Trapped in time: Fossils of North America

StoryImage( ‘/Images/Story//Auto-img-118409277927316.jpg’, ‘Photo by Dr. Robert A. Hedeen’, ‘The Burgess Shale Formation lies under a belt of snow in the Canadian Rockies of British Columbia.‘);
StoryImage( ‘/Images/Story//Auto-img-118409286112644.jpg’, ‘Photo courtesy of Falcon-Lang‘, ‘A 300 million-year-old fossilized rain forest was recently found in a coal mine in Illinois—what scientists call a natural Sistine Chapel—revealed sprawling tree trunks and fallen leaves. One of the fossils coating the mine’s ceiling was a pteridosperm, an extinct seed-producing fern-like plant. ‘);

The bedrock under our part of Illinois is made of a limestone called dolomite that was laid down 450 million years ago when this region was covered by a warm, tropical sea, during what geologists call the Silurian period. Our limestone foundation has a rich variety of the fossilized remains of creatures from that time.

The remains of trilobites, cephalopods, brachiopods, snails, crinoids, various worms, and a host of other forms may be readily found by the fossil collector wherever the bedrock is exposed. Soft tissued animals, however, were poorly fossilized in this type of rock.

Another rich fossiliferous area in Illinois is the famous Mazon Creek formation south of the Rockford area. The formation is composed mainly of shale (geologists call it Francis Creek shale) and dates from the Carboniferous period 350 million years ago.

This was the age when plants conquered the land in the form of giant fern trees, horsetails and others. It was during this period that a large portion of the world’s petroleum was formed along with vast quantities of coal. Unfortunately, the practice of strip mining for coal has wreaked havoc with the environment in some areas, but progress has been made in restoring stripped areas to their former condition.

Paleontologists come from far and wide to study the organisms preserved in the Mazon Creek shale. The formation is extensive and includes parts of Grundy, Kankakee, Will, and Livingston counties.

A much more recent, famous fossil area is the La Brea Tar Pits some 7 miles west of the old city limits of Los Angeles. This area is from the Pleistocene era, only some 2 to 3 million years ago.

Petroleum formed millions of years ago found its way to the surface and created the tar pits, which served as effective traps for animals that happened to fall into them. When a petroleum spring comes to the surface, evaporation of the volatile oils produces first a pool of sticky tar and then one of viscous asphalt.

This happened at a place called Rancho La Brea, and many mammals and birds were trapped in such a pool. They are among the best preserved fossils we have of animal life during the Pleistocene Era. Because the city of Los Angeles has grown up around it, it no longer entraps the wild fauna of the region. The L.A. Fire Department, however, is sometimes called out to rescue an individual who has gotten his feet stuck in the asphalt.

Perhaps the most interesting and important fossil area today is called The Burgess Shale Formation. This formation is between Mts. Field and Wapta, at an elevation of 3,000 feet, in Yoho National Park in the Canadian Rockies of British Columbia.

The paleontological treasure was named a World Heritage Site in 1981. It was discovered in 1909 and is dated at about 530 million years old, making it much older than other important fossilized areas. The 530 million date places it in the ancient Cambrian period of geological history, when all of the major animal groups we know today evolved with explosive force.

Originally, the area was covered by a warm, shallow sea, which gradually disappeared. Mud collected in low areas and trapped much of the fauna, and eventually the mud was transformed into shale. In this manner, many soft-bodied animals not previously known to science were preserved in remarkable condition.

Many new species, genera, and families of animals have been described from specimens taken from the Burgess Shale. This knowledge has greatly increased our understanding of the evolutionary pathways many of our present day animal groups have taken.

Fossil collecting in the area by the amateur is strictly forbidden, but one can take a ranger-guided tour of the area after a strenuous hike of several miles uphill.

If this brief account of the Burgess Shale Formation has stimulated an interest in any of my readers, I refer you to a book written by the late paleontologist and evolutionary scholar Stephen Jay Gould. W.W. Norton Company published his Wonderful Life: The Burgess Shale and the Nature of History in 1989.

What we learn from the past enhances our understanding of the future.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

from the July 11-17, 2007, issue

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