CONODONTS FROM THE WINNESHIEK LAGERSTÄTTE, ST. PETER SANDSTONE (ORDOVICIAN) OF NORTHEAST IOWA

The Winneshiek Lagerstätte is a recently discovered Middle Ordovician Konservat-Lagerstätte from a new shale unit within the St. Peter Sandstone exposed near Decorah in northeast Iowa, USA. Abundant well-preserved conodonts form a prominent component of the Lagerstätte. Although coniform elements occur, the conodont biota from the Winneshiek fauna primarily comprises taxa grouped within the Chirognathidae and Coleodontidae. Conodonts from the fauna are preserved as both separated elements and natural assemblages. To date, at least 4 or 5 different kinds of partial or complete assemblages have been recovered, including the first known apparatuses of the enigmatic coleodontids, which are comprised of 6 elements: two pairs of coarsely denticulated Archeognathus elements and one pair of elongated finely denticulated Coleodus elements. These associations indicate that Archeognathus and Coleodus are synonymous. Conodont assemblages associated with high-carbon soft tissue remains also occur in this fauna. Although more confident evidence is still required, these specimens may represent parts of early conodont animals. Three fossil preservation characteristics are noteworthy in the conodont fauna. First, conodont elements from the fauna commonly preserve complete basal structures. Second, some conodont elements from this fauna are preserved with bromalitic materials, indicating they may have experienced the digestive tracts of carnivores such as eurypterids. Detailed studies on these materials will reveal additional information relevant to paleoecologic, taphonomic, and systematic interpretation. Third, conodont elements from the Winneshiek Lagerstätte display different colors, ranging from amber to white. In most cases, conodont color lightening is associated with varying degrees of dissolution in which conodont basal structures or even whole elements are dissolved, leaving a clearly defined basal void or element mold. Because of the color variation, the conodont alteration index (CAI) is not directly applicable. These variations are probably related to the different degrees of decomposition from anaerobic bacteria, or other diagenetic or biologic processes. Some of the white-colored elements may be bromalites.