Abstract

Sauropterygians were the stratigraphically longest-ranging clade of Mesozoic marine reptiles with a global fossil record spanning ∼180 million years. However, their early evolution has only been known from what is now the Northern Hemisphere, extending across the northern and trans-equatorial western margins of the Tethys paleo-ocean1 after the late-Early Triassic (late Olenekian, ∼248.8 million years [Ma] ago), and via possible trans-Arctic migration to the Eastern Panthalassa super-ocean prior to the earliest Middle Triassic (Olenekian–earliest Anisian, ∼247 Ma). Here, we describe the geologically oldest sea-going reptile from the Southern Hemisphere — a nothosaur (basal sauropterygian5) from the Middle Triassic (Anisian, after ∼246 Ma) of New Zealand. Time-scaled ancestral range estimations thus reveal an unexpected circum-Gondwanan high-paleolatitude (>60° S) dispersal from a northern Tethyan origination center. This coincides with the adaptive diversification of sauropterygians after the end-Permian mass extinction and suggests that rapid globalization accompanied their initial radiation in the earliest Mesozoic.