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).
Kear et al. (Mon,) studied this question.