Remains of Life from Another Time and Place
What is a fossil?
Fossils are the remains of past life that have been preserved in sediments (such as clays and peats) or rocks (such as slates and sandstones). There are many ways in which an organism can be preserved as a fossil, including positive or negative impressions, internal or external molds or by complete mineralogical replacement (see Fossils: Window to the Past: An introduction for the general reader to types of fossils, conditions leading to fossilization, and the information contained in fossils [University of California Museum of Paleontology]).
A fossil may represent only one part of the organism, or it may preserve the majority of that organism's anatomy. Most fossils consist of hard body parts such as shells, teeth and bones. These body parts have the best chance of surviving in the rock record because they are the most resistant to weathering. Soft-bodied organisms can also be preserved (often as impressions), but this type of preservation requires a more specialized set of circumstances and is relatively rare in the rock record.
Other types of fossils, known as trace fossils, preserve the activities of some of these less represented soft-bodied forms. Some animal tracks, trails and burrows made over 400 million years ago are similar to those made by modern animals, such as marine worms and jellyfish, and allow us to infer their presence in the geological past, even if their bodies have not been preserved.
In all, a fossil provides a link between biology and geology that gives us a "snapshot" of the past.
Every fossil has a tale to tell about past life and the world in which it lived. Each fossil represents an organism that was once part of a living community, surviving in an environment that at times resembled our own present day settings, and at other times would have been very foreign to us. How do paleontologists and other geoscientists who study ancient life decipher these stories? What are the clues that allow them to reconstruct past environments and their inhabitants? Follow this link to find out how to bring the past to life.
Where are fossils found?
Fossils are most often found in sediments and sedimentary rocks. Igneous rocks, which form from molten rock, almost never contain fossils (some exceptions include fossil-bearing boulders encased in lava, but not completely melted and fossils preserved in ash layers and clastic debris flows produced by volcanoes). A significant number of fossils have also been recovered from metamorphosed sedimentary rocks. Metamorphic rocks have been subjected to extreme temperature and pressures that at times produce significant distortion and deformation of the rock unit. In areas where the metamorphism is weak, identifiable fossils have often survived, although they may show some of the wear and tear of these stresses.
A look at Maine's bedrock map shows the areas most influenced by intense metamorphism and igneous activity and explains why most of Maine's fossil bearing units occur in the northern and eastern parts of the state.
Certain types of sedimentary communities are better represented in the fossil record than others. This is particularly true when one compares the number of fossiliferous marine environments preserved to those from the terrestrial realm. The reason for this large bias is due to the fact that, overall, the marine basins represent areas of deposition, whereas the terrestrial regimes are predominantly areas of erosion. In the marine setting, sediments are carried in by rivers and streams and deposited on the seafloor, essentially locking them away from significant and frequent erosional events. There may be some reworking of the sediments within the basin due to local events (storms, underwater currents) or more regional ones (drop in sea level), but overall, these sediments remain in the basin, as do the organic remains they encase. In contrast, those sediments and associated organics deposited in a terrestrial setting, along the banks of rivers and streams or in freshwater lakes, often have to contend with a greater number of erosional agents and events, such as wind-driven weathering, subaerial exposure, dessication, higher energy regimes (leads to the destruction of organic remains) and often greater fluctuations in the local water chemistries (affects weathering potential). Furthermore, riverine and lacustrine settings are shorter-lived phenomena than oceans which means that these settings represent less time and therefore preserve fewer organisms than the longer-lived oceanic basins.
Environmental settings also play a role in the types of communities that inhabit them. For example, warm, clear, open ocean waters often foster the growth of species-rich carbonate communities, such as the coral reef settings of present day. The same held true for fossil communities in the past. Likewise, settings that were plagued with a certain degree of uncertainty (such as the intertidal mud flats, areas that received significant incursion of sediments or that were characterized by murky, cooler waters) were often inhabited by a less diverse community, one that had adapted to living in those conditions, but was not necessarily flourishing in that regime.
Maine's Fossil Record
Maine's fossil record covers a vast span of time. Fossils preserved in bedrock date from 500 to 360 million years ago. Most fossils contained in these rocks are marine animals such as brachiopods, gastropods, bivalves, corals, trilobites, and crinoids. Unfortunately there is a gap in Maine's fossil record, from about 360 million years ago to about 1 million years ago. No fossil-bearing rocks exist in Maine that have been dated to this period of time. Because of this, no DINOSAUR fossils have ever been discovered in Maine.
In the recent geologic past, immediately following the last Ice Age, as the glaciers melted the sea flooded coastal Maine well inland. Sand and mud from the glaciers blanketed this sea floor, and fossils preserved in this sediment are usually 11-12,000 years old. Most of the fossils preserved in this marine clay are shells of marine invertebrates (such as bivalves, gastropods, and brachiopods), although fossil wood and some vertebrates (including mammoth, walrus, and seal remains) have also been found.
Where to view Maine fossils
General Anatomy and Classification:
The Audubon Society Field Guide to North American Fossils. 1982. Knopf, Inc.
Boardman, R. S., et al., 1987, Fossil Invertebrates. Blackwell Science Publishers.
Feldman, R.M., ed. 1996. Fossils of Ohio. Ohio Division of Geological Survey, Bulletin 70.
General Paleontology and Geology:
Parker, S., 1990. The Practical Paleontologist. Fireside Book, Simon and Schuster, Inc., New York.
Dixon, D. 1992. The Practical Geologist. Fireside Book, Simon and Schuster, Inc., New York.
History of Life:
Fortey, R. A., 1998, Life: A Natural History of the First Four Billion Years of Life on Earth. Knopf, Inc.
Gould, S. J. (ed.), 1993. The Book of Life: an Illustrated History of the Evolution of Life on Earth. Norton and Co.
Churchill-Dickson, L., in prep. Maine's Fossil Record. Maine Geological Survey.
Other interesting fossil sites
The University of California Museum of Paleontology website contains extensive links to other paleontology sites on the web. A great place to start your exploration of paleontology.
Last updated on April 25, 2012