Fossil stereom elemental (EDS) linescan data

<p>Echinoderms are a diverse and ecologically important phylum of marine animals in both ancient and modern ecosystems. In the fossil record, echinoderms are most often found as individual elements of a complex multi-element skeleton that is generally only preserved under exceptional circumstances. Interestingly, echinoderm ossicles are often reported as part of the Cambrian “small shelly fossil” phosphatic faunas. In this paper, we investigated the preservation of stem ossicles of the abundant and distinctive Ordovician crinoid echinoderm <em>Cincinnaticrinus </em>in the area around Cincinnati, Ohio utilizing thin sections and insoluble residues that yielded abundant “phosphatized” material similar to Cambrian occurrences. Although the specimens from residue appeared to have had their skeletons replaced by phosphate, examination of thin sections revealed that the phosphate is a micro-mold of the original calcareous skeleton that preserves minute details of the original stereom in reverse, confirming at least some stages of the “Lamboy Model” for echinoderm ossicle “phosphatization”. We then examined a specimen of the extant isocrinid <em>Endoxocrinus</em> from Florida, comparing the galleried stereom in stem segments of the two crinoids. Despite being separated by hundreds of millions of years, we find these structures remarkably similar. Phylogenetic evidence indicates through-going ligaments were present in the common ancestor of these taxa and suggests an even earlier evolutionary origin for these structures. Preservation by phosphatic micro-molding is not considered “exceptional” but has the potential to allow detailed studies of the complex skeletal features that support the soft tissues of these animals.</p>

<p>If the phosphate in these <em>Cincinnaticrinus</em> ossicles occurs as early-diagenetic micromolds of the internal stereomic voids, and if the original calcite of the skeleton is preserved, then the calcite (original skeletal material) from within the ossicle boundaries and surrounded by phosphate should be free of detrital sedimentary particles, while the phosphatic filling should contain detrital sedimentary particles. Furthermore, the calcite outside the boundaries of the original ossicle originated as syntaxial overgrowth cement, which formed after the death of the echinoderm, and should contain some detrital material.</p>

<p>Common detrital minerals include quartz (SiO2) and a number of aluminosilicate minerals. Elements uniquely (in these rocks) associated with silicate minerals are silicon and aluminum, with lesser amounts of potassium.</p>

<p>Therefore, if phosphatized echinoderm ossicles retain their original spongy stereomic calcite skeleton, then that calcite should be relatively free of aluminum, silicon, and potassium while both the phosphate in the stereomic interstices and the overgrowth calcite that surrounds the ossicle should contain more aluminum, silicon, and potassium.</p>

<p>SEM/EDS lines scans were used to test this hypothesis. We examined polished thin sections in a Hitachi S3400N scanning electron microscope (SEM) at Purdue University Fort Wayne. We used compositional backscatter electron imaging (BSE-COMP). These images were used to guide the selection of linear transects using an Oxford SEM-mounted energy dispersive spectrometry (SEM-EDS) and processed the data using Oxford Aztec nano-analysis software, with the “quantline” settings. Each resulting “line scan” consists of 500 or 1000 analysis points. For each points, the quantline algorithm calculated the weight percent of several elements.</p>

<p>Several line scans were made within and between each of the three phases (putative original skeletal calcite, micromoldic phosphate, and external syntaxial overgrown cement). When compared back to the backscatter electron images, these line scans demonstrate that putative skeletal calcite is free of silicon and aluminum “spikes” which reflect the presence of detrital mineral grains.</p>