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 ichthyopterygian1,6. 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 successors2,3 within the earliest Mesozoic marine predator communities.
