CONODONT ASSEMBLAGES FROM THE MIDDLE ORDOVICIAN WINNESHIEK LAGERSTATTE, NORTHEAST IOWA

The Middle Ordovician (Whiterockian) Winneshiek Lagerstätte is preserved in a new stratigraphic unit included within the St. Peter Sandstone of northeast Iowa. It consists of greenish-brown to dark-gray sandy laminated shale with abundant organic carbon and pyrite. Fossils are extraordinarily preserved, including soft-body remains of several taxa. Recent research indicates that the shale was deposited within a local impact-related circular basin.

Conodonts, many occurring as natural assemblages, are an important constituent of the fauna that contains a variety of invertebrate and vertebrate taxa. To date, identified conodont assemblages include chirognathids (Erismodus, Erraticodon, Chirognathus), coleodontids (Coleodus, Steroconus), multioistodontids (Multioistodus), protopanderodontids (?Oneotodus, ?Glyptoconus), oistodontids (“Acodus”), and possibly others. The largest assemblages (1.3 cm long) are bimembrate containing six elements previously assigned to Coleodus and Archeognathus. Conodonts within the Winneshiek fauna commonly preserve complete basal plates, and some assemblages are associated with dark-brown to black organic materials. Denticles are usually amber in color, but color alteration is observed including white-colored mineral replacement.

Several conodont assemblages occurring within vermiform organic films are of special note. Two or three of these display similar fin-like structures, and one is associated with a 3-D segmented vermiform body, but further study is needed to confirm the nature and significance of these occurrences. The presence of conodont assemblages and soft-bodied fossils in the Winneshiek Lagerstätte provides high potential, with further collecting, for discovery of the oldest known conodont animal fossils.