MINOR ELEMENT DISTRIBUTION IN SKELETAL ELEMENTS OF MODERN ECHINOID (ECHINODERMATA) GENERA

Echinoderms are composed of distinct categories of skeletal elements that grow at different rates during organismal development. These elements are composed of biologically templated, microporous, high-Mg calcite that is precipitated in disequilibrium with the organism’s surroundings. Previous studies on several echinoid genera suggest that carbon and oxygen isotope composition of different skeletal element categories show differences of up to several per mil differences across the animal (Weber and Raup, 1968a; 1968b; Limbeck et al., 2017). These isotopic differences have been hypothesized to reflect fractionation driven by the rate at which different skeletal elements grow and the existence of these ‘vital effects’ calls into question the use of echinoderm skeletal elements as proxies for understanding the composition of ancient seawater. There is, however, only limited work to explore the extent to which this disequilibrium precipitation of echinoderm elements affects minor and trace element composition.

This study examined minor element distribution in four genera of echinoids (Lytechinus, Eucidaris, Arbacia, and Clypeaster) from across the Gulf of Mexico to determine if the growth rate pattern from isotope composition was also present for minor elements. Among the minor elements that were examined, only Mg and Sr showed strong chemical differences that grouped different skeletal elements among the four genera. Slow growing skeletal elements associated with the test and Aristotle’s lantern, group together with higher concentrations of Mg and Sr, while the rapidly growing spines have the lowest concentration of Mg and Sr. The concentration of Mg within the skeletal elements is consistent with the patterns derived from Mg partition coefficients for biogenic calcite. While Sr records similar patterns to Mg, these patterns are less distinct and may vary even within specimens of the same genus and species. These findings are in line with previous studies of isotope composition of skeletal elements wherein growth rate plays a substantial role in the chemical composition of echinoderm skeletal elements.