MASS EXTINCTIONS AND MARINE SCLEROBIONT COMMUNITY EVOLUTION IN THE PHANEROZOIC

Marine sclerobionts (organisms living on or in hard substrates) have a fossil record almost as long as life itself, and Phanerozoic sclerobiont communities have been particularly well described in the past few decades, yet we still know little about their responses to mass extinctions. When we examine the five major mass extinctions of the Phanerozoic, significant changes in sclerobiont community structure occur only after the massive end Permian extinction. At that time Paleozoic-type encrusting faunas dominated by bryozoans and echinoderms were replaced after a long recovery period by faunas of primarily cemented bivalves and corals. Paleozoic bioeroders are predominantly the ubiquitous tubular boring Trypanites, but in the Mesozoic borings diversified greatly with the rise of bioeroding sponges, bivalves and barnacles. At all the other mass extinctions the sclerobiont communities on either side share the same basic ecological structure, despite some taxonomic turnover. Sclerobiont communities show a strong structural conservatism and stability compared to their soft substrate equivalents. We need, though, detailed paleoecological and systematic studies of sclerobionts through mass extinction intervals to test these observations. There are advantages to using sclerobiont assemblages in this work because they are preserved in situ, often showing symbiotic and competitive relationships, and they are generally well preserved. However, studies are complicated because finding equivalent hard substrates below and above an extinction boundary is difficult. Substrates such as invertebrate shells may change dramatically in composition and abundance after a mass extinction, necessarily forcing modifications in the encrusting and boring communities inhabiting them and obscuring evolutionary changes in sclerobiont communities themselves. In some cases changes in nutrient levels and other conditions raised rates of bioerosion during and after mass extinctions, further destroying hard substrates and the record of any associated sclerobionts. Nevertheless there are opportunities to study sclerobiont community changes on surviving skeletal taxa such as thick-shelled mollusks and corals, and on inorganic hard substrates such as hardgrounds and rockgrounds.