More than 300 million years ago, during the Carboniferous Period, much of northern Illinois outside Chicago — including what is now the Mazon Creek ("muh-ZAHN") fossil site — was alive with ancient creatures thriving in lush, tropical swamps, river deltas and shallow seas.
Now, researchers at the University of Missouri's College of Arts and Science are collaborating with geologist Gordon Baird to reanalyze his massive fossil collection from Mazon Creek — currently housed at the Field Museum in Chicago — which includes 300,000 siderite concretions from around 350 different localities.
The Mazon Creek fossil beds are renowned for their exceptional preservation of both plants and animals, made possible by their unique geological setting. The fossils are encased in siderite — an iron carbonate mineral — forming abundant concretions that have become a treasure trove for scientists and avocational fossil hunters alike.
Thanks to decades of research at Mazon Creek, including foundational fieldwork by Baird and colleagues in the late 1970s, we now have an extraordinary view of life along that ancient coast.
A snapshot of ancient life
Baird's original work at the Mazon Creek fossil site helped distinguish two major faunal assemblages, or groups of animal remains. These assemblages helped scientists understand the ancient environments where the fossils originated. They were a marine assemblage comprised of life in offshore coastal waters, and a mixed assemblage from a river delta along the shoreline, where freshwater organisms and washed-in terrestrial plants and animals were preserved together.
Now, Mizzou's team has confirmed a slightly more nuanced view of Baird's original findings, using modern data analysis techniques coupled with advanced imaging at Mizzou's X-ray Microanalysis Core.
"We found three readily identifiable paleoenvironments, including the unique characteristics of a benthic marine assemblage representing a transitional habitat between the nearshore and offshore zones," said Jim Schiffbauer, Marie M. and Harry L. Smith Endowed Professor of Geological Sciences. "These ancient environments were each dominated by specific groups of animals, for example freshwater animals nearest to shore, jellyfish and sea anemones further offshore, and marine clams and worms in the transitional zone."
The fossils formed during a phase of sea-level rise and flooding of what used to be large coal swamps.
"The different environments affected how quickly and deeply organisms were buried, and in what specific geochemical conditions fossilization may have started," Schiffbauer said. "That, in turn, shaped where certain microbes lived and helped form the minerals that make up the concretions surrounding these fossils today."
Next steps
In current and future research, Schiffbauer and Baird are using this information to create a sedimentological model to show how the Mazon Creek ecosystem connects to the Colchester coal layers below — where coal mining led to the fossil site's original discovery.
"Given that multiple episodes of rapid coastal drowning events occurred in the U.S. midcontinent during the Carboniferous Period, refinement of information from the Mazon Creek locality will lead to a deeper understanding of similar deposits in other coal basins," said Baird, who is now an emeritus professor at State University of New York at Fredonia.
Mizzou's new collaborative analysis with Baird, colleagues from the private sector and the University of Toronto is the most comprehensive and data-driven picture of what Mazon Creek's ancient ecosystem looked like long ago. This knowledge contributes significantly to our understanding of the Carboniferous Period's biodiversity and paleoecology.
"It offers a real snapshot of the incredible diversity present in the late Carboniferous Period and allows for inferences about the complexity of food chains and how this ecosystem functioned," Schiffbauer said. "Now, we have an unparalleled and statistically supported look at the interconnected terrestrial, estuarine and marine life of the Carboniferous Period."
The study, " 283,821 concretions, how do you measure the Mazon Creek? Assessing the paleoenvironmental and taphonomic nature of the Braidwood and Essex assemblages ," was published in the journal Paleobiology.
Other co-authors are John Warren Huntley and Tara Selly at Mizzou; Charles Chabica at Northeastern Illinois University; Marc Laflamme at University of Toronto Mississauga; and A. Drew Muscente at Princeton Consultants, Inc.
