Scientists in the U.S. are trying to figure out what a fossil dubbed "Godzillus" used to be.
The 68-kilogram fossil recovered last year in Kentucky is more than two metres long. To the untrained eye, it looks like a bunch of rocks or a concrete blob.
Experts are trying to determine whether it was an animal, mineral or a form of plant life from a time when the Cincinnati region was under water.
Scientists at a Geological Society of America meeting viewed it Tuesday at the Dayton Convention Center in Ohio.
"We are looking for people who might have an idea of what it is," said Ben Dattilo, an assistant professor of geology at Indiana University-Purdue University Fort Wayne.
Scientists say the fossil is 450 million years old. University of Cincinnati geologist Carl Brett said it's the largest fossil ever extracted from that era in the Cincinnati region.
"This is the ultimate cold case," said Ron Fine, the Dayton, Ohio, an amateur paleontologist who spotted the fossil on a hillside last year and gave it its name.
"Like Godzilla, it's a primordial beast that found its way to the modern era," Fine said. Now 43, he's been collecting fossils since age 4, and said he saw part of this one on a hillside off Kentucky 17 nearly a year ago.
"Most fossils around here are small, the size of your thumbnail or your thumb," he said. "This thing's huge."
He said it could be an early form of seaweed or kelp.
"This one has us stumped," said David Meyer, another UC geology professor. Fine shared his find last September at a meeting of the Dry Dredgers, a group of amateur geologists.
Meyer, who wrote a book called A Sea Without Fish about the era, said the fossil has intricate patterns that remind him of "goose flesh. Some of its surface also looks like scales. But this thing is not boney. It is not a fish."
He guesses it could have been something like a sponge.
"Cincinnati was covered by a sea, 100 to 200 feet deep," Meyer said. "Primitive shellfish lived in it. But no fish."