CONODONT BEDDING PLANE ASSEMBLAGES OF AMORPHOGNATHUS TVAERENSIS FROM THE PHI KAPPA FORMATION (ORDOVICIAN) IN THE TRAIL CREEK REGION OF CENTRAL IDAHO

The Ordovician Phi Kappa Formation in the Trail Creek region of central Idaho consists of weakly metamorphosed, siliceous black shale with sandstone beds in some intervals. At the Little Fall Creek section the Phi Kappa has yielded abundant conodonts on bedding plane surfaces at multiple levels. One of the most productive levels is 233m from the base of our measured section. This level is well constrained biostratigraphically by Climacograptus bicornis Zone graptolites (Late Ordovician, upper Sandbian), and yields an abundant conodont fauna of Amorphognathus, Periodon grandis, Protopanderodus sp., and Panderodus sp. on bedding planes. The conodonts are commonly preserved as molds, which complicates specific identification of many of the individual elements. A new species of Amorphognathus, Amorphognathus viirae, was described by Paiste et al. (2022) from the upper Sandbian in the upper half of the Amorphognathus tvaerensis Zone, an interval correlated with the Climacograptus bicornis Zone. Based on the range of this new species a new Atlantic Realm Conodont Zone succession was proposed. We do not identify Amorphognathus elements from the 233m level of the Phi Kappa Formation at Little Fall Creek as Amorphognathus viirae because the denticle row on the dextral Pa element in upper view is not shaped as a sinuous curve, which is the diagnostic character of A. viirae. It is possible that this is a taphonomic artifact of bedding plane preservation, but the large number of complete specimens makes it unlikely. In addition to the abundant individual elements present on bedding planes at this level, there are several associations of elements that we interpret to be natural assemblages. Each of these natural assemblages represent the partial remains of an individual conodont animal. Newly recovered material supports the idea that Amorphognathus had a 15 element apparatus and provides direct evidence of element positions of Amorphognathus within multiple natural assemblages.