Data di Pubblicazione:
2019
Citazione:
THE MICROSTRUCTURE OF MODERN AND FOSSIL BRACHIOPOD ARCHIVES / F. Ye ; tutor: L. Angiolini ; co-tutors: U. Brand, S. Shen; coordinator: E. Erba. Università degli Studi di Milano, 2019 Feb 07. 31. ciclo, Anno Accademico 2018. [10.13130/ye-facheng_phd2019-02-07].
Abstract:
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 carbon
Tipologia IRIS:
Tesi di dottorato
Keywords:
brachiopod; microstructure; palaeontology; biomineral archives; fossil; evolutionary history; Scanning Electron Microscopy; stable isotope compositions
Elenco autori:
F. Ye
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