IMPACT OF METAZOAN REEFS ON MIDDLE ORDOVICIAN SEASCAPE HETEROGENEITY (Invited Presentation)

Reef-building is an important type of ecosystem engineering. Reef builders generate structural complexity and habitat heterogeneity, which underpin reefs’ high biodiversity and probable role as centers of speciation over long timescales. In modern reefs, the type and diversity of reef-building organisms influence the diversity of reef-dwelling communities. The Middle Ordovician (~ 470-458 Ma) saw a major marine biodiversification, accompanied by a global shift from microbial- to metazoan-dominated reefs, often dominated by tabulate corals, stromatoporoids and bryozoans. This novel metazoan reef type invaded soft-substrate marine settings, an example of the global impact of metazoan ecosystem engineering on early Paleozoic shallow marine environments.

The exceptionally well-exposed reef-bearing carbonate rocks of the Mingan Archipelago National Park Reserve, Quebec, present an excellent opportunity to investigate reef composition and growth in a Middle Ordovician (Darriwilian, ~467-458 Ma) shallow marine setting. Reefs in the Mingan Formation are typically matrix-rich, metre-scale mounds containing abundant, diverse skeletal fossils. Previous study of the stratigraphy and facies associations of the Mingan Formation, and the paleokarst horizons which punctuate it, provides good constraints on relative water depth, and paleoenvironment of reef horizons, while excellent preservation allows detailed study of reef textures and fossil assemblages.

Using field mapping of reef compositions, point counts, and hierarchical cluster analyses, we quantified and visualized the heterogeneity of reef compositions in the Mingan Archipelago. Detailed sedimentological data were also used to constrain the environmental contexts in which the reefs developed. The increasing diversity and abundance of metazoan reef builders generated high spatial heterogeneity in reef composition at scales of meters to kilometers.

Investigation of the many forms of habitat heterogeneity in reefs promises insights into how this form of ecosystem engineering has changed over time. Given the ecological and evolutionary importance of reefs, such studies may provide constraints on the mechanisms by which marine diversity is generated and sustained.