STABLE-ISOTOPE ANALYSIS OF TRILOBITE CUTICLE (PSEUDOGYGITES LATIMARGINATUS) FROM SOUTHERN ONTARIO, CANADA

The Late Ordovician was a time of shifting climatic and biological conditions with the onset of glaciation on Gondwanaland resulting in the end-Ordovician mass extinction event that greatly affected trilobite communities. The Ordovician trilobite Pseudogygites latimarginatus was collected from the organic-rich black shales of the Lower Whitby Formation deposited in the Utica Basin ~450 Ma. The Utica basin was located in the foreland of the Taconian Orogeny and contained widespread black shale deposition over much of northeastern North America. This euxinic environment led to high abundances of P. latimarginatus in a relatively monospecific community, indicating opportunistic ecological strategies similar to other trilobites such as Elrathia kingii (Gaines and Droser, 2003). Scanning electron microscopy of trilobite cuticle indicated pristine calcite and a lack of secondary diagenetic features. Isotopic analysis of coexisting trilobite cuticle and brachiopod shell from the Lower Whitby Formation indicated that trilobite cuticle, like brachiopod calcite, is precipitated in isotopic equilibrium with ambient seawater. The few isotopic datasets on trilobite cuticle indicate that they are homogeneous. High-resolution isotopic analysis indicates that this is not the case and that δ¹³C values can vary as much as 2.5‰ within 2 cm of the pygidium of one specimen. It is suggested that the presence of symbionts (chemo- and/or photo-) is the most plausible explanation, however other mechanisms such as cuticle morphology, cuticle precipitation regimes, and diagenesis cannot be explicitly dismissed. δ¹⁸O values of both trilobite cuticle and brachiopod shell are significantly negative giving rise to anomalously high paleotemperatures. Using a δ_w value of -3‰, the high paleotemperatures decline significantly and agree with other regional and global datasets for the Late Ordovician. A secular curve constructed from trilobite cuticle would need to investigate whether chemo-photo-symbionts are present on trilobite cuticle, thus potentially affecting the usefulness of such curves. However, stable-isotope investigations on trilobite cuticle can provide valuable insights into the paleobiology and paleoecology of this extinct species.