THE MIDDLE ORDOVICIAN WINNESHIEK LAGERSTÄTTE - AN UNUSUAL SETTING FOR EXCEPTIONAL PRESERVATION

Two attributes demonstrate the unusual nature of the Middle Ordovician (Whiterockian, Darriwilian) Winneshiek Lagerstätte in northeast Iowa. First, the Lagerstätte was deposited in a shallow marine near-shore environment, most likely with a brackish influence. Second, multiple lines of evidence indicate that the present distribution of the Winneshiek Shale, which hosts the Lagerstätte, is limited to a 5.5 km diameter meteorite impact crater, the Decorah Impact Structure. The restricted environment resulted in a faunal composition dramatically different from normal marine shelly faunas: common Paleozoic taxa including trilobites, graptolites, corals, echinoderms, bryozoans, and non-linguloid brachiopods are all absent. Low oxygen levels and quiet conditions at the bottom of the crater provided conditions for the preservation of taxa living in the water column, including non-biomineralized remains.

The Winneshiek fauna is comprised of conodonts, a diversity of arthropods, linguloids, rare fishes and a variety of coprolites and cololites. Conodonts are the most abundant fossil group in the Winneshiek fauna, but the taxonomic diversity is relatively low. Unusually large conodonts (with individual elements commonly >10 mm) with robust basal plates and fibrous denticles likely represent a new taxon of Conodonta. Arthropods from the Winneshiek include phyllocarids, eurypterids, ostracods and a small number of other new forms. They are characterized by biomineralized and organic cuticles and sometimes preserve gut traces. Eurypterids from this fauna are the earliest record from Laurentia. At least two diffent phyllocarids are present, the largest with valves more than 80 mm long and a phosphatic cuticle. Abundant algae and possible plant tissues are also recognized. Coprolites and cololites indicate that predators were feeding on different elements of the fauna including conodonts and more rarely phyllocarids. A number of coprolites contain concentrations of rounded sand grains. Thus, the Winneshiek Lagerstätte provides a unique window on Ordovician life in Laurentia, augmenting the normal fossil record.