A 300-Million-Year Identity Crisis

Tully Monster 3D model. Photo from the Illinois State Museum. 
What exactly is a Tully Monster?

“We still don’t know what he is – or was,” then-Field Museum curator Mary Carman, a leader in the effort to designate the Tully Monster as the official Illinois State Fossil, told the Chicago Tribune in 1987. “He makes a brief appearance in the fossil record and suddenly disappears. He didn’t evolve into anything, nor was anything ancestral to him, but he is a one-in-a-million fossil. That is why he would make a great state fossil for Illinois. People come here from all over the world to hunt for their own Tully.”

So, what’s a Tully Monster? And where did it come from?

Hundreds of millions of years ago, the globe appeared much different than the one we are familiar with. Over those millennia, the continents have crashed together, moved apart and gradually shifted into the world map we know today. This shifting also caused relics to be left behind in unexpected places, like the strange-looking marine mammal that somehow wound up in the middle of North America, and is today Illinois’ State Fossil.

During the Pennsylvanian subperiod of geological history, between 323 million and 298 million years ago, the area now known as Illinois was mostly underwater, or part of a swampy coastal plain. During this time, a very peculiar animal swam in these seas. It was symmetrical, with a distinct anterior and posterior (something akin to a snout and a tail), but had no skeleton or shell. Its two eyes were mounted on stems protruding from either side near the front of its body. It was approximately a foot long with vertical fins and a proboscis with a jaw holding as many as eight miniature teeth.

The creature, whatever it was, was likely carnivorous and lived in open water, but it would sometimes find itself washed ashore in shallow, marshy water. Much like that which characterized modern-day Illinois during this time period.

At least some of these creatures died in the mud of these marshes and began to decompose. But before they could do so, a chemical reaction occurred between the creature’s body and the groundwater which surrounded it causing a crust of the iron-rich mineral siderite to form. The siderite, or ironstone, then made an impression in the mud, which was preserved alongside those of many other similar creatures who met their demise on that marshy shore.

Over time, the continents continued to move, the sea dried up, land closed in and the preserved impressions sank deeper underground. At the same time, other plant and animal life which had become buried in these swampy areas began the extremely long process of being converted to coal. In fact, it became one of the richest coal seams in the world, with its Illinois component stretching from the southern tip of the state, through about two-thirds of the state’s land area northeastward toward Lake Michigan. This included a section of land in present-day Grundy County along Mazon Creek.

It would be three hundred million years later when the fossilized impressions of the aquatic creature again saw daylight.

As glaciers melted 10,000 or so years ago, Mazon Creek was formed, and began cutting through the hardened shale southwest of Lake Michigan, exposing material which had been buried for millions of years. In the 1850s, some of these fossils came to the surface along Mazon Creek, and the area; which today includes portions of Grundy, Will and Kankakee Counties, as well as the Mazonia-Braidwood State Fish and Wildlife Area; quickly developed an identity as being a prime ground for fossil-hunting. But it was also home to an abundance of coal, and by the 1920s strip-mining operations were tearing open the ground along Mazon Creek and depositing off to the side large piles of shale and other material deemed to be waste.

These “spoil heaps” turned out to be a jackpot for fossil hunters. They now had access to much more material to sift through in search of fossils, and over the years many Illinoisans took advantage of the opportunity, including a pipefitter and amateur archaeologist from Lockport named Francis J. Tully.

Tully was searching for fossils one day in 1958 when he found an impression of a creature he had never seen before.

Photo from the Illinois State Museum
“I found two rocks that had cracked open from natural weathering,” he told the Chicago Tribune in 1987, “They held something completely different. I knew right away. I’d never seen anything like it. None of the books had it. I’d never seen it in museums or at rock clubs. So I brought it to Chicago to the Field Museum to see if they could figure out what the devil it was.”

Tully took his find to George Langford at the Field Museum. Langford and a museum curator named Eugene Richardson were familiar with the Mazon Creek fossils and were accustomed to such inquires. But this one had them stumped. The two men worked in a museum with 381,000 fossils of invertebrates, but neither had ever seen this creature before. Lacking any other name for it, they soon christened it Tully’s Monster.

Richardson later Latinzed the nickname, and the fossil became Tullimonstrum gregarium, but the original moniker stuck. Scientific papers were written about the find and the bizarre creature in 1969, 1979 and 1991, but the Tully Monster defied easy classification.

“We could not even decide which phylum to put it in,” Richardson wrote years later. “And that was a serious and embarrassing matter.”

That dispute continues today, as scholars disagree on whether the monster is a kind of arthropod, a worm, a mollusk, a snail or even possibly a vertebrate after all. This lack of consensus is yet another characteristic that makes the Tully Monster unique, and a point of local pride in the communities along Mazon Creek, including at least one which has a restaurant named after the creature.

Paleontologist Victoria McCoy of the University of Leicester, part of a team which sought in 2016 to determine exactly what the Tully Monster is was more blunt than Richardson. “I would rank the Tully Monster just about at the top of the scale of weirdness,” she said.

Since that day more than six decades ago, more Tully Monster fossils have been found, but nowhere else in the world besides Illinois. This too is a matter of some local pride, and in the late 1980s, Carman and other geologists began an effort to have the Tully Monster designated as Illinois’ official State Fossil.

In 1987, legislators in the 85th General Assembly took up the cause. The legislation passed that year proposed presenting the question of the State Fossil to Illinois schoolchildren in a yes-or-no referendum. Governor Jim Thompson vetoed the bill, calling it “un-American” to present no other fossils for the students to choose from. In 1989, the 86th General Assembly tried again, this time passing a bill with no referendum included. Thompson signed it into law on September 1, 1989. Illinois then joined a group of about two dozen states which officially designated a State Fossil, but the move came too late for Francis Tully, who had passed away in 1987 at the age of 75.

Thirty years later, the dispute over what the Tully Monster was might have been put to rest by a 2016 article in the journal Nature in which McCoy and her colleagues determined that the creature did in fact have a spinal cord and was, therefore, a vertebrate fish like an eel or a lamprey. It allowed scientists to place the creature within its proper phylum.

Tully Monster reconstruction by the Field Museum. 

The Mazon Creek area has proved to be a bonanza for paleontologists, who have identified more than 500 plant and animal species from the Pennsylvanian period. The Illinois State Museum maintains an extensive collection database. The area is described as a “Lagerstatten,” from the German for “mother lode,” which is used to describe those rare areas of the earth where such a diverse collection of plant and animal specimens are so well preserved. Specimens from Mazon Creek can be found in collections all over the country, but the largest is at the Field Museum.

Maybe the debate over what the Tully Monster is has finally been settled, or maybe not. The one word that seems to occur most frequently in the reporting on the creature is “unique.”

Unique in the animal kingdom.

Unique to the state of Illinois.

And definitely the most unique State Fossil in the nation.