SYNCHROTRON IMAGING OF BURGESS SHALE FOSSILS: EVIDENCE FOR BIOCHEMICAL COPPER (HEMOCYANIN) IN THE MIDDLE CAMBRIAN ARTHROPOD MARRELLA SPLENDENS

The Burgess Shale, a fossil Lagerstätte famous for its diversity of advanced Cambrian animals, onlaps a submarine fault scarp cutting the margin of a drowned carbonate platform in southeastern British Columbia. We conducted reconnaissance synchrotron X-ray fluorescence imaging of a handful of unweathered specimens belonging to several soft-bodied taxa. Using the HXMA beamline at the Canadian Light Source Synchrotron, this imaging affords an advantage over conventional elemental mapping in that much smaller quantities can be detected. Our analysis revealed that the composition of each fossil is distinct from its surrounding mudstone. Fe and S concentrations, for example, are especially high in the fossils, indicating precipitation of diagenetic pyrite influenced by decay. Marrella splendens Walcott 1912 is an elaborate, non-mineralized arthropod virtually always preserved compressed with a dark stain emanating from the thorax. This stain represents body fluids (most likely blood) that were expelled upon death or leaked out during decay. Elevated concentrations of Cu were observed in all five specimens scanned, mostly in the dark stain but also in other regions co-localized with the fossil, but not in other taxa or the matrix. We interpret this Cu enrichment as being due to the remnants of Cu-containing hemocyanin. Hemocyanin occurs widely in mollusks and arthropods, especially chelicerates and malacostracan crustaceans. It is utilized for transporting oxygen in the respiratory cycle. Because hemocyanin is a relatively poor oxygen-binding protein molecule (compared to iron-containing hemoglobin), Marrella likely lived in well-oxygenated waters. Given other evidence for the in situ nature of the Burgess Shale animals, rather than their transport from shallower water, this argues for fully oxygenated conditions along the fault scarp in a water depth of several hundred meters. Cu enrichment specifically in fossilized Marrella may be because this animal possessed comparatively more blood in the circulatory system of the thorax than contemporaneous Burgess Shale animals, or because of selective preservation of the blood components in the stain. The molecular clock age of the hemocyanin gene family coincides approximately with the Middle Cambrian age of the Burgess Shale, some half a billion years ago.