Age constraints confirm antiquity of the earliest Triassic oceanic reptile fossils

Abstract The geologically oldest sea-going reptile fossils occur in Lower Triassic (Smithian, ca. 250 Ma) strata of the Lusitaniadalen Member ([LM], Vikinghøgda Formation) on the Norwegian Arctic island of Spitsbergen. Yet some recent studies have dismissed this biostratigraphically and chemostratigraphically demonstrated record with a counterclaim that fully pelagic reptiles must have originated up to ~1 m.y. later (mid-Spathian, ca. 248.7 ± 1.0 Ma) because of radiometric dating on a deposit in China. Here, we resolve this contention using a comprehensive suite of stratigraphic, isotopic, elemental, and petrographic analyses that unanimously age-correlate the LM marine reptile fossils with the middle-to-upper Smithian Euflemingites romunduri–Wasatchites tardus ammonoid zones. These underlie a stratigraphic hiatus preceding the Smithian–Spathian boundary (ca. 249.236 Ma). The LM fossil-bearing layers do not extend down-sequence beyond the Dienerian–Smithian boundary (ca. 250.626 Ma). Thus, the oldest-known oceanic reptile remains are unambiguously constrained at ca. 1.24–2.63 Ma after the cataclysmic end-Permian mass extinction (ca. 251.867 Ma).