July 11. Mazon Creek

Pennsylvanian shrimp Acanthotelson stimpsoni,
from Dugger Formation, Indiana.

Another remarkable Pennsylvanian fossil assemblage is found at Mazon Creek, Illinois. It’s a lagerstatte, one of those natural collections that’s remarkable in its state of preservation. The Pennsylvanian rock unit that these fossils are found in is a shale – but the fossils aren’t in shale but rather are in concretions within the shale. The concretions are called ironstone, oval to circular concentrations of iron in the form of siderite, iron carbonate that seem to have preferentially focused on the fossils, encasing and entombing them. Possibly the bacteria that began to decompose plant and animal parts generated carbon dioxide which combined with iron – which must have been anomalously high in concentration in the water to make iron carbonate. The concretion is still mostly shale or siltstone, but the iron dissolved from the rock makes the concretion enclosing a fossil harder than the general rock, and they weather out nicely. Collectors split the concretions to reveal plant and animal fossils in wild diversity. 

The overall setting for the Mazon Creek rocks was probably a large river delta, with sediment and iron eroding off the uplifting Allegheny and Appalachian Mountains to the east. It was a tropical environment.

Over 400 species of plants and more than 300 species of animals have been found in Mazon Creek concretions. Leaves such as parts of fern fronds are common, as well as seeds and cones from plants. The animals include jellyfish, worms, shrimp, snails, and fish, plus centipedes, insects, spiders, and beetles – the oldest known beetle is from Mazon Creek, and it was described in 2009.

Tully Monster, collection of Mike Hamilton, photo by Steve Henderson

One interesting animal, found only at Mazon Creek, is called the Tully Monster. It was an invertebrate with no hard parts that ranged in size from about 8 to 35 centimeters, or three to 14 inches. It had a long proboscis with teeth at the end, with which it presumably fed on small animals or debris in muddy water, and a linear bar with possible eyes on each end. We flat-out do not know what the Tully Monster is. We don’t know its phylum or its affinities, beyond a suggestion that it might be some variation on some of the worm-like themes seen in the Burgess Shale of Cambrian age.

—Richard I. Gibson

Oldest beetle
Tully Monster 

Pennsylvanian shrimp Acanthotelson stimpsoni, from Dugger Formation, Indiana. Collected by Richard Gibson. Photo by Steve Henderson. One half of this fossil is in Montana, the other half is in Georgia. Both photos used by permission from Steve Henderson.