evolution of Fossil Ecosystems. Chicago: The University of Chicago Press. |