CAM photosynthesis may have conferred an advantage during the Permian–Triassic mass extinction event

The Permian–Triassic mass extinction represents the most severe loss of biodiversity in Earth history and profoundly reorganized terrestrial ecosystems. On land, this crisis was followed by a marked floral turnover, with herbaceous lycophytes dominating Early Triassic vegetation. Here we show that these pioneer (so-called disaster) taxa that rapidly colonized stressed post-extinction environments, possessed specialized physiological traits that promoted survival under extreme conditions. Independent phylogenetic analyses show that Early Triassic lycophytes are closely related to modern Isoetales, a group characterized by exceptional ecophysiological flexibility. Their carbon isotope signatures resemble those of extant Isoetes that use crassulacean acid metabolism (CAM) photosynthesis, indicating a similar physiological strategy in deep time. Coupling these results with climate simulations suggests that CAM photosynthesis could have conferred a substantial advantage under Early Triassic super greenhouse conditions. Together, our findings identify CAM physiology as a potential mechanism enabling plant survival and ecosystem recovery following Earth’s largest mass extinction.