Episode 110 – Mazon Creek Fossil Beds

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Listen to Episode 110 on PodBean, YouTube, Spotify, or your favorite podcast place!

Go digging in the right place in Illinois and you’re liable to find some of the best Carboniferous fossils in the world. This episode, we discuss the history, the geology, and the incredible – and sometimes bizarre – plants and animals of the Mazon Creek Fossil Beds.

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Fossils of Mazon Creek

Since at least the mid-1800s, geologists and fossil collectors have recognized that the banks of Mazon Creek (Mazon River) in Northeast Illinois are a source of exceptional fossils. Throughout the 1900s, as extensive mining operations dug into the geologic formation known as the Francis Creek Shale, it became clear that these Mazon Creek fossils were also exceptionally widespread, found over an area of some 150 square kilometers. Today, thousands and thousands of Mazon Creek fossils are held in museum collections and private collections, making this one of the best-sampled fossil localities in the world.

The Mazon Creek fossil beds are considered a conservation lagerstätte, a site of exceptional preservation. Plant and animal fossils are often preserved here with full-body impressions and even soft tissues like leaves, skin, and organs.
Images: fossil plants and shark by James St. John [CC BY 2.0]

The Francis Creek Shale was deposited in a deltaic environment, a vast region where freshwater flowed into an ancient shallow sea around 310 million years ago during the Pennsylvanian Period, at a time when Illinois was very near the equator. The fossils of Mazon Creek represent animals that lived in these tropical waters, as well as land-dwelling plants and animals washed into the delta by the flowing rivers.

Mazon Creek fossils are typically found inside round nodules of ironstone (siderite). These nodules might have formed due to the unique chemical qualities of the water in the ancient delta. Paleontologists often crack these open to reveal entire bodies of ancient animals!
Image: Esconites (worm) by James St. John [CC BY 2.0]

The Mazon Creek Ecosystem

Several hundred species of ancient plants and animals have been identified in the Mazon Creek beds. These include many soft-bodied animals that rarely fossilize, such as jellyfish and worms, as well as fossils that represent milestones in evolutionary history, including some of the earliest widespread ferns, some of the earliest known insect larvae, and some of the earliest true reptiles. With such a variety of fossils over such a wide area, Mazon Creek provides an excellent opportunity to explore multiple communities, terrestrial, freshwater, and marine. It has even been suggested, given the abundance of shark egg cases and juvenile fish preserved here, that this habitat might have acted as a nursery for certain aquatic animals.

What a weirdo! The Tully Monster has a soft body, stalk eyes, and a long clawed proboscis. Most Mazon Creek fossils are pretty recognizable, but Tullimonstrum looks like it belongs in the Burgess Shale.
Artist’s reconstruction (left) by PaleoEquii [CC BY-SA 4.0)
Fossil photo (right) by James St. John [CC BY 2.0]

Without a doubt, the most famous fossil species from Mazon Creek is the Tully Monster, Tullimonstrum gregarium. Since its discovery in the 1950s, paleontologists have debated what exactly this animal is. It has such a strange combination of characters that it’s proven difficult to identify what phylum it belongs to. Although the most common suggestion is that it is some kind of strange mollusc, other scientists have suggested it might belong to the nematodes or annelid worms, and some recent research has been amassing evidence that the strange Tully Monster might actually be a cousin of vertebrates.

Learn More

Check out this overview of Mazon Creek Flora, with lots of plant pictures!

Clements et al 2019. The Mazon Creek Lagerstätte: a diverse late Paleozoic ecosystem entombed within siderite concretions (technical)

Recent research suggests the Tully Monster is a vertebrate:
Ancient ‘Tully monster’ was a vertebrate, not a spineless blob, study claims (non-technical, includes link to paper)
The eyes of Tullimonstrum reveal a vertebrate affinity (technical, paywalled)
But other researchers disagree:
The mysterious ‘Tully Monster’ fossil just got more mysterious (non-technical)
The ‘Tully Monster’ is not a vertebrate (technical)

This 2016 episode of Brain Scoop features Tully Monster research and cracking open some Mazon Creek nodules!

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