A NEW SOFT-BODIED MIDDLE ORDOVICIAN FAUNA FROM THE ST. PETER SANDSTONE IN NORTHEAST IOWA

A new Lagerstätte has been discovered from a recently recognized shale unit within the Ordovician St. Peter Sandstone of northeast Iowa, USA. The fauna includes conodont assemblages and a variety of other fossils, many preserved with associated soft tissue impressions. The greenish-brown to dark-gray laminated shale interval locally occurs between the Tonti and Readstown members of the St. Peter Sandstone and ranges in thickness from zero to 38 meters. An upper Middle Ordovician age is indicated for the new shale unit.

Well preserved conodonts form a prominent component of the fauna. Almost all conodont elements from the fauna preserve basal plates or jaw-like basal structures. In some cases the basal structure is dissolved leaving a clearly defined cavity. Four or more different types of conodont assemblages have been discovered, consisting of either a single element form taxon or elements previously assigned to different form taxa. Of particular note, two specimens show conodont assemblages associated with integument-like organic films and impressions. One is associated with coniform conodont elements and another occurs with paired conodont elements. These specimens likely represent parts of two different conodont animals. This is only the fourth occurrence worldwide that is known to preserve soft-bodied tissues associated with conodont assemblages.

In addition to conodonts, jawless ostracoderm fish, early eurypterids, phyllocarid crustaceans, linguloid brachiopods, and other indeterminate fossil forms are also found in the fauna. Soft body tissues, impressions, or even three-dimensionally preserved organisms are not rare in the unit. This new Lagerstätte will substantially advance our understanding of the paleobiology of various extinct organisms, including early vertebrates, and the paleoenvironment in which they lived and died. This discovery is especially surprising, as the St. Peter Sandstone, noted for its supermature quartz sandstone lithology, has yielded few diagnostic fossils over its vast geographic extent.