A GIANT PRIONIODINID CONODONT FROM THE MIDDLE ORDOVICIAN WINNESHIEK LAGERSTäTTE OF NORTHEASTERN IOWA, USA

Well preserved conodonts are the most abundant fossils in the Middle Ordovician Winneshiek Lagerstätte of Iowa, but some specimens show diagenetic color alteration. Many conodont elements preserve both denticle(s) and attached basal plates. Bedding plane conodont assemblages are common, although many of them are preserved in coprolites or gut traces of carnivorous animals. The diverse Winneshiek conodont assemblages will provide a clearer understanding of a number of multi-element Ordovician conodont taxa, especially Coleodus, Stereoconus, Multioistodus, several prioniodinid (chirognathid) genera, and others. Many Winneshiek conodont elements are remarkably large, reaching up to 11 mm in Coleodus. A new prioniodinid taxon also includes exceptionally large and robust elements up to 16 mm long. Preserved assemblages of this taxon contain up to 14 or more elements, the largest of which is found in an assemblage of 8 elements dispersed across 30 mm. Most elements of this new taxon display a prominent central cusp and numerous fibrous denticles (lacking white matter) distributed along elongate processes. Some elements have more than 30 denticles, the largest one has at least 37 denticles. S-elements commonly show three processes: two major denticulate processes and a shorter third one with denticles or nodes. The presence of three denticulate processes allies this taxon most closely with Erraticodon. The new taxon also shares some similarities with the robust multi-denticulated “Erismodus” (?Erraticodon) horridus from the similar aged Joins Formation of Oklahoma, but it is not conspecific. Elements of the new taxon commonly preserve large robust basal plates, some more than 2 mm thick, with ridges and grooves that roughly radiate from the base of the cusp. Thin sections of the basal plate show prominent growth lamellae. The largest known conodont element is an isolated 18.8 mm long Promissum pulchrum from the Upper Ordovician Soom Shale of South Africa, but the largest elements recorded from preserved apparatuses of P. pulchrum are 14 mm long. The Winneshiek Lagerstätte includes some of the largest conodonts known from the fossil record.