Reconstruction of the oldest sea-going reptile from the Southern Hemisphere. Nothosaurs swimming along the ancient southern polar coast of what is now New Zealand around 246 million years ago. Artwork by Stavros Kundromichalis. (CREDIT: Stavros Kundromichalis)

UPPSALA, Sweden — Scientists have unearthed the geologically oldest sea-going reptile from the Southern Hemisphere. The fossil, a nothosaur (a type of sauropterygian reptile) dating back to the Middle Triassic period (around 246 million years ago), was found in New Zealand. It sheds light on the surprising global dispersal of these ancient creatures, challenging previous beliefs about the early evolution of sauropterygians.

Sauropterygians were a group of marine reptiles that ruled the oceans for an astonishing 180 million years during the Mesozoic era, the age of the dinosaurs. These creatures, which included the well-known plesiosaurs, were previously thought to have originated and evolved primarily in the Northern Hemisphere, specifically along the northern and western margins of the ancient Tethys Ocean. However, the discovery of this nothosaur fossil in New Zealand has turned that notion on its head.

The fossil, cataloged as “GNS CD 540” and revealed in the journal Current Biology, was found in a loose boulder along the Balmacaan Stream at the base of Mt. Harper in New Zealand’s South Island. The rock encasing the fossil belongs to the Balmacaan Formation, which has yielded a rich array of invertebrate fossils that help date the specimen to the mid-lower to upper Anisian stage of the Middle Triassic, around 246 million years ago.

“The nothosaur found in New Zealand is over 40 million years older than the previously oldest known sauropterygian fossils from the Southern Hemisphere. We show that these ancient sea reptiles lived in a shallow coastal environment teeming with marine creatures within what was then the southern polar circle,” explains Dr. Benjamin Kear from The Museum of Evolution at Uppsala University, in a media release.

Original fossil of the New Zealand nothosaur vertebra. The oldest sea-going reptile from the Southern Hemisphere.
Original fossil of the New Zealand nothosaur vertebra. The oldest sea-going reptile from the Southern Hemisphere. (CREDIT: Benjamin Kear)

Methodology: Unraveling the Fossil’s Secrets

To uncover the significance of this fossil, the research team, led by Benjamin P. Kear from Uppsala University, employed a multidisciplinary approach. They carefully analyzed the morphology of the preserved vertebra using micro-computed tomography (mCT) imaging, which revealed characteristic features of eosauropterygians, a group that includes nothosaurs, pachypleurosaurs, pistosaurs, and plesiosaurs.

The team then scored the fossil into a previously published tip-dated phylogeny of sauropterygians, which allowed them to estimate the ancestral ranges and reconstruct the dispersal history of the group. This process involved comparing the fossil to known sauropterygian species and determining its evolutionary relationships and geographic origins.

Results: A Surprising Southern Journey

The analyses unanimously placed GNS CD 540 within the monophyletic clade Nothosauroidea, which comprises nothosaurs and their close relatives, simosaurids and pistosauroids. The results suggest that nothosauroids originated in the northeastern Tethys Ocean during the earliest Triassic, around 248 million years ago, and subsequently expanded their range into the northwestern Tethys.

Remarkably, the discovery of GNS CD 540 in New Zealand indicates that nothosaurs had achieved an extended extra-Tethyan dispersal, circumscribing the entire peninsular landmass of eastern Gondwana to reach the southern polar periphery of the Panthalassa Ocean by at least the early-Middle Triassic. This finding challenges previous assumptions that adverse paleoenvironments prevented Triassic sauropterygians from dispersing towards the poles.

Reconstruction of the New Zealand nothosaur. The oldest sea-going reptile from the Southern Hemisphere.
Reconstruction of the New Zealand nothosaur. The oldest sea-going reptile from the Southern Hemisphere. (CREDIT: Johan Egerkrans)

Study Limitations

While this study provides compelling evidence for the early globalization of sauropterygians, the authors acknowledge some limitations. The fossil record of Southern Hemisphere sauropterygians is still relatively sparse, with GNS CD 540 being the only known specimen from the Middle Triassic of this region. More fossils from a broader range of locations would help to further solidify the findings and provide a more comprehensive understanding of sauropterygian dispersal patterns.

Additionally, the species-level geographic sampling of the phylogenetic dataset used in the study is incomplete, which may influence the reconstructed ancestral ranges and dispersal routes. As more fossils are discovered and added to the dataset, the evolutionary and biogeographic picture may become clearer.

Discussion: Rewriting the History of Marine Reptile Evolution

The discovery of GNS CD 540 has significant implications for our understanding of early marine reptile evolution and dispersal. It demonstrates that sauropterygians had achieved a global distribution much earlier than previously thought, coinciding with their adaptive diversification after the end-Permian mass extinction around 252 million years ago.

This finding highlights the importance of exploring new geographic areas for fossils, as they can yield surprising discoveries that challenge long-held assumptions about the evolution and distribution of ancient life. The presence of a nothosaur in the high-paleolatitude waters of the Southern Hemisphere suggests that these marine reptiles were more adaptable and widespread than previously recognized.

Furthermore, this study underscores the need for continued research into the early evolution of marine tetrapods and the factors that influenced their dispersal patterns. As more fossils are discovered and analyzed, we can gain a more comprehensive understanding of how these fascinating creatures responded to changing environmental conditions and spread across the globe.

“The beginning of the Age of Dinosaurs was characterized by extreme global warming, which allowed these marine reptiles to thrive at the South Pole. This also suggests that the ancient polar regions were a likely route for their earliest global migrations, much like the epic trans-oceanic journeys undertaken by whales today. Undoubtedly, there are more fossil remains of long-extinct sea monsters waiting to be discovered in New Zealand and elsewhere in the Southern Hemisphere,” Kear concludes.

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