Paleontologists decided it was a strange jellyfish called Essexella asherae. But the creature’s anatomy was unlike any living jellyfish.
Roy Plotnick, a paleontologist at the University of Illinois Chicago, turned over an Essexella specimen while doing research.
Immediately, the true identity of the seemingly amorphous blob began to take shape.
What scientists thought was a floating jellyfish has turned out to be another ocean creature.
Essexella fossils date back to the Carboniferous period, when northern Illinois lay just above the equator. A local river delta emptied into the sea, creating a network of brackish wetlands home to sea scorpions, centipedes and early amphibians. Many of these creatures were buried by mudslides, protecting their remains from scavengers and decay. In the 19th century, coal miners began mining an area, known as Mazon Creek, for fuel, and fossils turned up in their tailings.
Collectors have been finding the remains of these creatures in the Mazon Creek fossil beds for more than a century. Most of the fossils are buried in sandstone nodules. Breaking through these concretions reveals the tracks of soft-bodied animals that look like bug-eyed aliens. In the 1950s, a local collector named Francis Tully discovered the track of a torpedo-shaped creature with a snout-like mouth. The taxonomic identity of the “Tully monster” has puzzled researchers ever since.
Essexella was equally puzzling. Thousands of nondescript fossils were found in Mazon Creek, often sold at local flea markets or even thrown away.
Scientists published the first detailed scientific description of the spots in 1979. Essexella fossils are composed of two structures: a barrel-shaped, textured region and a smooth bulb. Researchers postulated that the textured area represented a skirt-like curtain that enveloped the jellyfish’s tentacles. The rounded region was the jellyfish’s bell.
But over time, this description seemed strange to many researchers.
“We were shoehorning it into the jellyfish model,” says Dr. Plotnick.
No living jellyfish has curtains around their tentacles. Such a curtain would make swimming and feeding difficult. The uniform shape of the fossil spots also puzzled Dr. Plotnick. “If this were a jellyfish that had fallen to the seabed, it would splash in all directions, like an old mop on the floor,” he explains.
Dr. Plotnick tried other hypotheses to explain the blobs — such as gelatinous, barrel-shaped creatures called salps or colonial congregations of tiny creatures known as siphonophores — but each new identity failed to explain Essexella’s anomalous anatomy.
In late 2016, Dr. Plotnick and a colleague, James Hagadorn, a geologist at the Denver Museum of Nature and Science, investigated the mother lode of the stains. They were at Chicago’s Field Museum, a Mazon Creek fossil repository that boasts the largest collection of Essexella in the world. Most had been donated by amateur collectors too intrigued to leave the fossils on the scrap heap.
The scientists searched through drawer after drawer of stained specimens. They lined up several fossils on a table to photograph and compare. One of the stains caught Dr. Plotnick’s eye. As he turned the fossil, he was struck by the clarity provided by the change in perspective.
“It looked like the bottom of an anemone,” Dr. Plotnick said. “It was one of the few times I’ve had the classic eureka moment.”
As Dr. Plotnick reviewed the anatomy of sea anemones, the ambiguous spots became clearer. “Everything that worried us about it being a jellyfish now makes sense,” he said.
Rather than being the bell of a jellyfish, the rounded region of the Essexella was the base of an anemone’s burrow. The textured barrel was not a curtain enclosing tentacles, but the body of the anemone. Some specimens are so well preserved that scientists could see the muscles the anemone used to bend and contract.
Dr. Plotnick, Dr. Hagadorn and their team redescribed Essexella as an ancient anemone last year in the journal Papers in Palaeontology. Because of their soft bodies, ancient anemone species are known only from 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. Dr. Plotnick maintains that these animals once roamed the bottom of the Mazon Creek estuary.
It’s not the only time palaeontologists have turned the scientific script on its head to clarify the identity of a strange fossil. Reconstructing any ancient animal is tricky. After millions of years underground, fossils have been deformed, eroded, crushed, scattered and stamped into slabs of stone.
Sometimes, the mere preservation of a fossil is enough to baffle researchers. For decades, paleontologists puzzled over why armored dinosaurs called Ankylosaurs were almost always fossilized upside down. In 2018, a team postulated that the heavily armored animals often turned belly-up due to bloat when their carcasses floated in the sea.
And then there are the evolutionary oddballs that are hard to decipher no matter the orientation of their fossils. In 1869, paleontologist Edward Drinker Cope mistakenly placed the skull of an Elasmosaurus, a marine reptile, at the end of the creature’s tail instead of its elongated neck. Othniel Charles Marsh, another paleontologist, seized on Cope’s mistake, sparking a rivalry that would become the so-called Bone Wars.
Stranger still was Hallucigenia. For decades, researchers couldn’t figure out this creature, a worm covered in tentacles and stilt-like spines. Then they realized that its head was actually its tail, and vice versa. “That was fun and not a mere detail,” said Jean-Bernard Caron, a paleontologist at the Royal Ontario Museum and co-author of a 2015 study that determined a bulb on one end of Hallucigenia was the creature’s head. Better-preserved fossils of a related animal in China also revealed that Hallucigenia, like Essexella, was originally reconstructed upside down.
“It’s clear that Hallucigenia has been around a lot,” Dr. Caron said.
While Dr. Caron’s work helped straighten out Hallucigenia, a recent paper casts doubt on his 2012 description of Pikaia, an enigmatic worm-like creature from the Burgess Shale in Canada that was assumed to be an early precursor to vertebrates. The new study suggests that a mysterious tubular organ that researchers thought ran down Pikaia’s back (and might have been a primitive nerve cord) is actually the animal’s gut cavity, which runs along its belly.
“The animal is now upside down!” said Dr. Caron. Another fossilized creature got a new story by turning over.
#fossil #mystery #solved #twist
