Paleontologists flip the script for anemone fossils

Billions of sea anemones grace the floor of Earth’s oceans—yet they are among the rarest fossils because their squishy bodies lack easily fossilized hard parts. Now a team of paleontologists has discovered that countless sea anemone fossils have been hiding in plain sight for nearly 50 years.

In a recently published paper in the journal Paleontology Papers, Roy Plotnick and his colleagues at the University of Illinois at Chicago report that fossils long interpreted as jellyfish were actually anemones. To do this, they simply turned the ancient animals upside down.

“Anemones are basically upside-down jellyfish. This study shows how a simple shift in a mental image can lead to new ideas and interpretations,” said Plotnick, UIC professor emeritus of earth and environmental sciences and lead author of the study.

The fossils are from the 310-million-year-old Mazon Creek Fossils in northern Illinois. Mazon Creek is a world-renowned Lagerstätte, a term used by paleontologists to describe a site with exceptional fossil preservation. An ancient delta enabled the detailed preservation of Mazon Creek’s soft-bodied organisms because millions of anemones and other animals were quickly buried in muddy sediments.

“These fossils are better preserved than Twinkies after an apocalypse. That’s partly because many of them were pulled to the sea floor as they were buried by a stormy mud avalanche,” said study co-author James Hagadorn, an expert on the unusual fossil preservation. at the Denver Museum of Nature and Science.

By far the most common fossil at Mazon Creek is the form known to local recreational fossil collectors as “the blob,” according to Plotnick, who notes that such blobs were so common and often unlimited that many were discarded or sold for a few dollars. at local flea markets. However, professional collectors donated almost all specimens to museum collections.

In 1979, Bradley University professor Merrill Foster made the first detailed study of the droplets. He decided they were jellyfish and named them Essexella asherae. Foster reported that these jellyfish had a unique feature found in no living jellyfish. This was a hard “curtain” that hung from the umbrella-like bell – the upper part of a jellyfish – similar to a skirt that covered their arms and tentacles, due to their barrel-like forms.

Plotnick said Foster also suggested that a small snail sometimes found in the skirt was a predator, similar to the snails that prey on jellyfish in modern oceans.

In their new paper, paleontologists took a new look at Essexella by examining thousands of museum specimens.

“It quickly became apparent that not only was it not a jellyfish, but it turned out to be clearly an anemone, possibly one that burrowed into the sea floor. The ‘bell’ was actually an enlarged muscular leg used to propel the anemone to the bottom.” Plotnik said.

The hard “curtain” was the barrel-shaped body of the anemone. Another fossilized jellyfish species that looked like a daisy turned out to represent rare anemones squished from top to bottom, like one might step on an aluminum can.

“Although most of these fossils are preserved as decomposed blobs that look like a piece of used chewing gum on the pavement, some specimens are so perfectly preserved that we can even see the muscles that the anemones used to bend and contract their bodies.” , the study said. co-author Graham Young, a fossil jellyfish expert from the Manitoba Museum.

The researchers explain that the wide variety of preservation seen in the Essexella samples was due to the different lengths of time the dead anemones sat on the sea floor before being buried. The snail was not a predator, but a scavenger of corpses.

“When jellies like Essexella wash up on the beach, they become a veritable beach buffet, nibbled on by snails and other creatures like we see in this fossil deposit,” Young said.

The team also suggested that a common trace fossil from the same period, long thought to be an anemone burrow, was made by an Essexella-like animal. Because Essexella is so abundant, it may have lived in large aggregations on the sea floor, they report.