Ordovician change in the nature of seafloor carbonates saw rapid decline of previously widespread flat pebble conglomerates and the Palaeozoic peak abundance of hardgrounds. The effective disappearance of flat pebble conglomerates, widely attributed to physical disruption of substrate by bioturbation, is reinterpreted as reflecting increased depth of carbonate precipitation below the Taphonomically Active Zone such that early lithified carbonates were less frequently reworked by scour. With deeper, more stable zones of cementation, exhumed limestones formed hardgrounds, whose mid-Ordovician acme supported rapid increase in epizoan diversity. Further deepening of cementation to below normal scour accompanied post-Ordovician decline in submarine hardgrounds.

Supplementary materials:

Database for Figure 2 is available at http://www.geolsoc.org.uk/SUP18896.

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