From the University of Illinois at Chicago:
Treasure trove of fossils found in Kendall County cave
Remnants from a cave embedded in a limestone quarry southwest of Chicago have yielded a fossil trove that may influence the known history of north central Illinois some 310 million years ago.
Initial research findings were presented April 12 by University of Illinois at Chicago earth and environmental sciences professor Roy Plotnick at a regional meeting of the Geological Society of America in Lawrence, Kan. …
“Finding this was pure serendipity,” said Plotnick. “We didn’t go out looking for it, but after finding it we said, ‘Wow, look at all of this!’
The cave is basically a trap for sediment, and things get preserved that usually may not get preserved.”
“The oldest conifers previously described are at least 2 million years younger,” said Plotnick.
The specimen is now in the collection of Chicago’s Field Museum.
The scientists think that a shallow sea covering today’s north central Illinois during the geological Ordovician period about 450 million years ago formed the limestone.
The caves were eroded in the limestone at the beginning of the Pennsylvanian period, about 315 million years ago.
Within a few million years, sand, mud and organic debris from plants and animals — some burned and turned to charcoal — washed into the cave through surface openings, where it remained preserved but not compacted since that time.