Patterns of turnover in Middle and Upper Ordovician brachiopods of the eastern United States: a test of coordinated stasis

A compilation of species occurrences in a chronostratigraphic framework of depositional sequences from a 250,000 km2 area in the eastern United States is used to test for coordinated stasis in Middle and Upper Ordovician articulate brachiopods. Two rapid pulses of turnover in brachiopod species separate three periods of relatively lower turnover (ecologic-evolutionary [EE] subunits) that range from 3 to 9 m.y. in duration. Turnover within these EE subunits is characterized by high levels of percent species origination (ca. 60%) and percent species extinction (ca. 80%) and low levels of percent species persistence (<10%), all of which fall outside the range reported for coordinated stasis. Turnover between EE subunits is characterized by low levels of percent species holdover and percent species carryover (ca. 10% or less) and is consistent with coordinated stasis, although turnover pulses are driven largely by pulses in either extinction or origination, and not by pulses in both, as reported for coordinated stasis. Taken together, although these data display a marked bimodality in turnover, high levels of turnover within EE subunits is inconsistent with a pattern of coordinated stasis. Turnover rates within these EE subunits are much higher than previous global estimates for Cambro-Ordovician brachiopods or Phanerozoic marine species and indicate that local extirpation and migration play a significant role in regional biodiversity dynamics. Despite the high level of turnover observed within these EE subunits, some level of ecologic stability occurs because abundant genera persist throughout entire EE subunits. Ordovician species in this study behaved relatively independently of other taxa and were not tightly integrated as suggested by the broadly overlapping taxon abundance curves, the shifting habitat preference of some taxa, the piecemeal turnover between EE subunits, and the continuous creation of new species associations due to background levels of turnover within EE subunits. Turnover within EE subunits was associated with relatively stable or only mildly fluctuating environments. Rapid turnover between EE subunits was caused by extreme perturbations to the regional or possibly global ocean-climate system.