Oldest southern sauropterygian reveals early marine reptile globalization

Sauropterygians were the stratigraphically longest-ranging clade of Mesozoic marine reptiles with a global fossil record spanning ∼180 million years¹. 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] ago²), and via possible trans-Arctic migration¹ to the Eastern Panthalassa super-ocean prior to the earliest Middle Triassic (Olenekian-earliest Anisian^3,4, ∼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 Ma⁶) of New Zealand. Time-scaled ancestral range estimations thus reveal an unexpected circum-Gondwanan high-paleolatitude (>60° S⁷) dispersal from a northern Tethyan origination center. This coincides with the adaptive diversification of sauropterygians after the end-Permian mass extinction⁸ and suggests that rapid globalization accompanied their initial radiation in the earliest Mesozoic.