Fossil blobs dating back 310 million years ago may not be jellyfish

Fossil beds dating back 310 million years ago

The fossils are from northern Illinois' Mazon Creek fossil beds dating back 310 million years. A site with outstanding fossil preservation is known as a Lagerstätte, and Mazon Creek has earned this designation.

Millions of anemones and other animals were quickly buried in muddy sediments in an old delta, which permitted the detailed preservation of the Mazon Creek soft-bodied invertebrates.

“These fossils are better preserved than Twinkies after an apocalypse. In part, that’s because many of them burrowed into the seafloor as they were being buried by a stormy avalanche of mud,” said study co-author James Hagadorn, an expert on unusual fossil preservation at the Denver Museum of Nature and Science.

The form known to local recreational fossil collectors as "the blob" is by far the most prevalent fossil at Mazon Creek, claims Plotnick, who adds that because these blobs were so dominant and frequently nondescript, many were discarded or sold for a few dollars at nearby flea markets. Yet, almost all of the specimens in museum collections were given by amateur collectors.

It looks like a jellyfish, but actually, it's not

“It quickly became obvious that not only it wasn’t a jellyfish, but turned upside down, it was clearly an anemone, probably one that burrowed into the seafloor. The ‘bell’ was actually an expanded muscular foot used to wiggle the anemone into the seafloor,” Plotnick said.

The anemone's barrel-shaped body served as a sturdy "curtain." Another species of fossil jellyfish resembling a flower was a unique anemone stomped as an aluminum can from top to bottom.

The varying times that dead anemones rested on the seafloor before being buried, the researchers said, are what caused the wide range of preservation documented in Essexella species. The snail was a scavenger on the carcasses rather than a predator.

Also, the scientists proposed that an animal resembling Essexella may have created a common trace fossil from the same time previously thought to be an anemone burrow. They speculate that due to Essexella's abundance, it may have resided in vast aggregations on the ocean floor.

Study abstract:

Sea anemones (Actiniaria) are among the rarest of recognized fossil organisms, even rarer than jellyfish. Here we demonstrate that the most abundant fossil in the Pennsylvanian Mazon Creek Lagerstätte of Illinois, Essexella asherae, is an infaunal or semi-infaunal anemone. Essexella is redescribed based on a taphonomic analysis of thousands of specimens, as well as associated medusae and trace fossils. Specimens of Essexella (also known as the ‘blobs’) were long believed to be medusae, but we reassign Essexella to the order Actiniaria and reinterpret the putative jellyfish Reticulomedusa as the pedal or oral disc of Essexella. We also implicate Essexella as a producer of Conostichus, a widespread plug-shaped trace fossil that occurs in coeval strata in the same region. Radiate structures comparable to the bases of Conostichus and the ichnofossil Bergaueria, as well as the pedal discs of modern anemones, characterize Reticulomedusa. Bona fide medusae are present in the Mazon Creek biota, and include Anthracomedusa turnbulli and Octomedusa pieckorum, whereas the soft-bodied fossil Lascoa mesostaurata is referred to Problematica.