SHELL ULTRASTRUCTURE AND ONTOGENY OF ORGANOPHOSPHATIC BRACHIOPODS FROM THE LOWER CAMBRIAN OF SOUTH AUSTRALIA

Phosphatic linguliform brachiopods are an abundant constituent of the lower Cambrian shelly faunas of South Australia and are typically represented by thin-shelled and fragile forms. Exceptions to this include the multi-layered and thickened valves of Eodicellomus elkaniiformis, Minlatonia tuckeri, and Curdus pararaensis Holmer and Ushatinskaya in Gravestock et al. 2001. While scanning electron microscopy (SEM) has been an effective means of visualizing outward shell morphologies and characteristics for taxonomic description, our understanding of the internal ultrastructure and ontogeny of these thick-shelled brachiopod taxa has been limited. As revealed by fractured specimens, distinct layered muscle platforms present in both valves exhibit several large internal void spaces that were likely filled with a chitinous organic matrix in life. Using combined SEM and x-ray tomographic microscopy (micro-CT) we document the entire shell ultrastructure and morphology of these respective taxa for the first time. Micro-CT permits detailed visualization of individual accretionary layers, which allows us to 1) characterize the growth of the secondary and tertiary layers of the shell, 2) document the shape and size of void spaces and determine whether these are being overemphasized due to taphonomic processes, and 3) estimate volumetric contributions of baculate and lamina shell layers to the overall shell structure. These new insights into the shell construction of early brachiopod genera contribute additional details and characters necessary for further taxonomic and phylogenetic analyses.