Multiple paths to recovery after the Permian-Triassic mass extinction

The current biodiversity landscape results from hundreds of millions of years of evolution, yet the accumulation of biodiversity has been punctuated by mass extinctions. A key question is how surviving lineages rebuilt diversity after extinction events. Here, we examine the morphological and taxonomic recovery of three clades (i.e., ammonoids, brachiopods, and ostracods) that experienced selective extinction during the Permian-Triassic mass extinction (PTME) event. Using a deep learning method-based tool for automatic extraction of morphological features, combined with quantitative taxonomic diversity measures, we find two main paths of biotic recovery following the PTME: refilling mode, in which ammonoids and brachiopods rebound in biodiversity by refilling the vacated morphospace with limited innovation, leading to a partial recovery of their previous disparity and taxonomic diversity, and expansion mode, in which ostracods underwent an adaptive radiation, with their morphospace expanding rapidly during the Early-Middle Triassic and then stabilizing post-Triassic. Their ecological niches expanded substantially during the post-Paleozoic, and their diversity exceeded its pre-extinction level during the Jurassic. These findings suggest that rapid morphological diversification could facilitate ecological expansion and thereby foster long-term growth in taxonomic diversity. The PTME played a fundamental role in reshaping marine ecosystem structure by triggering different recovery trajectories among survivors.