Abstract
Hard substrate communities can impact coral reef growth by adding or removing calcium carbonate when they act as encrusters or bioeroders, respectively. Although such sclerobiont communities are known across the Phanerozoic, the Triassic saw a substantial increase in reef macrobioerosion. This study provides the first quantitative assessment of sclerobionts in a Late Triassic (Norian) reef coral community from the Nayband Formation in central Iran and establishes some post-Paleozoic trends in reef bioerosion. Sclerobionts were common on the dominant coral Pamiroseris rectilamellosa and covered between 0 and 26.4% of total coral surface area among the 145 colonies investigated. Encrustation was significantly more prevalent (1.78% of total area) than bioerosion (0.36% of total area). The underside of corals was 3.5 times more affected by sclerobionts than the surfaces. This suggests that the sclerobionts preferentially colonized dead parts of corals. The main encrusting taxa were polychaeta worms (76.2%), followed by bivalves (11.9%). The main bioeroders were of Polychaeta (51.4%), Porifera (22.2%), and Bivalvia (20.7%). The lack of a correlation between encrustation and bioerosion intensity suggests that the amount of sclerobionts is not simply a function of exposure time before final burial. Our review of the published literature suggests a declining trend in the relative importance of bivalves, polychaetas and cirripeds in reefs, whereas sponges increased in importance from the Triassic until today.