Three examples of the Tully Monster, on exhibit at the Field Museum in Chicago.
In 1958, amateur fossil collector Francis Tully was searching through an old mining pit in northeastern Illinois when he came across some odd-looking fossils in the Mazon Creek Formation layers of rock, about 300 million years old, from the Carboniferous geological era. The fossils were apparently an invertebrate, at most 6-8 inches long, with two triangular fins at one end and an elongated necklike projection at the other. Sometimes two budlike appendages could be seen at the sides.
When Tully took some of the fossils to the Field Museum in Chicago for identification, the entire staff was puzzled. The animal, now dubbed “Tully’s Monster”, did not resemble any known creature, living or dead. In 1966, Field Museum paleontologist Eugene Richardson formally described the fossils and assigned them the scientific name Tullimonstrum gregarium, but he didn’t even try to place them in any known group of animals, stating only, “While this obscure but plentiful animal is being studied, I prefer not to assign it to a phylum.” In 1969, after studying some more newly-found fossils, Richardson co-authored another paper with colleague Ralph Gordon Johnson, but they still could not decide to which biological group the Tully Monster belonged. “There is no compelling reason,” they wrote, “to assign Tullimonstrum to any of the known phyla.”
Richardson and Johnson were able to uncover a few more details about the animal’s structure. The two budlike appendages were actually eyestalks. The small headlike bulb at the end of the long necklike projection bore a small grasping claw with eight toothlike hooks at the end, but no mouth opening. It is likely that the Tully Monster used its claw to capture prey, then used its long flexible proboscis to carry the prey to its mouth, like an elephant’s trunk. The body appeared to be segmented. The entire animal was soft-bodied–there were no external plates or scales. Later fossils indicated that there may have been three fins at the rear, not two. Apparently the Tully Monster was an active swimmer, and was most likely a predator that may have pursued its prey, or may have used its long clawed proboscis to probe in the bottom muck in search of prey.
Based on the fossils that were found alongside it (jellyfish, sea cucumbers, and sharks) the Tully Monster seems to have been fossilized in shallow warm seas, and a number of land plant fossils found with it indicated that the shoreline was nearby, and terrestrial deposits were occasionally washed in. Several specimens had their back ends bitten off, apparently by predators, perhaps sharks. It is unknown if the Tully Monster was a normal inhabitant of the shallow areas in which its fossils have been found, or if it was an inhabitant of deeper water that was sometimes washed in to shallower areas after death (intact specimens are rare, which may indicate that the dead bodies were carried some distance by water and started to rot before they sank and were buried).
Since then, several other paleontologists have speculated about what groups the Tully Monster might be related to. In 1979, paleontologist Merrill Foster hypothesized that it was a mollusk, distantly related to whelks and limpets. More recently, one paleontologist speculated thatTullimonstrum was related to the Cambrian invertebrate Vetustovermis, but no one was quite sure which group Vetustovermis itself belonged to. Others opined that the Tully Monster might be related to Opabinia, a Cambrian member of the celebrated Burgess Shale fauna, 500 million years old. Opabinia also had triangular fins and a grasping claw at the end of a long proboscis, but no other fossils have ever been found that could link the 500 million-year old Opabinia with the 300 million-year-old Tully Monster.
Some of the speculations were downright silly. In 1965, self-styled British monster hunter Fredrick W. Holiday declared that the Loch Ness Monster was a form of giant worm, and the eight-inch-long Tully Monster was its ancient ancestor. Not surprisingly, nobody took his suggestion seriously. A short time later, a “retired British Colonel” sent a letter to the Field Museum declaring that back in the 20’s he had been stationed in Kenya and had seen similar-looking creatures living in swamps, called “dancing worms” by the local Kikuyu natives, that, he said, gave a type of milk. That report turned out to be a hoax.
In 1989, the Illinois State Legislature designated Tullimonstrum gregarium as the official State Fossil of Illinois, but neither the legislators nor the scientists had any idea what sort of animal it was. Despite the fact that over 100 specimens have now been found (though none of them outside Illinois), the Tully Monster remains just as much a scientific mystery today as it was when the first fossil was found nearly 60 years ago.