Geology
Strange fossil remains mystery
Sunday, May 5, 2002
{Dale Gnidovec}
For The Dispatch
What was about 10 inches long, had fins on each side of its tail and a long claw-bearing tube on its head, and lived 310 million years ago? Don't know? Neither does anyone else. 

The animal I described is known from thousands of fossils found in the ironstone concretions at Mazon Creek, the famous fossil site just south of Chicago. Discovered in 1958 by farmer and amateur fossil collector Francis Tully, in 1966 it was given the scientific name Tullimonstrum gregarium -- Tully's abundant monster. At some localities its fossils make up as much as 18 percent of the animals found. The Tully Monster is found at no other site in the world, and nothing like it is known from the rest of the fossil record. 

The Tully Monster was an ocean-living creature; its fossils are found only in the marine Essex fauna at Mazon Creek. It was soft-bodied, much longer than wide, with indistinct segments. Individuals ranged from 3 inches to just over 1 foot long. 

The back half was reminiscent of a squid, with a bluntly pointed tail and a pair of fins. Towards the front of its body were two round eyelike structures connected by a stiff transverse bar. A long flexible tubelike proboscis extended from the head, ending in a clawlike structure. The claw bore as many as 14 small sharp stylets or "teeth.'' Some specimens have lighter-colored center lines that might represent the animal's digestive, nervous or circulatory system. 

I've got some good specimens of Tully Monsters at the Orton Geological Museum, if anyone wants to see them. 

Although the Tully Monster has been known to scientists for decades, its biological relationships remain a mystery. Some scientists have put it in its own group or phylum, but that seems like the easy way out. Others have suggested it was related to heteropod gastropods, which are swimming snails without shells. Still other paleontologists have suggested its relatives are found among the annelids, which include the common earthworm, or leeches. 

Some scientists have suggested relationships with the sipunculids or "peanut worms.'' Another group of worms with vague similarities is the echiurians, which look (to me anyway) like part of an intestine. 

The Tully Monster is the official state fossil of Illinois. I've never been that fond of trilobites, considering them little more than big pill bugs, but the Tully Monster makes Ohio's state fossil, the large Ordovician trilobite Isotelus, look positively beautiful. 

Dale M. Gnidovec is curator of Ohio State University's Orton Geological Museum. 

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