The Tullimonstrum gregarium, commonly known as the Tully Monster, is an extinct soft-bodied organism that lived approximately 300 million years ago. This enigmatic creature has puzzled paleontologists because its body plan is unlike any other known animal, fossil or living. Its unique appearance and resistance to classification led to its designation as the official state fossil of Illinois. The long-standing scientific controversy surrounding its place on the tree of life has only recently begun to find a tentative resolution.
Discovery and the Mazon Creek Fossil Record
The history of the Tully Monster begins with Francis Tully, an amateur fossil collector who discovered the first specimen in 1955 near Braidwood, Illinois. Tully brought the unusual fossil to the Field Museum of Natural History, where scientists were unable to classify the bizarre creature. It was informally nicknamed the “Tully Monster,” a name that stuck when it was formally described as Tullimonstrum gregarium in 1966.
All known specimens originate from the Mazon Creek Lagerstätte, a world-famous fossil site in northeastern Illinois. This region is unique because of the specific conditions that allowed for the preservation of soft-bodied organisms. When the animals died, their bodies were rapidly buried in fine mud that formed ironstone concretions, or hard nodules of rock. This rapid encasement protected the delicate tissues, creating detailed impressions of the creature inside.
The Uncanny Appearance
The Tully Monster was a free-swimming marine predator that grew to an average length of about 8 to 14 inches. Its body was torpedo-shaped and segmented, terminating in a triangular or spade-shaped tail fin used for propulsion. This streamlined structure suggests it was an active hunter in the ancient coastal waters of what is now Illinois.
The most striking features of the animal were located on its head. A rigid, horizontal bar extended sideways from the main body, with a dark, bulbous organ interpreted as an eye on each end, giving it a hammerhead-like profile. Extending forward from its head was a long, narrow proboscis, a flexible appendage used for grasping.
At the very end of this proboscis was a specialized structure resembling a jaw or claw, equipped with eight sharp, small teeth. This feeding apparatus indicates the Tully Monster was a carnivore, likely using its grasping proboscis to capture small prey in the water column.
The Great Taxonomic Debate
For decades following its discovery, the Tully Monster was considered one of the most perplexing “problematic fossils” in paleontology. Its strange combination of features defied placement into any major, established animal phylum, fueling intense scientific argument. Scientists proposed numerous possible identities, classifying the creature at various times as a type of worm, a mollusk, or even a relative of the extinct arthropod Opabinia.
The difficulty lay in the organism’s lack of universally recognized traits. It did not possess the hard shells of mollusks, the jointed legs of arthropods, or the typical internal structures of vertebrates. The unique arrangement of its eyes on a transverse bar and the presence of a proboscis with terminal teeth were features that simply did not align with existing animal groups. The creature’s seemingly unrelated anatomical components made it impossible to confidently link it to any living group.
Current Scientific Consensus
The decades-long mystery appeared resolved in the mid-2010s when new studies proposed the Tully Monster was a primitive vertebrate. Researchers analyzed over 1,200 specimens and identified features such as a structure resembling a notochord, the flexible rod that forms the rudimentary backbone in all chordates. Analysis of the eyes also found two distinct shapes of melanosomes, pigment-containing organelles previously thought exclusive to vertebrates.
This evidence placed the Tully Monster as a stem-lamprey, a relative of modern jawless fish. This classification was based on interpreting internal structures, including the possible presence of gill pouches and cartilaginous elements known as arcualia, which are found in lamprey skeletons.
However, the debate remains unsettled, as subsequent studies have strongly challenged the vertebrate hypothesis. Researchers using advanced 3D scanning argued that the supposed notochord was not structured like a vertebrate’s. Furthermore, the creature displayed segmentation in its head that is not found in any vertebrate lineage. These counter-arguments suggest the Tully Monster is most likely an invertebrate, possibly related to non-vertebrate chordates like lancelets or a highly modified protostome.