DEPOSITIONAL CONTROLS ON THE EXTINCTION OF NORTH AMERICAN SILURIAN TRILOBITE ASSOCIATIONS: AN ANALOG TO MIDDLE CAMBRIAN-EARLY ORDOVICIAN BIOMERES

The Silurian marine rocks of central North America exhibit a distinct series of depositional sequences related to periodic glacial-eustatic events. These large-scale sequences occur in strata representing reef and non-reef environments within cratonic shelf or basinal settings. Each sequence exhibits a similar carbonate-rich succession terminated by an unconformity. They begin with an argillaceous transgressive phase, the end of which is associated with the initial development of a prominent positive ∂¹³C_carb excursion peaking in overlying clean carbonates. Biostratigraphic evidence demonstrates the temporal equivalence of these excursions with those of the global standard.

Environmentally ubiquitous trilobite extinction events are associated with the onset of the ∂¹³C_carb excursions. These extinctions impact the composition of well established trilobite associations as all taxa disappear and are commonly replaced by morphologically-similar members of new associations. The association of trilobite extinctions with ∂¹³C_carb excursions in the same position in these depositional sequences has been identified in late Ordovician, late Llandovery, Wenlock and Ludlow rocks of the study area. Co-occurring organisms such as pentamerid brachiopods and rhombiferan cystoids may have similar extinction patterns. Evidence suggests that related depositional sequences, ∂¹³C_carb excursions, and trilobite extinctions also occur in the Silurian of Europe (Gotland, Welsh Borderland).

In middle Cambrian through early Ordovician marine rocks, trilobite biomeres display a remarkably similar relationship of extinctions, ∂¹³C_carb excursions, and depositional sequences. Biomere boundaries display broad patterns of abrupt large-scale trilobite extinction and recovery, which are associated with the onset of positive ∂¹³C_carb excursions within transgressive strata overlying prominent unconformities. Anoxia has been suggested as a cause of biomere extinctions, but evidence of Silurian anoxic conditions in these events is limited. In general, the similarities in the depositional nature of the extinction events of both time periods suggest that Silurian trilobite associations are analogous to Cambrian and Ordovician biomeres, even though they exhibit a much reduced abundance and diversity.