THE MICROSTRUCTURE OF MODERN AND FOSSIL BRACHIOPOD ARCHIVES

Due to their high biodiversity and widespread distribution in the Phanerozoic oceans, brachiopods are very important tools for research in palaeontology and related fields in Earth Sciences to investigate the past and present global change. Their biominerals have been considered the best carbonate archives of proxies for extending climate and environmental records on a broad geographical scale over long periods of time. Their fidelity as archives is supported by the following: 1) they record the physical and chemical composition of the seawater in which they live without, or with very limited, vital effects; 2) they precipitate a low-Mg calcite shell, which withstands post-depositional alteration; and 3) they are low metabolic and physiologically unbuffered animals sensitive to change in the physicochemical composition of the ambient seawater. However, there is still insufficient knowledge of the microstructures of these biomineral archives and their biomineralization processes during the evolutionary history of the phylum. The aims of the present thesis, focused on solving these issues, are to: 1) examine the micro- and morpho- structural diversity of modern and fossil brachiopods, 2) assess the microstructure variation in different environmental conditions; and 3) reconstruct the evolutionary changes and fabric differentiation of the main brachiopod classes through geological time. A multidisciplinary approach was used for the microstructural analyses: 1) a comprehensive dataset was established based on detailed microstructural observations of modern and fossil brachiopods analysed by Scanning Electron Microscopy (SEM); 2) new measurement methods were developed based on SEM observations to quantitatively describe the morphology and size of the structural units (fibres) of the shell secondary layer, the thickness of the primary layer, and the density and size of endopunctae of modern brachiopod shells; 3) new measurement methods were developed to describe the structural units (laminae and fibres) of fossil brachiopod shells; 4) statistical analyses of the acquired data were performed, i.e. independent-sample t-tests, frequency distribution plots, principal component analysis, and symmetric and asymmetric variants analyses; 5) stable isotope compositions (δ13C and δ18O) were tested from the secondary shell layer along shell ontogenetic increments in both dorsal and ventral valves of modern brachiopod shells; and 6) Transmission Electron Microscope (TEM) and Electron Backscatter Diffraction (EBSD) were performed in collaboration with other researchers to investigate the micro- and nanoscale features of modern brachiopod shells. Through these approaches, details of microstructural patterns were described and compared of twenty-nine specimens of six recent brachiopod species [Notosaria nigricans (Sowerby, 1846), Liothyrella neozelanica (Thomson, 1918), Liothyrella uva (Broderip, 1833), Magasella sanguinea (Leach, 1814), Gryphus vitreus (Born, 1778), Calloria inconspicua (Sowerby, 1846)] from different environmental conditions. Based on the morphology and size of the shell secondary layer fibres, the following conclusions were reached: 1) There was no significant difference in the shape and size of the fibres between ventral and dorsal valves of the same specimen; 2) An ontogenetic trend in the morphology of the fibres was found, as they become larger, wider, and flatter with increasing age. This change in size and shape indicated that the animal produced a fibrous layer with a different organic content during the ontogeny. 3) The relationship between size and shape of fibres and environmental conditions was clear when comparing two species of the same genus (L. neozelanica, L. uva) living in seawater with different carbonate saturation state and temperature, i.e. the fibres of L. uva are narrower and rounder than those of L. neozelanica. This in turn indicated a higher shell organic content in L. uva. Additional investigations were performed on the species Magellania venosa (Dixon, 1789), grown in the natural environment and in controlled culturing experiments in different pH conditions (7.35 to 8.15 ±0.05), and led to following conclusions: 1) Under low pH conditions, M. venosa produced a more organic-rich shell with larger and higher density endopunctae, and smaller secondary layer fibres, when subjected to about one year of culturing. 2) Increasingly negative δ13C and δ18O values were recorded by the shell produced during culturing and are related to the CO2–source in the culture setup. 3) Both the microstructural changes and the stable isotope results supported the value of brachiopods as robust archives of proxies for studying ocean acidification events in the geologic past. Finally, the measurements made on the size of structural units (laminae/fibres) of Cambrian to Devonian fossil brachiopod shells coupled with very detailed qualitative micro-scale observations, allowed the following conclusion: 1) The fossil organocarbonate brachiopod shells produced two main secondary layer fabrics: a laminar fabric in the Strophomenata, and a fibrous fabric in the Rhynchonellata. The Strophomenata laminar fabric shells appeared to be more variable and complex in their structural organization, but the thickness of the laminae was rather uniform and much thinner than that of the fibres. The Rhynchonellata fibrous fabric was more simple and uniform in its organization, but the size of the fibres was much more variable and comparable to the fabric of modern brachiopods. 2) Brachiopods with a fibrous secondary layer were mostly associated with biconvex shells, whereas brachiopods with a laminar secondary layer are associated with a variety of shell shapes. 3) Detailed microstructural studies were shown to be a very useful tool to construct the phylogenetic tree of the Phylum Brachiopoda. For example, the recorded gradual change in thickness of laminae from Billingselloidea to Productida could be important evidence to support the hypothesis that taxa with laminar microstructure diverged from the Billingsellida. Microstructural observation on the Chonetidina suggested that their shells had already evolved a laminar fabric during the Devonian. In summary, this new multidisciplinary and quantitative approach to describe the microstructure of brachiopod shells is a powerful tool to interpret microstructural variations of brachiopod shells in different ontogenetic stages and environmental conditions. Moreover, using the microstructure of brachiopod shells as a biomineral archive is a very promising tool for studying climate and environmental change and reconstructing the state of the oceans over the long history of geological time, and may be used to constrain the evolutionary history of the Phylum Brachiopoda.