CONODONT APPARATUSES FROM THE MIDDLE ORDOVICIAN WINNESHIEK SHALE, IOWA, USA

Conodont apparatuses are extremely rare in the Ordovician, being known from only a handful of localities in the entire world. The discovery of relatively common examples in the Middle Ordovician (Darriwilian) Winneshiek Shale in northeastern Iowa is therefore of special interest. The Winneshiek Shale is a greenish brown to dark gray laminated sandy shale with a thickness of 18-27 m, which was deposited in a meteorite crater known as the Decorah Impact Structure with a diameter of 5.6 km. Due to its unusual setting, fossils are exceptionally preserved in the shale, which is a Konservat-Lagerstätte containing a restricted-marine fauna including jawless fish and plants, and dominated by conodonts and arthropods. Winneshiek conodonts are excellently preserved in three dimensions and usually with prominent basal plates. More or less complete multi-element apparatuses are relatively common and provide unique materials for morphological and taxonomic studies.

Many Winneshiek conodont apparatus elements are of huge size (up to 10 mm or more) suggesting that these conodont animals may have been 0.5-1.0 m in length. Two of the most common apparatus types are referred to Archeognathus primus Cullison, 1938 and to a new genus and species. The former has a 6-element apparatus composed of two pairs of archeognathiform (P) and one pair of coleodiform (S) elements. The 15-element apparatus of the latter contains only ramiform elements of alate (one element) and angulate or bipennate, and tertiopedate types (seven pairs). Elements of both taxa are hyaline and provided with robust basal plates. They are the first hyaline conodont apparatuses described from the Ordovician. Compared with the few previously described Ordovician conodont apparatuses, these apparatuses from the Winneshiek are very distinctive. Whereas the Archeognathus apparatus is quite unlike any previously described conodont apparatus, that of the new genus and species is superficially similar in its 15-element architecture to that of prioniodiform conodonts but differs in the placement, appearance, and presumed function of the elements. Based on their apparatuses, both genera appear to occupy an isolated taxonomic position. Some other conodont apparatuses comprising hyaline elements also occur in the Winneshiek fauna.