Wilderness Underfoot: The magnificent fossil jellyfish of Illinois

StoryImage( ‘/Images/Story//Auto-img-114728817030892.jpg’, ”, ‘They call it a "blob" – the most common fossil jellyfish of Illinois, Essexella asherae. Here it is shown as it would have appeared in life.’);
StoryImage( ‘/Images/Story//Auto-img-114728821030896.jpg’, ”, ‘A tiny jellyfish from the Coal Age – Octomedusa pieckorum (above) is often found fossilized in clusters. The “bell” of this creature was less than an inch across. Anthracomedusa turnbulli (bottom) is a relative of modern sea wasps — the most deadly living invertebrates. There is no reason to doubt that the tentacles of the ancient creature contained the same extremely toxic stingers that modern sea wasps have.’);
StoryImage( ‘/Images/Story//Auto-img-114728822130896.jpg’, ”, ”);

Today, the only way to find these mysterious creatures in our state is in aquariums or in fossils

Jellyfish are among the earth’s most ancient organisms. They have roamed the seas in abundance for at least 600 million years. Even so, their evolutionary record is spotty because they don’t fossilize well.

It is remarkable that a jellyfish could even fossilize at all. The gooey tissue of this animal would normally have been eaten by predators or decomposed by bacteria long before it could have been preserved.

Most fossils are composed of the hardest parts of prehistoric organisms, such as bones, teeth, shells, woody plant fiber or leathery leaves. For a jellyfish to be preserved would have required its sudden burial under fine sediment with just the perfect chemistry to prevent the animal from quickly rotting.

Apparently, those conditions existed in Illinois about 300 million years ago, when inland seas flooded the Midwest, with northernmost shores near the area that is now Kankakee, Ill. In ancient times, this zone was a river delta system much like the area where the Mississippi River spills into the Gulf of Mexico. Today, that fossil locality is known as the Mazon Creek region, and the richest source of jellyfish fossils in that region is near the town of Braidwood. There, rock hounds can find jellyfish fossils by the handful in designated collecting areas among the spill piles from long-abandoned coal mines.

No other fossil locality has as many jellyfish species as Illinois, and none can match the fine detail and quality of ours—although a quarry in Mosinee, Wis., has a superbly preserved “fossil beach” with much larger (though less detailed) jellyfish lying in rippling waves of sand. Only a few other locations exist in the world where very soft creatures such as jellyfish have been fossilized.

From the May 10-16, 2006, issue

Enjoy The Rock River Times? Help spread the word!