Paper No. 260-12
Presentation Time: 4:40 PM
BEDDING PLANE CONODONT OCCURRENCES FROM DARK SHALES IN THE PHI KAPPA FORMATION AT THE LITTLE FALL CREEK SECTION, TRAIL CREEK REGION OF CENTRAL IDAHO, AND THE PALEOBIOLOGICAL SIGNIFICANCE OF NEW PARTIAL NATURAL ASSEMBLAGES OF AMORPHOGNATHUS TVAERENSIS (Invited Presentation)
The co-occurrence of conodonts and graptolites on bedding planes in Ordovician shale sequences enhances the overall correlation precision between platform and deep-water successions. These co-occurrences are relatively common, and we have reported them from shale successions in Tarim (western China), Alabama, Idaho, and Oklahoma in North America, central Scotland, and Bornholm, Denmark, where these successions contain key conodont zonal indicator species and have increased our ability to tie together conodont and graptolite zonal schemes. The Little Fall Creek section spans the mid Dapingian to upper Sandbian and we have collected conodonts from upper Dapingian, upper Darriwilian, and Sandbian strata. In addition to the biostratigraphic significance of finding conodonts in association with graptolites, some conodonts are in bedding plane associations that yield paleobiological information about the conodont animal. We have discovered bedding plane associations/partial natural assemblages of Pygodus serra, Periodon aculeatus?, and Amorphognathus tvaerensis from the Little Fall Creek section of the Phi Kappa Formation in central Idaho. Some of these bedding plane associations are difficult to interpret as the rocks containing them have been altered through a complex tectonic and thermal history. However, some associations demonstrate high potential for apparatus reconstruction and anatomical interpretation. In particular, new bedding plane partial natural assemblages of A. tvaerensis of the balognathid clade indicate that 1) the position of S and M elements is consistent within the clade, and 2) the number of P elements in the clade is variable, and Amorphognathus has one pair of P1 and one pair of P2 elements. If our material is representative, it suggests that additional pairs of P elements, a character state that Promissum, a genus also in the balognathid clade with excellent bedding plane assemblages from the Soom Shale, is not shared with Amorphognathus, and that Amorphognathus likely has a 15 element apparatus.