Earliest Triassic ichthyosaur fossils push back oceanic reptile origins
Benjamin P. Kear, Victoria S. Engelschiøn, Øyvind Hammer, Aubrey J. Roberts, Jørn H. Hurum
Abstract
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 Ma 2 , and postdate a landmark turnover of amphibian-dominated to reptile-dominated marine ecosystems spanning the late Smithian crisis (LSC) 3 , ∼249.6 Ma 4 —less than ∼2.3 Ma after the EPME. Here, we report ichthyopterygian (the group including ‘fish-shaped' ichthyosaurians 1 ) remains from the Arctic island of Spitsbergen that predate the LSC in later-middle to early-late Smithian 5 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.