Unique find: The oldest fossil of its kind in the Southern Hemisphere

“This is the reptilian equivalent of a sea lion,” says Jørn Hurum, professor of palaeontology at the University of Oslo.

“Imagine a cross between a sea lion and an otter with a slightly long neck. It has webbed limbs and a mouth full of sharp teeth for catch fish and squid,” he says.

This is how Hurum describes the deceased owner of the bone that he and his colleagues now present in the scientific journal Current Biology.

However, it is not the animal itself that makes the discovery special. The fossil comes from a nothosaur, which is a well-known marine reptile from the Triassic period.

But the age and location of the fossil make it truly unique:

“This is the oldest fossil of a marine reptile that has been found in the Southern Hemisphere,” says Hurum.

It holds significant importance for our understanding of evolution on Earth following the greatest known catastrophe that life has endured.

A catastrophic event

252 million years ago, the Earth turned into an inferno.

The largest known volcanic eruption in history filled the atmosphere with greenhouse gases, raising global temperatures by 10 to 15 degrees. In some regions, ocean temperatures reached up to 40 degrees Celsius, leading to the extinction of around 90 per cent of all marine species.

Life was dealt a heavy blow and was down for the count.

However, this did not last long.

In the more habitable conditions near the north and south poles, a new chapter in the evolution of life began:

The marine reptiles.

Went from land to water

The starting point was reptiles that lived on land.

While many large sharks and bony fish died out, some of these land-dwelling reptiles moved towards the water.

Within a relatively short time – a few million years – several reptiles adapted to life in water. Their bodies streamlined, limbs evolved into flippers or fins.

These water-adapted reptiles became the new predators of the sea. Finds from Svalbard, North America, and China indicate their rapid spread across the Northern Hemisphere.

But what about in the south?

So far, there has been no evidence of marine reptiles on the Southern Hemisphere in the tens of millions of years following the extinction event. Did these animals arrive there much later?

This is where the new fossil comes in.

Mysterious fossil

The fossil was actually found a long time ago, Benjamin Kear from the University of Uppsala says. He has worked with Hurum for many years and made many discoveries in Svalbard. But Kear originally comes from Australia.

“In late 2018, I was alerted by a colleague, Ewan Fordyce from Otago University in New Zealand, about the occurrence of a mysterious marine reptile fossil from the South Island that was found by geologists in 1978,” Kear writes to sciencenorway.no.

The fossil consisted of a single, well-preserved vertebra from a marine reptile. However, the discovery was made in loose sediments in a river. The researchers had an idea of which geological layer and which time it originated from, but this was never confirmed.

As a result, the bone was overlooked and remained forgotten in New Zealand's national palaeontological collections.

“Sadly, Ewan passed away before I could visit to see the specimen, but we finally managed to organise a loan of the fossil during our research visit and fieldwork in Australasia last year,” Kear writes.

The bone was thoroughly cleaned and examined at the Norwegian Center for Paleontology at the Natural History Museum, University of Oslo. That was when it became clear how special it was.

246 million years old

Remnants of sediments and fossil snails on the bone revealed that the vertebra was a staggering 246 million years old – just six million years younger than the extinction event.

There was no doubt that the bone came from a nothosaur, says Aubrey Roberts, researcher at the University of Oslo and the Natural History Museum in London.

“We’ve found more complete skeletons of nothosaurs in various locations across the Northern Hemisphere. This enables us to recognise the individual bones, she says.

A single bone can even reveal which nothosaurus species we’re talking about.

“The vertebrae are very different from species to species, so it’s quite easy to compare and see which group they belong to,” says Roberts.

So, the bone from New Zealand belonged to a known type of sea-dwelling reptile, but it lived 40 million years before other such reptiles found in the Southern Hemisphere.

Spread early

“We were surprised that it was so old,” says Hurum. “This must mean that the nothosaur managed to spread very far, very early.”

Previous nothosaur finds in the Northern Hemisphere suggest that the animal evolved here. They then spread southward along the coasts of Pangea – the enormous continent that once encompassed all the landmasses on the planet.

And this movement from north to south must have happened quickly.

Currently, researchers do not know how the animals made their way southward, through the dangerously warm seas at the equator. It is not even known which side of the giant continent they travelled along.

Yet, Hurum remains hopeful of uncovering better answers in the future.

Barely scratching the surface

The professor anticipates discovering new fossils in the Southern Hemisphere that will provide deeper insights into the migrations of marine reptiles.

The reason we have mostly found fossils in the north is likely because that's primarily where we have been searching.

“We’ve barely scratched the surface. Only in recent years have people started searching in South America, Africa, and other southern areas. We know there’s much to be found there and that the findings will change our knowledge,” Hurum says.

Our narrative of the evolution of life will change. New fossils will emerge that will require us to think differently, Hurum predicts, who finds this prospect exciting.

“Palaeontology is truly one of the few fields where you can rewrite textbooks with a hammer and chisel,” he says.

Reference:

Kear et al. ‘Oldest southern sauropterygian reveals early marine reptile globalization,’ Current Biology, 2024. (Abstract)

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Translated by Alette Bjordal Gjellesvik

Read the Norwegian version of this article on forskning.no

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