BRACHIOPODS, BIOMERES, AND BIOFACIES: INTERPRETING THE CAUSE OF LATE CAMBRIAN-EARLIEST ORDOVICIAN EXTINCTION IN A SHALLOW CRATONIC SEA, CENTRAL TEXAS

The Late Cambrian and earliest Ordovician were times of low global diversity compared to the preceding Cambrian Explosion and the following Great Ordovician Biodiversification Event. During this time Laurentian trilobites were subjected to a series of apparent extinction events which define Laurentian stages and delineate informal stratigraphic subdivisions, “biomeres”.

Many studies have invoked sea level fall and subsequent rise and accompanying changes in temperature or oxygenation as a cause, while others suggest that shifting and merging biofacies played a role. Trilobites appearing in strata above the extinctions are interpreted as deeper water, “exotic” trilobites that replaced shallow water taxa. An assumption of these models is that patterns of replacement and recovery are clues to the cause of the extinction. Others interpret these phenomena as artificial clustering of first and last appearances associated with specific tracts within stratigraphic sequences. Work on these horizons and the “biomeres” between has focused on trilobites, to the neglect of linguliform brachiopods, also abundant in these rocks.

Through detailed stratigraphic sampling from uppermost Steptoean through the Sunwaptan and into the Skullrockian Stages in cratonic central Texas, we demonstrate that while linguliforms experienced complete turnover at the genus/species level in concurrence with the trilobite extinctions, the nature of the replacement fauna after the first extinction event is in contrast to the second. After the Steptoean/Sunwaptan extinction event the replacement fauna was endemic to the Laurentian craton, while after the Sunwaptan/Skullrockian event the replacement fauna was cosmopolitan and had ties to the Great Basin, Kazakhstan, and Iran. The pattern demonstrated at the lower boundary is in contrast to that seen in trilobites and does not support merging biofacies, but the pattern observed at the upper boundary is more like that documented for trilobites.

Our results highlight the difference between an extinction event and its causes versus a recovery event with the appearance of a replacement fauna. In the case of merging biofacies, they could be the same event, but in most cases they are not. Future work on this interval would benefit from better integration of brachiopod and trilobite data.