In northern Illinois, there is an area that contains a large amount of fossils. This area is known as Mazon Creek.
Polychaete annelid, Fossundecima Credit: UC Museum of Paleontology
Tullimonster (Tullimonstrum) Credit: UC Museum of Paleontology
Three hundred million years ago, what is now northern Illinois was part of a swampy marsh that included a river delta. Located near the equator, it had a tropical climate. As animals died in this area, they were covered in mud and sediment. This mud and sediment eventually formed shale. Within the shale, a particular mineral formed around many of the buried animals, called siderite. This helped preserve the hard and soft tissue of these animals, creating fascinating fossils that are present today.
