Earliest Triassic ichthyosaur fossils push back oceanic reptile origins

Reptiles first radiated into oceanic environments after the cataclysmic end-Permian mass extinction (EPME)1Nakajima Y. Shigeta Y. Houssaye A. Zakharov Y.D. Popov A.M. Sander P.M. Early Triassic ichthyopterygian fossils from the Russian Far East.Sci. Rep. 2022; 12: 5546Crossref PubMed Scopus (3) Google Scholar, 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)3Scheyer T.M. Romano C. Jenks J. Bucher H. Early Triassic marine biotic recovery: the predators’ perspective.PLoS One. 2014; 9e88987Crossref Scopus (105) Google Scholar, ∼249.6 Ma4 —less than ∼2.3 Ma after the EPME. Here, we report ichthyopterygian (the group including ‘fish-shaped’ ichthyosaurians1Nakajima Y. Shigeta Y. Houssaye A. Zakharov Y.D. Popov A.M. Sander P.M. Early Triassic ichthyopterygian fossils from the Russian Far East.Sci. Rep. 2022; 12: 5546Crossref PubMed Scopus (3) Google Scholar) remains from the Arctic island of Spitsbergen that predate the LSC in later-middle to early-late Smithian5Hansen B.B. Hammer Ø. Nakrem H.A. Stratigraphy and age of the Grippia niveau bonebed, Lower Triassic Vikinghøgda Formation.Spitsbergen. Norw. J. Geol. 2018; 98: 175-187Google Scholar deposits up to ∼250 Ma. Unexpectedly, however, their large size and spongy internal bone structure indicate a fully pelagic ichthyopterygian1Nakajima Y. Shigeta Y. Houssaye A. Zakharov Y.D. Popov A.M. Sander P.M. Early Triassic ichthyopterygian fossils from the Russian Far East.Sci. Rep. 2022; 12: 5546Crossref PubMed Scopus (3) Google Scholar,6Houssaye A. Nakajima Y. Sander P.M. Structural, functional, and physiological signals in ichthyosaur vertebral centrum microanatomy and histology.Geodiversitas. 2018; 40: 161-170Crossref Scopus (13) Google Scholar. 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 successors2Motani R. Jiang D.J. Tintori A. Ji C. Huang J.D. Pre- versus post-mass extinction divergence of Mesozoic marine reptiles dictated by time-scale dependence of evolutionary rates.Proc. R. Soc. B. 2017; 28420170241Crossref PubMed Scopus (30) Google Scholar,3Scheyer T.M. Romano C. Jenks J. Bucher H. Early Triassic marine biotic recovery: the predators’ perspective.PLoS One. 2014; 9e88987Crossref Scopus (105) Google Scholar within the earliest Mesozoic marine predator communities.