Development of Intertidal Biotas Through Phanerozoic Time

Changes in the biodiversity and organization of intertidal biotas from rocky, sandy, and muddy shores are summarized on the basis of information in the fossil record for 1622 extinct and extant species through Phanerozoic strata from the Cambrian to the Pleistocene at 361 localities around the world. To enter the database, each fossil species qualified as intertidal in origin based on sedimentological, geological, and other spatial criteria. Among the study sites documented in the scientific literature, 45% are considered former rocky shorelines. Another 31% represent former muddy shores and 24% are indicative of former sandy shores. Rocky-shore biotas demonstrate the greatest change in biodiversity, with species per Cenozoic study site nearly 2.5 times more than found on average at Paleozoic study sites. Key elements of the modern rocky shore biota were in place by Oligocene time and reflect much the same kind of ecological crowding found in that setting today. Coastal mudflat biotas show only a minor increase in biodiversity based on body fossils, although evidence from trace fossils implies an increase in ecological crowding through time. Sandy-shore biotas are the most conservative and least diverse in their development. The influence on intertidal habitat space by global tectonics, sea-level change, and relationship to other ecosystems is considered. When we go down to the low-tide line, we enter a world that is as old as the Earth itself – the primeval meeting place of the elements of earth and water. Rachel Carson, The Edge of the Sea