Rejuvenescence suggests recurrent coral responses in Miocene and modern reefs near upper thermal tolerance limits

Understanding coral-climate interactions through deep time is essential for evaluating reef resilience under ongoing climate change. Rejuvenescence is a rare and poorly documented phenomenon in fossil and extant reef-building scleractinian corals, involving polyp contraction followed by regeneration, and has previously been described as a survival strategy for coping with acute thermal stress in a living species. Here, we report evidence of rejuvenescence from a fossil reef in the Makran region of Iran, dated to the Miocene Climatic Optimum, and place it within an ecological and evolutionary framework. Its occurrence in both Miocene Iranian Acanthastrea cf. polygonalis and modern Mediterranean Cladocora caespitosa, despite their phylogenetic, temporal, and biogeographic separation, suggests a recurrent morphogenetic response associated with inferred thermal stress and potentially linked to metabolic downregulation, while its evolutionary origin remains unresolved between deep homology and convergent evolution. This putative role of rejuvenescence in promoting survival appears to be restricted to individual colonies or species and does not translate into thermal resilience at the reef-community scale. Accordingly, current evidence does not support rejuvenescence as a mechanism enhancing coral reef climate resistance but instead identifies it as an indicator of conditions approaching upper thermal tolerance limits in reef ecosystems across geological timescales.