ELECTRON BACKSCATTER DIFFRACTION AND CLOUDINA: A NEW LOOK AT AN OLD BIOMINERALIZER

The Ediacaran period contains the first appearance of benthic organisms with biomineralized skeletons, yet the manner in which many of these organisms developed their skeletons is still poorly understood. Among the first of these early biomineralizers was Cloudina, a metazoan with a millimeter-scale tubular, semi-conical skeleton. Here we use electron backscatter diffraction to examine Cloudina specimens from several localities in the White-Inyo region of eastern California and western Nevada.

Electron backscatter diffraction (EBSD) allows in situ phase identification and crystallographic information to be obtained from the biogenic material in a field of view of microns to nanometers. With respect to Cloudina, our purpose for EBSD is twofold: 1. To thoroughly characterize the shell microstructure of Cloudina and, 2. To quantify the amount of diagenetic alteration within the Cloudina shell material from the White-Inyo region.

To characterize the shell microstructure of Cloudina, image quality maps were created to indicate the regions of shell material that are indexed with high confidence. Limiting data collection to these high confidence regions minimizes error in subsequent analyses. These subsequent EBSD analyses include mapping the shell material according to crystal grain size, orientation, and phase. Misorienation plots estimate the level of crystallographic order within the shell material. Crystal orienation is deternined with respect to neighboring crystals and also with respect to location within the shell wall. These data provide valuable insight to how this organism possibly grew its shell, and future EBSD work will undoubtedly provide further insight into determining the organism’s life habit.

By indexing the Cloudina shell material for calcite and dolomite phases, EBSD provides a quantitative index for the level of dolomitic alteration among the White-Inyo specimens as well as the matrix in which they are preserved. In light of this alteration amongst the White-Inyo specimens, it is probable that many previous interpretations of early biomineralizing genera from this region are incorrect and several of the morphological characters distinguishing these genera from Cloudina are merely the result of the calcite recrystallization or dolomitic alteration of the shell material.