Diversification of rocky-shore biotas through geologic time

Changes in biodiversity of rocky-shore ecosystems from the early Precambria n (3,500 Ma) to the last interglacial epoch (125 Ka) are summarized on the basis of the fossil record associated with geological unconformities that r eflect coastal paleotopography. This analysis is derived from data reported in 130 published papers culled and updated from previous bibliographic rev iews. Minimum total diversity of fossil and extant species treated herein i s 655 species. The highest biodiversity from any single locality is a mollu sk-dominated biota of 62 species from San Nicolas Island on the Pacific coa st of North America dating from the last interglacial epoch. Diversificatio n was affected by mass extinctions, as rocky-shore ecosystems expanded and contracted through a combination of species attributed to Archaic, Paleozoi c, Mesozoic, and/or Modern biotas. Stromatolites dominated Precambrian rock y shores, but continued as the principal Archaic biota through to the Mioce ne. The Paleozoic rocky-shore biota is characterized by encrusting inarticu late brachiopods, tabulate corals, and polyplacophorans, as well as ichnofo ssils representative of boring sipunculid worms (ichnogenus Trypanites) and acrothoracican barnacles (ichnogenus Zapfella). Boring bivalves (ichnogenu s Gastrochaenolites), encrusting bivalves (including oysters and rudists), scleractinian corals, and coralline red algae, as well as terebratulid brac hiopods, are typical of an enhanced Mesozoic rocky-shore biota. The much ex panded biodiversity of the Modern rocky-shore biota is demonstrated by clin ging but mobile gastropods, fixed bivalves that adopted byssate and wedging habits, and by balanomorph barnacles. Adaptive innovations played critical roles in the long-term colonization of rocky-shore substrates, but the pri mary force behind the expansion of rocky-shore ecosystems through geologic time was selective biotic displacement from offshore low-energy to onshore high-energy settings. Rocky coastlines subjected to strong and persistent w ave shock are effective "safe places" where species living in the intertida l zone often find refuge from predators and other competitors. This thesis is tested by checking the offshore origins of successful rocky-shore groups including barnacles, bivalves, corals, and coralline red algae. Concepts o f keystone species and ecological locking in ancient rocky-shore ecosystems are explored. Latitudinal gradients and other geographic relationships amo ng Pleistocene rocky-shore groups are commensurate with the Recent record, but only vaguely apparent for groups dating from earlier periods such as th e Cretaceous. Time intervals for which even the most rudimentary data on ro cky-shore biotas are most sparse include the Paleocene, Triassic, and the D evonian.