At Mazon Creek in Illinois, collectors gather to search the riverside for ancient stones that hold a portal to another world. 300 million years ago, this place was a site where land met the sea and a Paleozoic delta played host to all forms of life and evolution.

This fossils are rare 300,000,000 year old imprints of a soft-bodied organism, the jellyfish Essexella. It is incredibly difficult to find fossils of soft creatures like jellyfish and cephalopods, as their remains decay quickly and leave behind no bones. The unique conditions at Mazon Creek though allowed for these fossils to form like fingerprints of history on stone.

Fossil Jellyfish Sizing:

Small- 1-1.75" in diameter

Large- 1.75-3" in diameter

Each fossil jellyfish comes as a split concretion pair, allowing you to easily open and discover the prehistoric imprint inside. Included are three (3) acrylic stands that you can use to display your jellyfish in different ways. Both large and small sizes ship inside our sturdy shipping cartons. Each half of the specimen is carefully wrapped in bubble wrap, and a small information card is included that also serves as the certificate of authenticity.

These fossils are incredible portals to the past and make fantastic display pieces. Show off the interior of the specimen or leave both halves joined to provide a surprise for visitors!

Please Note: As to be expected, there are wide variations in size and shape with this specimen. In addition, the fossil impressions are often quite soft. Clearly defined "arms" are exceedingly rare.

“While fossils from Africa, Europe, Australia, and India delight our eye [...], none approaches so closely to that parting of the ways as the tiny creatures found on the banks of Mazon Creek, in Illinois.“ -Paleontologist Roy L. Moodie

At Mazon Creek in Illinois, collectors gather to search the riverside for ancient stones that hold a portal to another world. 300 million years ago, this place was a site where land met the sea and a Paleozoic delta played host to all forms of life and evolution. The creatures that lived here were caught in powerful floods, bringing loads of sediment to cement them in time and preserve them for fossilization. Now, after eons, they have been uncovered and teach us about the amazing history of our planet.

The information gathered by the study of Mazon Creek is invaluable. The location’s prolific fossil beds were discovered in the mid 1800s, and since that time over 400 species of prehistoric plants and 320 species of animals have been identified. These animals have been split into two categories: the marine Essex species and the freshwater Braidwood species. Ocean fossils include jellyfish, clams, shrimp, and fish while the Braidwood species contain insects, arachnids, freshwater fish, and even proto-amphibians.

What makes Mazon Creek uniquely interesting to paleontologists is the detail in which soft-bodied organisms like jellyfish and insects have been preserved. Since soft tissues are removed by scavengers and decomposition occurs before fossilization, plus jellyfish and insects lack tough bones and teeth, information about these animals is sparse. However, at Mazon Creek, rapid flood cycles deposited large amounts of sediments at once. This covered the creatures bodies and allowed the organic material to convert to fossils.

After the organic material was covered, some bacteria were still able to consume pieces. This decomposition would end up being helpful to the fossilization process though, as it gave off carbon dioxide that mixed with the iron-rich groundwater in the delta. Over time, this would produce concretions of tough ironstone around the fossil, encasing them in a protective bubble without much compression. Because of this, some of the fossils recovered from Mazon Creek form without compression, casting them in three dimensional space.

The steps of this process can be seen in the graphic shown:

a) Creature is deposited in sediment.

b) Bacterial decomposition produces CO2 mixing with groundwater to form the ironstone siderite.

c) Ironstone formation grows into a concretion which protects the fossil.

d) The fossil of the organism is sealed within the concretion and waits for millions of years before being found by paleontologists.

These fossils come from Essexella, an extinct jellyfish named after the marine fossil group. Essexella belonged to the class Scyphozoa and would have had two distinct phases in its life cycle. First, larvae explored the area around the delta, searching for a place to anchor and form a polyp. This polyp would grow over time, eventually emitting several segments from its main body which formed into medusae, the bell and tentacle form we think of when we imagine jellyfish.

Essexella likely had 8 arms that sprouted from its mouth. Each of these arms contained a suctioning “mini-mouth” of its own that helped the jellyfish feed. The species’ modern relatives include all edible species of jellyfish, so perhaps Essexella could have a delicious snack today if it hadn’t gone extinct!

Fossils of a unique and rather strange creature known as Tullimonstrum have been discovered at Mazon Creek. This animal, known colloquially as the Tully Monster, was a soft bodied creature about a foot in length with a long proboscis. At the end of this appendage was a set of teeth on a jaw-like structure. The Tully Monster also had eye stalks emerging on either side of its main body.

Scientists are unsure of whether the animal could be classified as a vertebrate or not and beyond a brief amount of physical details little is known about the bizarre creature. Due to its lack of hard structures for fossilization, it’s possible that Mazon Creek is the only place evidence of the creature exists.