Endemic Fossils Record the Pre‐Mesozoic History of Panthalassan Terranes

The paleogeographic origins of accreted terranes remain among the most challenging problems in global tectonics. Here, we use the biogeography of late Paleozoic symbiont‐bearing fusulinid foraminifera to reconstruct the origins and migration pathways of (proto)terranes now incorporated into the Cordilleran systems of North America, Japan, northeastern Russia, and New Zealand. We show that the distribution of narrowly endemic taxa, particularly the Gifuellinae and Lepidolininae, delineates a belt of carbonate atolls and microplatforms that developed near the Panthalassa–Paleo‐Tethys Junction (PPTJ) during the Capitanian. These biogeographic signals, preserved within limestone blocks incorporated into Mesozoic accretionary complexes, constrain the pre‐Mesozoic positions of seamount chains and carbonate platforms that formed along major Panthalassan plate boundaries. Patterns of endemism among fusulinid assemblages suggest the existence of isolated atoll systems along the ancestral Panthalassan plate boundaries, which were subsequently dispersed by Late Triassic–Jurassic and Cretaceous oceanic spreading and terrane transport. Our results demonstrate that fusulinid biogeography provides a powerful and independent framework for reconstructing terrane evolution and oceanic plate dynamics through deep time, particularly where the original oceanic record has been obscured or destroyed by subduction. This approach offers important constraints on the origin, migration history, and paleogeographic relationships of circum‐Panthalassan terranes that are difficult to resolve using geochemical, tectonic, and geophysical data alone.