Illinois State Museum

Missing Pieces - Food For Dinosaurs?

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There are a few areas in Illinois with deposits that formed during the Age of Dinosaurs. We have yet to find any dinosaur bones, but Cretaceous plant fossils from one area show us what some dinosaurs in Illinois may have been

The Great Cretaceous Seaway

During the Late Cretaceous Period, an inland sea spread from the Gulf of Mexico to the Arctic Ocean, and from Iowa in the east to Colorado in the west. This body of water is known as the Cretaceous Interior Seaway. It was the end result of a global pattern of rising sea levels.

The seaway was home to plesiosaurs, mosasaurs, turtles, sharks, and bony fish, as well as ammonites and bivalve mollusks. Along its western margins, dinosaurs were abundant. Pterosaurs and birds flew overhead.

The southern part of the seaway, the Mississippi Embayment, covered much of what is now the lower Mississippi River valley and extended into southern Illinois. Rivers flowing west from the Appalachian Mountains into the embayment formed richly vegetated deltas. The McNairy Formation in Pulaski County is composed of sand, silt, and clay from one of these deltas. The plant fossils exhibited here are from that formation.