Abstract

Summary. Reptiles first radiated into oceanic environments after the cataclysmic end-Permian mass extinction (EPME)¹, 251.9 million years (Ma) ago. The geologically oldest fossils evincing this adaptive transition have been recovered from upper-Lower Triassic (lower Spathian) strata, ∼248.8 Ma², and postdate a landmark turnover of amphibian-dominated to reptile-dominated marine ecosystems spanning the late Smithian crisis (LSC)³, ∼249.6 Ma⁴ -less than ∼2.3 Ma after the EPME. Here, we report ichthyopterygian (the group including 'fish-shaped' ichthyosaurians¹) remains from the Arctic island of Spitsbergen that predate the LSC in later-middle to early-late Smithian⁵ deposits up to ∼250 Ma. Unexpectedly, however, their large size and spongy internal bone structure indicate a fully pelagic ichthyopterygian^1,6. Given this unambiguous occurrence ∼2 Ma after the EPME, these pioneering seagoing tetrapods can now be feasibly recast as mass extinction survivors instead of ecological successors^2,3 within the earliest Mesozoic marine predator communities.