CLIMATIC TRANSITIONS AND THE REORGANIZATION OF NORTH AMERICAN BRACHIOPOD FAUNAS AT THE MID-CARBONIFEROUS BOUNDARY

Late Serpukovian rock units in North America, just prior to the mid-Carboniferous boundary, record the transition from global greenhouse to icehouse climate. This climatic transition coincided with a minor global extinction event, in which brachiopod faunas, in particular, experienced substantial turnover and reorganization.

The exact link between the greenhouse-icehouse transition and the reorganization of North American brachiopod faunas is unclear. One consequence of the shift in climatic conditions is a transition of depositional regimes from massive thick-bedded carbonates (relative sea level stability) to mixed carbonate-siliciclastic deposition recording high-amplitude, high-frequency sea level changes. This could result in less cosmopolitan faunas because of environments that are compartmentalized in space and time. Furthermore, genera that were adapted to life in dominantly carbonate environments may have been less successful in the Late Carboniferous due to the greater influx of siliciclastic sediments in many environments. To evaluate these and other hypotheses, we drew from museum collections and literature compilations to construct a database of over 12,000 genus occurrences spanning the entire Carboniferous of North America. The lithological affinity of each genus was determined based on its membership in siliciclastic and carbonate collections. Analysis of these data indicates that sample-level diversity drops in carbonate environments throughout the early Carboniferous, reaching a low point in the Serpukhovian, coincident with the onset of icehouse conditions. The dip in sample-level diversity is, however, short-lived as previous levels are again attained by the middle-late Carboniferous. Diversity in siliciclastic environments remains stable or increases slightly during this interval and is not affected by the climatic transition. Samples from the transition interval show an increase in dominance yet a lower collection-membership for genera classified as generalists (an affinity with neither sliciclastic nor carbonate environments). While further characterization and confirmation of this pattern is necessary, it implies that the transition from greenhouse to icehouse conditions may have favored more specialized genera for a brief time.