A giant bowfin from a Paleocene hothouse ecosystem in North America

Aquatic biodiversity changed dramatically at the start of the Paleogene. Although comparatively little is known about global freshwater ecosystems that appeared just after the Cretaceous-Paleogene mass extinction, available data suggest that they were buffered from the worst effects of the extinction event. Here, we describe a nearly complete skeleton of a large-bodied ray-finned fish from a wetland ecosystem that existed fewer than 10 Myr after the end-Cretaceous extinction in western North America. With a maximum length likely exceeding 2 m, dagger Amia basiloides sp. nov. is one of the largest species in Holostei, a once species-rich clade of ray-finned fishes now survived by the nine living species of gars and bowfins. High-resolution computed tomography scans illuminate the anatomy of dagger A. basiloides and suggest it was an analogue of living large-bodied, piscivorous freshwater fishes found in the Southern Hemisphere and southern North America. When considered in a phylogenetic context, dagger A. basiloides shows that close relatives of living bowfins rapidly achieved gigantism in the Early Paleogene of North America after the largest members of an ancient clade closely related to bowfins went extinct. Although the Cretaceous-Paleogene boundary likely induced turnover of freshwater vertebrate predatory guilds, holostean faunas remained ecologically comparable across the extinction due to exceptionally fast episodes of convergence.