The oldest marine reptile fossil in the Southern Hemisphere! How did this Nothosaurus cross the Cape of Good Hope?

Produced by: Science Popularization China

Author: Destroyed Sunny (Paleontologist)

Producer: China Science Expo

Editor's note: In order to expand the boundaries of cognition, the China Science Popularization Frontier Science Project has launched a series of articles on the "Unknown Realm", which provides an overview of the exploration results that break through the limits in deep space, deep earth, deep sea and other fields. Let us embark on a journey of scientific discovery and get to know the amazing world.

“…the crew gazed upon this magnificent cape that had been hidden for who knows how many centuries. They had not only discovered a promontory, but a new world. In view of the dangers and storms they had experienced before, Bartolomeu Dias and his companions named it Cape of Storms. After they returned to the kingdom, King João II gave it another glorious name - Cape of Good Hope.” - João de Barros (1496 – 1570)

Cape of Good Hope

(Photo source: veer photo gallery)

A magical dragon

The Ashburton Lakes region in the central part of New Zealand's South Island is a famous tourist destination, where a series of alpine lakes reflect the surrounding mountains, creating a beautiful contrast. In the lake region, the gentle Harper Mountain is not conspicuous. In Maori, Harper Mountain is also called Mahaanui, which is a huge stone that symbolizes the warship of the hero Maui in mythology.

The hillside is covered with black gravel, and if you are lucky, you may find one or two decorative snail fossils among them - usually they belong to the genus Mellarium. These small snails are the iconic fossils of the Anisian period of the Middle Triassic in New Zealand, about 245 million years ago.

Harper Mountain reflected in the lake

(Image source: Wiki media commons)

245 million years ago, New Zealand Island was part of Pangaea, the last supercontinent in Earth's history. The main plates on the modern Earth squeezed together to form a huge ancient land spanning north and south. It is not far from the Antarctic Circle in the Triassic period (about 60°S), and is a huge cape in the southeast of Pangaea.

To the north of the cape, the Tethys Ocean was expanding, pushing the Tibetan, Iranian, and Turkish land masses slowly across the equator.

Further east, the South China and North China plates are about to merge, but most of the land in modern East Asia is still in the sea - for modern people, this is a strange world, but in this vast world, many creatures we are familiar with are emerging - gymnosperms, dinosaurs, pterosaurs, and our ancestors - but at this time, all this has not yet settled, and the arid red land is the most common scenery.

In New Zealand's Middle Triassic, a befuddled marine reptile rests in a shallow sea. We can't trace its death - only a single vertebra remains of the Nothosaur, named GNS GD 540, hidden in the rubble of Mount Harper.

Its existence has surprised paleontologists, and in a study published in Current Biology, researchers have confirmed that GNS GD 540 is the oldest marine reptile fossil found in the Southern Hemisphere.

This ancient creature belonged to a group that had set out from the Tethys Ocean and drifted past this cape southeast of Pangaea. Just as Dias crossed the Cape of Good Hope and discovered the Indian Ocean hundreds of millions of years later, this group of Nothosaurus also crossed ancient New Zealand and discovered Pangaea.

Magic dragon restoration

(Image source: Wiki media commons)

Photo and CT scan image of the GNS GD 540 fossil of Nothosaurus

(Image source: Reference 1)

Schematic diagram of the body shape of the dragon

(Image source: Wiki media commons)

Different Paths Between North and South

Nothosaurs are not a group of fossils that people are familiar with. These marine reptiles, which were usually only three or four meters long, lived in the Triassic period and had a typical long neck and long tail. But compared with other marine reptiles active in the Mesozoic era, they were not special in morphology.

Unlike ichthyosaurs or plesiosaurs, Nothosaurus's aquatic adaptations are not prominent, its limbs are not fully finned, and its tail has not evolved into a flat end similar to that of fish. Some scholars believe that Nothosaurus would retain a certain degree of land locomotion like modern turtles, and would leave the ocean and rest on land during periods of reproduction or feeding.

The fossilized footprints of the Nothosaurus preserved in Luoping, Yunnan suggest that this marine reptile may have been in a transitional state between semi-aquatic and aquatic. Its swimming posture was similar to a "dog paddle", with its forelimbs paddling but touching the mud on the seabed.

All this evidence shows that Nothosaurus was not a good swimmer. Most of the time, it just wandered in the shallow sea, searching for fish or ammonites with its needle-like teeth. It was not easy for them to cross the deep sea, but even so, Nothosaurus showed a surprising breadth of distribution - whether it was the Tethys Ocean or the Pan-Ocean, we have found records of Nothosaurus fossils.

The possible movement mode of Nothosaurus. According to the Nothosaurus footprints found in Luoping, Yunnan, the movement of this species is mainly driven by the forelimbs, and its mode is between "underwater walking" and "underwater flying".

(Image source: Reference 2)

Just as GNS GD 540 crossed the southeastern corner of Pangaea and approached the Antarctic Circle, another type of nothodontid also traveled northward, leaving the warm tropical waters and entering the Arctic Ocean.

Corosaurus (named after Corus, the god of the northwest wind in Roman mythology), which lived in the Early Triassic period, is one of the oldest nothosaurs discovered so far. However, unlike its relatives, it did not live in the warm Tethys Ocean on the east side of the supercontinent, but on the northwestern edge of the supercontinent, which is now the low-latitude shallow sea in Wyoming, USA.

Although the ancient latitude reconstructed from the fossil site is relatively low, considering the obstacles of Pangaea, paleontologists believe that this group of Nothosaurus most likely bypassed the broken landmass in the northeast of Pangaea, entered the Arctic, and then went all the way south to Wyoming. This path bypassed the entire northern part of Pangaea. For Nothosaurus, which had poor swimming ability, such a large-scale radiation was undoubtedly a feat.

Schematic diagram of the paleogeography of the Middle Triassic. The red star indicates the discovery site of Nothosaurus GNS GD 540; the blue star indicates the discovery site of Nothosaurus. The solid red arrows indicate the confirmed migration routes of Nothosaurus, and the dotted lines indicate possible migration routes.

(Image source: Reference 1)

For most reptiles, high latitudes are not an area worth exploring. Modern reptiles rarely reach areas above 60° north and south latitude. For them, who are temperature-changing, the cold climate at high latitudes will seriously affect their metabolism and normal physiological functions.

Therefore, compared with the distribution and activity patterns of modern reptiles, the Triassic Nothosaurus seems to be heading in a completely different direction - they actively expanded their distribution range and even crossed the Arctic Seaway. This tolerance to low temperatures seems to indicate that Nothosaurus may have a stronger metabolic capacity, or it may have warm fat like its distant relative, the ichthyosaur.

But in any case, the Mitralisaurus on the west side of Pangaea and GNS GD 540 on the southeast side of the continent together prove one thing - after the origin of the Tethys Ocean, many marine reptiles represented by Nothosaurus seem to have begun to radiate to high latitudes. They do not seem to favor the tropics, because the tropics in the Triassic period were as hot as if burned by fire.

Burning World

After the Permian mass extinction, the Earth entered the most intense greenhouse period of the Phanerozoic. Under the influence of the huge amount of greenhouse gases emitted by the Siberian Large Igneous Province, the global average temperature in the Early Triassic may have reached 50°C. In this environment, the surface sea temperature near the equator was much higher than today.

Although higher temperatures are beneficial for reptiles to maintain body temperature and metabolic activity, extreme high temperatures are clearly unacceptable for vertebrates. Most animals will suffer severe thermal damage to their proteins in an environment above 45°C. For marine organisms, this threshold is even lower - because under high temperature conditions, the solubility of oxygen will further decrease, so high-temperature seawater is often hypoxic.

Under the influence of this hot climate, both terrestrial and marine organisms tend to migrate to cooler high-latitude environments. As an important driving force for biological migration and evolution, the high temperature environment of the Triassic may be the main driving force that prompted some marine reptiles to embark on a drifting journey. At the same time, the high-latitude areas of the Triassic were not as icy and snowy as they are today, and the oceans were blocked by glaciers. The ice-free poles of the Triassic were full of unobstructed seaways, so Nothosaurus and other marine reptiles could reach various places they wanted to reach along the edge of the continent.

Reconstructed curve of global average temperature in the Phanerozoic Eon. It can be seen that around 240 million years ago, the Earth's temperature reached the extreme value of the entire Phanerozoic Eon.

(Image source: Reference 6)

The distribution pattern of marine and terrestrial organisms in the Early Triassic period. The area in red is the "death zone" where there is a lack of biological fossil records.

(Image source: Reference 5)

It wasn't just marine reptiles that embarked on a migration journey like Nothosaurus. On land, terrestrial reptiles also showed a strong north-south distribution. The center of Pangaea, which is now central Africa and northern South America, seemed to be an insurmountable forbidden zone for life, and almost no fossils of large vertebrates were found there.

Although the high temperatures in the early Triassic period did not last long, the long greenhouse climate lasted for almost the entire Mesozoic Era. It was during this long greenhouse that dinosaurs, birds, and mammals gradually appeared on the historical stage, and the biological features of the Paleozoic Era were gradually replaced. Under the scorching sun of the Triassic period, a brand new world was taking shape.

Unfortunately, our protagonist, Nothosaurus GNS GD 540, never had the chance to see this new world. In the extinction event at the end of the Triassic period, Nothosaurus withdrew from the stage of history along with most marine reptiles. Just like Bartolomeu Dias hundreds of millions of years later, he once again set out on a journey to find India, and forever rested in the howling storm on that cape.

The sailboat that Dias sailed when he passed the Cape of Good Hope in 1488. Twelve years later, when he set sail again to pass the Cape of Good Hope, his fleet encountered a strong storm and his sailboat sank in the storm.

(Image source: Wiki media commons)

References:

1.Kear, BP et al. Oldest southern sauropterygian reveals early marine reptile globalization. Curr Biol 34, R562-R563 (2024).

2.Zhang, Q. et al. Nothosaur foraging tracks from the Middle Triassic of southwestern China. Nat Commun 5, 3973 (2014).

3. Bardet, N. et al. Mesozoic marine reptile palaeobiogeography in response to drifting plates. Gondwana Research 26, 869-887 (2014).

4.Lindgren, J. et al. Soft-tissue evidence for homeothermy and crypsis in a Jurassic ichthyosaur. Nature 564, 359-365 (2018).

5.Sun, YD et al. Lethally Hot Temperatures During the Early Triassic Greenhouse. Science 338, 366-370 (2012).

6. Scotese, CR, Song, H., Mills, BJW & van der Meer, DG Phanerozoic paleotemperatures: The earths changing climate during the last 540 million years. Earth-Science Reviews 215 (2021).