PATHOLOGICAL PALMATOLEPIS FROM THE LATE DEVONIAN SWEETLAND CREEK SHALE

The type section of the Sweetland Creek Shale, located near Muscatine, Iowa, is a grey-green, hemipelagic shale of 3.9 m, deposited on the epeiric slope of the Iowa Basin during the upper Frasnian. This formation records the Lower Kellwasser Event (LKE) through a positive carbon isotope excursion and conodont extinction. Sweetland Creek is a condensed section rich in conodonts, where samples are dominated by Palmatolepis. Uniquely, samples from the LKE interval commonly yield aberrant morphologies of Palmatolepis and Polygnathus. Specimens of Palmatolepis display thickened, crenellated, orally-deflected platform margins. The morphology falls outside of the pathology classification established by Weddige. However, the consistent malformations observed, and the fact that Palmatolepis platforms do not occlude, suggest an explanation outside of typical feeding damage and repair.

This research tests the hypothesis that malformed LKE palmatolepids developed abnormal apatite crystallinity or composition. Volcanic or hydrothermal influx of metals has been proposed to drive anoxia during the LKE. Upwelling at Sweetland Creek could send dysoxic, metal-rich waters into the photic zone where conodonts lived. These solutes could be uptaken into conodont apatite, similar to modern fish in polluted environments, and affect crystallinity through increased amorphousness or pathological calcification.

This study combines qualitative description of conodont morphology and quantitative analysis of conodont geochemistry and crystallinity to better understand the LKE and the use of conodonts as recorders of seawater chemistry. Oil immersion optical microscopy is used to observe growth laminae for evidence of abnormal growth or repair. SEM images document aberrant conodont elements while EDS serves as a preliminary examination of elemental distribution. LA-ICP-MS analysis is used to quantify elemental composition and sample composition is compared within and between horizons across the LKE. XRD data assess apatite species, crystallite size, and unit cell volumes which can identify unusual substitutions and amorphous phases. Abnormal apatite within malformed conodonts may support their use as indicators of environmental crisis throughout the Paleozoic.