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, these fossils are lying in wait ready to be cracked open and explored.
📸 Identifying fossil ferns at Mini Museum!
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.
Fossils were first identified at Mazon Creek in the 1850s. At the time, Mazon Creek was a coal digging site, and amateurs would dig through “spoil piles”—piles of dirt and rock waste produced by mining extraction. Inside, these diggers would find concretions housing fossilized ferns and other plants.
The Carboniferous Period lasted from around 350 to 300 million years ago and is the source of many coal deposits in the modern day. This time was marked by a warm climate and a boom of plant and animal life across Earth's landmasses. Lush forests, massive insects, and even early reptiles were common finds.
Of the several hundred confirmed species identified at Mazon Creek, the majority are ferns. Ferns are vascular plants, defined by their complex tissue system that effectively distributes water and photosynthesis-produced compounds throughout the plant.
Some of the commonly identified ferns include Pecopteris subcrenulata (Lesquereux), which grew into large fronds with a pattern of straight and alternate lateral vein system. Another commonly identified species is Laveineopteris rarinervis (Bunbury) Cleal, with its uniquely dense system of veins making it easy to spot.
Given the vast diversity of plant life at Mazon Creek, scientists rely on small variations in things like vein systems and leaf pattern to be able to distinguish the many plants from each other. It’s a reminder of just how diverse the ecosystem at Mazon Creek was, and how invaluable a resource it remains today.
📸 DEFINITELY NOT A JELLYFISH! IT'S TULLIMONSTRUM, THE STATE FOSSIL OF ILLINOIS!
Outside of plants, many animal fossils have also been discovered at Mazon Creek. Particularly exciting is the presence of fossilized jellyfish—a rarity in paleontology due to the animal's soft tissue body.
Mazon Creek is even home to a unique creature known as Tullimonstrum. 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.
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.
📸 Cave Bear Tooth fossils from Mini Museum
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 here:
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.
📸 A pair of Fossil Fern Concretions!
Mazon Creek Fossils
When concretions from Mazon Creek are split open, you can find the fossilized impressions of prehistoric lifeforms. Mini Museum's collection contains several of these beautifully detailed specimens, which date back 300,000,000 years to the Carboniferous.
Both fossilized ferns and fossilized jellyfish are available in the collection below. These fossils come as a paired set, which you can open to reveal a beautifully detailed imprint of ancient life on either side.
Check them all out below!
Further Reading
Thomas Clements, Mark Purnell, Sarah Gabbott; The Mazon Creek Lagerstätte: a diverse late Paleozoic ecosystem entombed within siderite concretions. Journal of the Geological Society 2018;; 176 (1): 1–11.
Wittry, Jack. The Mazon Creek Fossil Flora. Esconi, 2006.
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.