ASSESSING TRENDS IN MINOR ELEMENT COMPOSITION OF MISSISSIPPIAN ECHINOIDS (ECHINODERMATA)

Echinoids evolved during the late Ordovician, and like all echinoderms, possess skeletal elements composed of microporous, high-Mg calcite that precipitates in disequilibrium with the organism’s surroundings. Poorly lithified and easily disaggregated shale from the Pennyrile Parkway locality of the Mississippian (Chesterian) Glen Dean Fm. contains extremely well preserved isolated skeletal elements of the echinoids Archaeocidaris and Lepidesthes. SEM analysis shows near pristine stereom and minimal secondary pyrite and dolomite mineralization. Fossil echinoids are not generally used in geochemical studies because of (1) documented isotope and elemental variation associated with growth-rate, and (2) assumptions that the microporous nature of echinoid elements will result in a mixture of skeletal and diagenetic phases. Here, we explore the latter question by determining the extent to which echinoid minor-element composition is preserved in the geologic record.

To compare fossil and modern geochemical patterns, we explore Mg and Sr concentration trends within individual skeletal elements. In modern echinoids, Mg and Sr composition show distinct differences between spines and Aristotle’s lantern elements. This same relationship is seen in the fossil skeletal elements, as spines and teeth have distinctly different Mg and Sr compositions. Additionally, the Mg and Sr concentrations of the fossil skeletal elements are significantly lower than those of modern echinoid elements, which has been suggested to result from expulsion of Mg with the transition from high-Mg calcite to low-Mg calcite.

To assess the role of diagenetic fluids in the minor element composition of the fossil skeletal elements, Mn and Fe concentrations were examined in the skeletal elements as well as from spar that had infilled a blastoid from the same locality. The spar had significantly higher Mn and Fe concentration than the skeletal elements indicating that the stereom of the skeletal elements has not been strongly influenced by infill with diagenetic spar. Overall, these trends suggest that minor element incorporation patterns seen in modern echinoids can survive, with modification, the fossilization process.