Fossilized “clumps” found in Illinois were a 310 million year old puzzle.
According to the criteria of
Paleontologists decided that it was a strange jellyfish called Essexella asherae. But the creature’s anatomy did not resemble that of any modern jellyfish.
Roy Plotnick, a paleontologist at the University of Illinois at Chicago, turned an Essexella specimen upside down while conducting research. Immediately, the true identity of the seemingly amorphous mass began to take shape.
What scientists thought was a free-floating jellyfish turned out to be an entirely different ocean creature.
Essexella fossils date back to the Carboniferous period, when parts of northern Illinois hovered just above the equator.
A local river delta emptied into the sea, creating a network of brackish wetlands that were home to sea scorpions, centipedes and primitive amphibians. Many of these creatures were buried by mudslides, protecting their remains from scavengers and decay.
In the 19th century, coal miners began excavating an area known as Mazon Creek, and fossils appeared in their waste piles.
Collectors have been finding the remains of these creatures in the Mazon Creek fossil beds for over a century. Essexella fossils turned up by the thousands and were often sold at flea markets or discarded.
Scientists published the first detailed scientific description of these masses in 1979. The Essexella fossils are composed of two structures — a textured, barrel-shaped portion and a smooth bulb. Researchers hypothesized that the textured area represented a skirt-like curtain that enveloped the jellyfish’s tentacles. The rounded portion was the jellyfish’s bell.
But as time went on, this description struck many researchers as odd. “We were really pushing it to fit the jellyfish model,” Plotnick said.
No living jellyfish have curtains around their tentacles. Such a curtain would make swimming and feeding difficult. The uniform shape of the mass fossils also puzzled Plotnick.
“If it was a jellyfish that fell to the seabed, it would just scatter in all directions like an old mop on the floor,” he said.
Plotnick tried other hypotheses to explain the masses — such as gelatinous, barrel-shaped creatures called salpids or colonial congregations of tiny creatures known as siphonophores — but none of the new identities could explain Essexella’s anomalous anatomy.
In 2016, Plotnick and a colleague, James Hagadorn, a geologist
at the Denver Museum of Nature and Science in Colorado, investigated the mother lode at the Field Museum in Chicago, a Mazon Creek fossil repository that has the largest Essexella collection in the world.
One of the masses caught Plotnick’s eye. As he turned the fossil upside down, he was struck by the clarity offered by the change in perspective.
“It looked like the underside of an anemone,” Plotnick said.
When he looked at the anatomy of the sea anemone, ambiguous masses took shape. Instead of being the bell of a jellyfish, the rounded part of the Essexella was the base of an anemone’s burrow. The textured barrel was not a curtain enveloping tentacles, but the body of the anemone.
Last year, Plotnick, Hagadorn and their team re-described Essexella as an ancient anemone in the journal Papers in Palaeontology. Because of their soft bodies, ancient anemone species are known primarily from just a handful of poorly preserved fossils.
With thousands of relatively well-preserved Essexella specimens, this once-puzzling species is now the best-known anemone in the fossil record.
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