Diversification of rocky-shore biotas through geologic time

Changes in biodiversity of rocky-shore ecosystems from the early Precambrian (3,500 Ma) to the last interglacial epoch (125 Ka) are summarized on the basis of the fossil record associated with geological unconformities that reflect coastal paleotopography. This analysis is derived from data reported in 130 published papers culled and updated from previous bibliographic reviews. Minimum total diversity of fossil and extant species treated herein is 655 species. The highest biodiversity from any single locality is a mollusk-dominated biota of 62 species from San Nicolas Island on the Pacific coast of North America dating from the last interglacial epoch. Diversification was affected by mass extinctions, as rocky-shore ecosystems expanded and contracted through a combination of species attributed to Archaic, Paleozoic, Mesozoic, and/or Modern biotas. Stromatolites dominated Precambrian rocky shores, but continued as the principal Archaic biota through to the Miocene. The Paleozoic rocky-shore biota is characterized by encrusting inarticulate brachiopods, tabulate corals, and polyplacophorans, as well as ichnofossils representative of boring sipunculid worms (ichnogenus Trypanites) and acrothoracican barnacles (ichnogenus Zapfella). Boring bivalves (ichnogenus Gastrochaenolites), encrusting bivalves (including oysters and rudists), scleractinian corals, and coralline red algae, as well as terebratulid brachiopods, are typical of an enhanced Mesozoic rocky-shore biota. The much expanded biodiversity of the Modern rocky-shore biota is demonstrated by clinging 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 primary 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 wave shock are effective “safe places” where species living in the intertidal 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 of keystone species and ecological locking in ancient rocky-shore ecosystems are explored. Latitudinal gradients and other geographic relationships among Pleistocene rocky-shore groups are commensurate with the Recent record, but only vaguely apparent for groups dating from earlier periods such as the Cretaceous. Time intervals for which even the most rudimentary data on rocky-shore biotas are most sparse include the Paleocene, Triassic, and the Devonian.