DIVERSIFICATION OF IDIOGNATHODUS P1 ELEMENTS IN THE UPPER PENNSYLVANIAN HUSHPUCKNEY SHALE (SWOPE CYCLOTHEM), MIDCONTINENT BASIN, USA

The Upper Pennsylvanian Hushpuckney Shale is well known for its diverse and abundant conodonts. The The Hushpuckney Shale represents the peak diversity for Idiognathodus following the Late Desmoinesian extinction. The extinction of the genera Neognathodus and Swadelina provided Idiognathodus with a broad range of available ecological niches. Idiognathodus P₁ elements recovered from the Checkerboard/South Mound, Exline, Critzer, and Hertha cyclothems exhibit variability in rostral lobe shape and size. In the Hertha cycle, expanded rostral lobe bearing species (I. swadei and I. turbatus) become less abundant than species bearing restricted rostral lobes (I. sulciferus and I. eccentricus).  New species like I. vorax and I. species 1 developed dorso-ventrally elongate rostral lobes rather than rostro-caudally expanded rostral lobes.

In the Hushpuckney shale, elongate rostral lobes and reduced rostral lobes become more common. Those species once known for their expanded rostral lobes (Idiognathodus. swadei and I. turbatus) exhibit weak development of their rostral lobes, mimicking the elongate lobes found in several new species (I. species S and I. species R). Several species exhibit severely reduced rostral lobes (I. species J, I. species E and I. species B). In addition to rostral lobe reduction, caudal lobes show evidence for reduction. Idiognathodus species P (I. cancellosus?)and I. species H do not have rostral lobes and have caudal lobes that have merged almost completely with the caudal adcarinal ridge. With decreasing morphological variability in lobes, P₁ elements show greater variation in platform ornamentation. Eccentric grooves (I. eccentricus, I. turbatus, and I. species R), medial nodosity (I. species B), long medial carina (I. species P and I. species H), medial grooves (I. species E), and chaotic platforms (I. species J) become increasingly more common. Flat, uniformly ridged platforms remain common when lobes are preserved (I. swadei and I. sulciferus). Juvenile specimens of I. species B show the first development of a true troughed platform. While it is not retained in adult specimens, the presence of a troughed platform implies that I. species B may be the ancestor of the descendent genus Streptognathodus.