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Fossil Jellyfish

Fossil Jellyfish

Limited Availability

Above: The front of the specimen card.

This specimen is an ichnofossil of an Essexella, an extinct genus of scyphozoan jellyfish dating to the Late Carboniferous Period (c. 300,000,000 years ago).

Above: One half of a Large Fossil Jellyfish specimen. No, we didn't actually go to the beach.

Such soft body structures are usually lost to time, but at Mazon Creek, Illinois, a unique fossil record captures cycles of rapid coastal flooding that combined layers of ocean sediment with iron-rich groundwater to produce tough ironstone nodules.

Above: Nodules containing Large and Small Fossil Jellyfish Specimens.

Locked within each nodule are the remnants of a river delta ecosystem that played host to hundreds of species, from insects and protomammals to ocean-going creatures such as shrimp and jellyfish.

Above: An assortment of Small and Large Fossil Jellyfish Specimens opened to display the ichnofossil within.

Fossil Jellyfish Sizing:

  • Small - 1-1.75" in diameter
  • Large - 1.75-3" in diameter

Each fossil jellyfish specimen includes BOTH halves of the concretion. 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.

We've also included three (3) small acrylic stands for display.

Above: All three stands included with each specimen.

Two of the stands allow you to show off the interior of the specimen or leave both halves joined to provide a surprise for visitors!

Above: Some suggestions for display.

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.

Above: Small and Large Fossil Jellyfish Specimens in Hand

About Essexella, Mazon Creek, and Ichnofossils 

"While fossils from Africa, Europe, Australia, and India delight our eye and stir our interest in their diversity of structure, 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

Above: Artist's Impression of Essexella (Source: Mini Museum)

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.

Above: Concretions from Mazon Creek displaying numerous creatures.

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.

One particularly exciting specimen is 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 eight or more 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 been edible as well if it hadn’t gone extinct!

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 before fossilization occurs and jellyfish or 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 creature's bodies and allowed the organic material to convert to fossils.

Above: The process of fossilization at Mazon Creek: 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. (Source: "Taphonomy of Middle Pennsylvanian Mazon Creek Area Fossil Localities, Northeast Illinois: Significance of Exceptional Fossil Preservation in Syngenetic Concretions")

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 organic material, 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.

Over time, the soft structures inside dissipate leaving an impression behind. Such impressions are called ichnofossils. These ghostly imprints of living animals provide us a wealth of information, including elements of locomotion, group dynamics, and ecology.

Bonus: The Tully Monster!

Above: Definitely NOT a Jellyfish! It's Tullimonstrum, the state fossil of Illinois!

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.

Further Reading

Baird, G. C., et al. “Taphonomy of Middle Pennsylvanian Mazon Creek Area Fossil Localities, Northeast Illinois: Significance of Exceptional Fossil Preservation in Syngenetic Concretions.” PALAIOS, vol. 1, no. 3, 1986, pp. 271–285.

Baird, G. C., et al. “Mazon Creek-Type Fossil Assemblages in the U.S. Midcontinent Pennsylvanian: Their Recurrent Character and Palaeoenvironmental Significance [and Discussion].” Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, vol. 311, no. 1148, 1985, pp. 87–99.

Moodie, Roy L. “Evolution's Most Romantic Moment.” The Scientific Monthly, vol. 11, no. 5, 1920, pp. 464–469.

Above: The back of the specimen card.


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