Trajectories of reef coral morphological traits during the early Paleogene hothouse reveal the predictive limits of fossil analogues in a rapidly changing climate

Ancient coral reefs offer critical perspectives on how reef-building organisms responded to past climatic perturbations. This study examines the morphological responses of reef-dwelling scleractinian corals during the early Paleogene (Paleocene–Eocene) hothouse in the Mediterranean region, encompassing the global reef crisis associated with the Paleocene–Eocene Thermal Maximum (PETM) and the Early Eocene Climatic Optimum (EECO), and the subsequent reef recovery phase. Using the newly compiled PalEoCoralTraits (PECT) dataset, we analyse a suite of morphological traits (colony size, growth form, corallite integration, budding type, corallite size and spacing, and septal number) across reef systems affected by long-term ecological stress and environmental change. Our results show that coral trait responses to early Paleogene hyperthermals only partially align with those observed in modern corals under current warming. While some traits (e.g., colony size) display resilient patterns, others (e.g., growth form, corallite integration) follow divergent trajectories. Increased branching correlates with the proliferation of mixed siliciclastic–carbonate settings, possibly reflecting adaptations to sedimentation and turbidity in mesophotic environments. Morphological disparity rose from the EECO to the late Eocene, coinciding with global cooling and the diversification of Symbiodinium clades, potentially supporting renewed reef development and functional innovation. Discrepancies between fossil and modern coral responses likely result from differences in the rate and magnitude of warming, the presence of Anthropocene-specific stressors, and variation in data resolution, sampling biases, and preservation quality. The decoupling of coral diversity and reef development in the Paleocene–Eocene further points to distinct boundary conditions or ecological thresholds. These findings highlight the value of the fossil record for identifying ecological tipping points, recovery dynamics, and trait-based strategies that shaped coral reef persistence under climatic stress. • A new dataset captures coral trait variation during the early Paleogene hothouse • Coral traits show partial alignment with modern responses to hyperthermal stress • Branching increased with mixed siliciclastic–carbonate reef environments • Coral trait disparity rose from EECO to late Eocene with reef recovery and cooling • Fossils reveal coral resilience across past climate-driven reef crises.