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

Abstract

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