Palaeozoic stromatoporoids and chaetetids analysed using electron backscatter diffraction (EBSD); implications for original mineralogy and microstructure

Palaeozoic hypercalcified sponges were ubiquitous Ordovician—Devonian reef builders but, despite their rich fossil record, original skeletal mineralogy and microstructure remain poorly understood. This study provides the first application of electron backscatter diffraction (EBSD) to analyse structure Silurian Devonian stromatoporoids. The two stromatoporoid taxa selected are typical stromatoporoids in showing poor preservation. A reference sample an exceptionally well-preserved chaetetid sponge from Carboniferous Buckhorn Asphalt Quarry (a lagerstätte renowned for its preservation microstructures) contains evidence that skeleton comprised distinct bundles single-crystal fibres, similar modern hypercalcifying sponges. Similar crystal fibres proposed here as stromatoporoids, acted precursors coarse fibrous calcitic overprinting recrystallisation is orientated normal growth layers, seen all viewed cross-polarised light. studied show pronounced microporosity micro-dolomite inclusions which circumstantial composition high-Mg calcite (HMC). We propose structures might be linked hydrated amorphous calcium carbonate (ACC·H2O) at time early diagenesis, occurs calcified possible HMC supports earlier views mineral Phanerozoic oscillations ratio Mg:Ca, expressed aragonite-calcite seas; thrived times calcite-seas.