GREEN GRAPTOLITES: GLAUCONITIC REPLACEMENT OF SOFT-TISSUES IN THE FOSSIL RECORD

Glauconite is an authigenic clay that forms at the sea water-sediment interface. It has been observed to commonly form in a chemical micro-environment, often with a semi-permeable barrier to facilitate elemental exchange. Though glauconite has been observed to replace fecal pellets and make internal shell molds (steinkerns), it has otherwise not been recognized to play a role in fossilization.

At least two types of green, mesh-like fossils were discovered in Mississippian-aged rocks in Illinois. Initially identified as glauconite-replaced frondose bryozoans, they created a real puzzle for understanding their preservation. No transition of calcium carbonate to glauconite has been previously recognized. A solution was reached when the specimens were correctly identified as dendroid graptolites. This was further supported by discovery in the samples of an additional graptolite species, Inocaulis. The transition of the organic graptolite remains to glauconite can be understood by analogy to the glauconitization of organic-rich fecal pellets. Additional occurrences of glauconitized fossils (molluscs from the Ordovician Gull River Formation) seem to fit this pattern—their organic periostracum was glauconitized while their shell was dissolved away.