The fossil record is filled with unusual creatures, but few have perplexed paleontologists like Tullimonstrum gregarium. Known as the Tully Monster, this animal from 300 million years ago presents a unique biological puzzle. Its body plan is so strange, with a combination of features not seen in any other known organism, that it defies easy categorization. This has made its place in the tree of life one of paleontology’s most persistent mysteries.
Discovery and Description
The Tully Monster was discovered in 1955 by amateur fossil collector Francis Tully in Illinois’s Mazon Creek fossil beds. Unable to identify his find, he brought it to paleontologists who were also stumped. The creature was nicknamed the ‘Tully Monster’ and officially named Tullimonstrum gregarium in 1966.
These animals lived approximately 300 million years ago in shallow coastal estuaries. The fossils reveal a soft-bodied creature averaging about 15 centimeters in length, though some were as small as three inches. Its body was elongated and cigar-shaped, ending in a triangular tail fin.
Perhaps its most peculiar features were its eyes and mouth. Two long stalks extended from its body, each with an eye at the end. A long, flexible proboscis projected from its head, tipped with a claw-like structure containing what are believed to be teeth. This combination of features has no clear modern or extinct relatives.
The Classification Conundrum
The core of the Tully Monster mystery is its classification. As a soft-bodied animal, it lacks a skeleton, the feature most often used to classify fossils. For decades, scientists proposed it could be related to mollusks, arthropods, or various worms, but no theory gained widespread acceptance due to the ambiguous evidence.
A development occurred in 2016 when a study proposed that the Tully Monster was a vertebrate. Researchers argued a faint line in the fossil was a notochord, a primitive backbone. This evidence suggested Tullimonstrum was a jawless fish related to modern lampreys, seemingly solving the puzzle.
However, this conclusion was challenged. A 2023 study using 3D imaging presented evidence contradicting the vertebrate hypothesis, suggesting the features were misinterpreted. One analysis pointed to head segmentation not seen in any vertebrate, arguing it was an invertebrate. The classification of the Tully Monster remains unresolved.
Significance as a Fossil
Beyond the debate over its identity, the Tully Monster is important for what it reveals about life’s history. Its existence is owed to the exceptional preservation conditions of the Mazon Creek Lagerstätte. This Illinois fossil bed is one of the few places where impressions of soft-bodied animals were preserved in fine detail within ironstone nodules.
The creature represents a unique body plan that appears to have no surviving relatives. Its anatomy highlights the diversity of life during the Carboniferous period and suggests the existence of evolutionary branches that have since vanished. Researching such fossils provides evidence of animal forms not preserved in other fossil sites.
In recognition of its unique origins and scientific importance, Tullimonstrum gregarium was designated the official State Fossil of Illinois. This status celebrates the creature as a symbol of the state’s deep prehistoric past. Its fossils have been found only in Illinois, making it a local treasure.