<p> This dissertation explores the internal geochemistry of Sepia officinalis cuttlebones and the rostra of several genera of extinct Belemnoidea with the aim of improving our ability to reconstruct the palaeoclimatic record. Sepia share several unique morphological features with Belemnoidea and may be their most suitable modern analogue. Consequently, a geochemical comparison between the internal skeleton of Sepia and Belemnoidea may help improve the understanding of the geochemistry of Belemnoidea rostra, which are often used to investigate Mesozoic palaeoenvironmental conditions.</p> <p> High resolution sampling of oxygen isotope values from five S. officinalis cuttlebones from the English Channel confirmed that S. officinalis cuttlebones provide an accurate record of water temperature that preserves seasonal temperature variation consistent with sea surface temperature records. Carbon isotope values show increasing values with growth until a sharp decline late in life, documenting a decrease in metabolism with ontogeny, followed by a late life increase. Trace element concentrations measured from two of these cuttlebones document a small but statistically significant correlation between oxygen isotope values and both Sr and Mn concentration, suggesting some degree of temperature control on their rate of incorporation. Mg concentration exhibits a small but statistically significant correlation with carbon isotope values, suggesting metabolism may exert some control on the rate of Mg incorporation.</p> <p> Due to the process of fossilisation, diagenesis may alter the geochemistry of Belemnoidea rostra, which is difficult to detect using stable isotope values alone. It is necessary to detect samples that have diagenetic overprinting of the original environmental signal so that past geochemical relationships may be accurately reconstructed. As Mn and Sr are enriched and depleted (respectively) in diagenetic fluids, they may be used as indicators of alteration. Following the removal of diagenetically altered samples, the carbon isotope values of Belemnoidea rostra sampled from the Green Beds at Carbondale River, Alberta, Canada document a statistically significant increase with ontogeny, suggesting a decrease in metabolic rate similar to S. officinalis. Contrasting with S. officinalis, oxygen isotope values do not show seasonal palaeotemperature cycles, possibly the result of latitudinal migration with favourable water temperatures. Trace element concentrations of Mg exhibit a small but statistically significant correlation with Belemnoidea oxygen isotope values, contrasting with Sr and Mn in S. ofjicinalis, suggesting some temperature control on the incorporation rate of Mg in belemnoidea calcite. The ranges in oxygen isotope and Mg values were used to investigate changes in surface and deeper water palaeotemperatures during the deposition of the Green Beds. Similarly, the range in carbon isotope values within individual Green Bed Belemnoidea rostra documented a positive isotope excursion in the Green Bed section, which may become a useful geochemical marker for the region.</p> <p> In most previous studies, palaeoenvironmental conditions have been reconstructed using a single sample from unspecified locations within Belemnoidea rostra. A high resolution sampling approach is necessary to capture the range of geochemical values preserved in individual rostra and may help to minimise the effects of sample bias in the geochemical record. Sampling radially from the apical line to the outside edge of an individual Belemnoidea rostrum is permits the collection of geochemical information spanning the organism's lifespan, and will improve the interpretation of the biology and environment while minimising sample bias.</p> / Thesis / Doctor of Philosophy (PhD)