Baculate shell structure in Early Palaeozoic linguliform brachiopods

Brachiopods are bivalved benthic marine invertebrates colonizing the seafloor since the Cambrian Period. The valves in the subphylum Linguliformea are composed of calcium phosphate (apatite). Linguliform brachiopods are important research objects for revealing the biomineralization patterns and evolution of mineralized skeletons as well as for gaining palaeoclimatic information stored in linguliform shell apatite. All these studies require understanding of taphonomic changes in biominerals and mineralized structures. The aim of this thesis was to study the structural and chemical variability of fossil linguliform brachiopod shells. The studied species are characterized by baculate shell structure that consists of porous laminae with small apatitic rods (bacula) alternating with compact laminae. Detailed studies of the valves of Cambrian Obolus ruchini and Ungula inornata as well as Devonian Bicarinatina bicarinata demonstrated remarkable variability in the shell structure and chemical composition. Narrow fibrils of 100–200 nm in diameter (nanofibrils) were discovered. These are similar to nanostructures shaping the baculate structure in modern linguliforms and likely represent originally non-mineralized (organic) fibrils preserved through rapid phosphatization. This suggests that the development of bacula has not much changed throughout the Phanerozoic. The studies of chemical composition and mineral morphology in the valves of Cambrian Ungula ingrica – one of the most characteristic fossil composing Estonian shelly phosphorite – revealed different chemical composition of the apatite in compact and baculate laminae. The apatite in compact laminae is morphologically and chemically similar to the shell apatite of modern linguliforms and may likely be used for biological or palaeoenvironmental interpretations. The apatite in baculate laminae is significantly different having only a limited potential for palaeoenvironmental analysis. The origin of this phase needs additional study. The successive stages of diagenetic changes in linguliform brachiopods are phosphatization, stabilization of original bioapatite, apatite transformations and structural homogenization. Estonian shelly phosphorite is mainly composed of the remains of linguliform shells and the new data on the structure and chemical composition of the shells may help to understand the properties and history of formation of phosphorite deposit, which may further provide information on technological characteristics of the phosphorite and help to solve environmental problems arising in course of mining.