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 But researchers on Wednesday announced they have finally solved
the mystery.
They analyzed numerous fossils of the creature, named Tullimonstrum
gregarium, and determined it was not a segmented worm or a
free-swimming slug, as once hypothesized, but rather a type of
jawless fish called a lamprey.
"I would rank the Tully Monster just about at the top of the scale
of weirdness," said paleontologist Victoria McCoy of Britain's
University of Leicester, who conducted the study while at Yale
University.
It boasted a torpedo-shaped body, a jointed, trunk-like snout ending
in a claw-like structure studded with two rows of conical teeth, and
its eyes were set on the ends of a long rigid bar extending sideways
from the head. Up to about 14 inches (35 cm) long, it had a vertical
tail fin and a long, narrow dorsal fin.
 A sophisticated reassessment of the fossils determined it was a
vertebrate, with gills and a stiffened rod, or notochord, that
functioned as a rudimentary spinal cord and supported its body. The
notochord previously had been identified as the gut.
"I've always loved detective work, and in paleontology it doesn't
get much better than this," said paleontologist James Lamsdell of
the American Museum of Natural History in New York. "Our re-study of
the specimens has shown that it is a very strange lamprey, a group
of eel-like vertebrates that live in rivers and seas today."
Tullimonstrum shared its shallow marine environment with fish
including sharks as well as jellyfish, shrimp, amphibians and
horseshoe crabs.
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"It fed by grasping things with the proboscis (snout) and scraping
bits off with its tongue. We don't know what it ate or if it was a
predator or scavenger," McCoy said.
It is called the Tully Monster in honor of amateur fossil-hunter
Francis Tully, who first found it in Illinois coal-mining pits in
1958 and brought it to experts at the Field Museum in Chicago.
"This puzzle has been gnawing at paleontologists," said Field Museum
paleontologist Scott Lidgard, whose museum holds 1,800 specimens of
Tullimonstrum, the official state fossil of Illinois. "I was blown
away when the results started coming in."
The research was published in the journal Nature.
(Reporting by Will Dunham; Editing by Sandra Maler)
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