In Iowa, many stratigraphic units are famous for abundant fossils. One of them is the Middle Ordovician Winneshiek Lagerstätte, which was recently found in Decorah.
This fossil fauna is characterized by many well-preserved fossils including some soft body and tissues. The fossil component indicates that the Winneshiek fauna lived in a special marine environment, and was dominated by many rare non-shelly taxa including the earliest eurypterid (left figure). This fauna thus opened a new window of the mid-Ordovician life, and was considered as “the discovery of the decade in early Paleozoic paleontology” by reviewers.
Studies have exposed that the Winneshiek Shale, which bears the Winneshiek Lagerstätte, is confined to a circular basin with a diameter of 5.6 km in Decorah area. Multiple lines of geologic evidence indicate that this basin was formed by a meteorite impact in Middle Ordovician, and known as the Decorah impact structure (right figure). This discovery not only provides new data to the study of the Great Ordovician Meteorite Shower, the exceptional crater-fill sequence preserved in this structure may also provide direct geologic and paleobiologic evidence to reconstruct the regional earth history which was eroded for ~20 million years by the Sauk – Tippecanoe megasequence boundary in the Midwest USA.