TRILOBITE SCLERITE RATIOS AS TAPHONOMIC PROXIES: COMPARING HABITATS IN THE ORDOVICIAN OF LAURENTIA

Because their multi-element exoskeletons released exactly one copy of each unique, durable sclerite in a 1:1 ratio upon death or molting, trilobites offer the potential to assess ancient taphonomic regimes. If sampling problems can be controlled for, any deviation from this ratio in samples from the sedimentary record can only be accounted for by the operation of biostratinomic processes. Hence, examining ratios of trilobite cranidia versus pygidia may yield information on the relative amount of taphonomic overprint affecting or even controlling the quantitative distribution of fossils in a sample.

Previous work indicates that trilobite distribution within a single monofacial section representing a shallow subtidal habitat (House Fm., Utah) is exceedingly volatile, with cranidia:pygidia ratios averaging nearly 7:1 and with great local variability. While this raises serious questions about the amount of ecological versus physical signal resident in these quantitative data, a general expectation would be that taphonomic overprint would vary with energy of the environment, and that lower energy, deep subtidal environments might yield samples close to the original 1:1 ratio.

We examined this assumption by counting 4,522 trilobite sclerites from samples derived from shallow subtidal, shelf-margin buildup, and deep subtidal habitats (identified on independent sedimentological criteria) within a relatively narrow stratigraphic interval (latest Ibexian to early Whiterockian) on the Laurentian paleocontinent. Samples were derived from the Fillmore Fm., Utah, the Shallow Bay Fm., Newfoundland, and the Table Cove Fm., Newfoundland.

All habitats yielded variable ratios which mostly deviated significantly from 1:1. Shallow subtidal ratios averaged 1.63:1, marginal buildups ranged from 1:1.19 to 3.91:1, and deep subtidal samples, which might have been expected to be least altered, averaged 2.43:1.

These results indicate that there may be no simple correlation between habitat and physical overprint. Even samples from low-energy deeper water deposits with little evidence of transport nevertheless may be considerably altered from their original shape-abundance profile. Work in progress involves testing these preliminary conclusions via more extensive sampling and the inclusion of additional habitats.