Response of nearshore marine assemblages to global change: a comparison of molluscan assemblages in Pleistocene and modern rhodolith beds in the southwestern Gulf of California, México

Marine life in the Gulf of California has experienced large changes in climate and sea level during the past several hundred thousand years. To evaluate the potential effects of these changes on marine mollusc species richness and similarity, modern molluscan assemblages associated with rhodolith beds in the southwestern Gulf of California were compared to their fossil counterparts from the oxygen isotope substage 5e (I.S. 5e) interglacial period, approximately 125 000 yr ago. A combined total of 219 taxa were found in I.S. 5e and modern assemblages. In general, raw and rarefied richnesses were significantly greater in I.S. 5e assemblages than in modern assemblages, and differences between assemblages suggest I.S. 5e are time-averaged deposits. Similarities between I.S. 5e and modern assemblages were lower than similarities within I.S. 5e assemblages. The results indicate that molluscan assemblages in the Gulf of California experienced some changes in taxonomic richness and composition during the environmental fluctuations of the past 125 000 yr, but not large scale taxonomic reorganization. The precise amount of taxonomic change is difficult to quantify due to time averaging.