This giant polar reptile once stalked an ancient super-ocean

Fossils show that the sea dragon-like New Zealand nothosaur had sharp teeth and reached 23 feet in length.

Posted on Jun 17, 2024 11:00 AM EDT

An illustration of nothosaurs swimming and eating along the ancient southern polar coast of present day New Zealand. These are the oldest known sea-going reptiles from the Southern Hemisphere. Stavros Kundromichalis

In today’s oceans, sea turtles, marine iguanas, saltwater crocodiles, and sea snakes are the primary reptilian residents amongst tons of mammals and fish. This was not always the case, as fossil evidence shows that about 252 million years ago reptiles dominated the seas. Now, an international team of scientists have put together another piece of this puzzle and identified the oldest fossil of a sea-dwelling reptile from the Southern Hemisphere. The vertebra fossil belonged to the sea dragon-like nothosaurus and was found in a stream bed on New Zealand’s South Island. The findings are described in a study published June 17 in the journal Current Biology.

[Related: New species of extinct marine reptile found with help from 11-year-old child.]

When reptiles ruled the seas

Millions of years before dinosaurs roamed the Earth, reptiles were the kings of Earth’s seas. 

The most diverse and geologically longest surviving group of these extinct marine reptiles are the sauropterygians. They have an evolutionary history spanning over 180 million years. Sauropterygians included the long-necked plesiosaurs–which looked like the popular image of the Loch Ness Monster.

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

The nothosaur was a distant predecessor of the plesiosaurs. They were roughly 23 feet long and used four paddle-like limbs to swim and flattened skulls with slender conical teeth inside their mouths that were used to catch fish and squid.

The nothosaurus vertebra found in this study dates back to when present day New Zealand was located on the southern polar coast of a vast super-ocean called Panthalassa. When a mass extinction called the Great Dying devastated marine ecosystems about 250 million years ago, the surviving reptiles found opportunity in Earth’s oceans. 

Scientists have found evidence of this evolutionary benchmark on the Arctic island of Spitsbergen, northwestern North America, and southwestern China. This single nothosaur vertebra fossil in the study is one of the latest finds from this time period and could shed new light on the history of ancient sea reptiles from the Southern Hemisphere. 

A new look at an old fossil

The New Zealand nothosaur was initially discovered during a geological survey in 1978, embedded in a boulder in a stream at the foot of Mount Harper on the South Island of New Zealand. The importance of this was not fully recognized until a team of paleontologists from Australia, East Timor, Norway, New Zealand, and Sweden collaborated to examine and analyze the vertebra and other fossils. 

“The nothosaur found in New Zealand is over 40 million years older than the previously oldest known sauropterygian fossils from the Southern Hemisphere,” Benjamin Kear, a study co-author and paleontologist at The Museum of Evolution at Uppsala University in Sweden, said in a statement. “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.”

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

The oldest known nothosaur fossils are roughly 248 million years old. They have primarily been found along an ancient northern low-latitude belt that stretched from the remote northeastern to northwestern borders of the Panthalassa super-ocean. 

Surfing the Panthalassa super-ocean

Paleontologists are still debating the origin, distribution, and timing of when nothosaurs reached these distant areas. Some prevailing theories suggest that they either migrated along northern polar coastlines, swam through inland seaways, or utilized currents to cross the Panthalassa super-ocean. Now, this new nothosaur fossil is throwing some cold water on these hypotheses.

“Using a time-calibrated evolutionary model of sauropterygian global distributions, we show that nothosaurs originated near the equator, then rapidly spread both northwards and southwards at the same time as complex marine ecosystems became re-established after the cataclysmic mass extinction that marked the beginning of the Age of Dinosaurs,” said Kear.

[Related: This Jurassic-era ‘sea murderer’ was among the first of its kind.]

When the Age of the Dinosaurs began, the Earth was going through an extreme period of global warming. The warm temperatures allowed these marine reptiles to thrive at Earth’s South Pole. Kear and the team believe that this suggests the ancient polar regions were likely the primary route for the nothosaurus’ earliest global migration. This is similar to the incredibly long migrations undertaken by today’s whales.

More study is needed to confirm this and will only come from digging up more remains of this ancient real-life sea dragon. 

“Undoubtedly, there are more fossil remains of long-extinct sea monsters waiting to be discovered in New Zealand and elsewhere in the Southern Hemisphere,” says Kear.

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