Large salamander fossils discovered in Namibia, will the history of tetrapod evolution be rewritten?

Produced by: Science Popularization China

Author: Destroyed Sunny (Paleontologist)

Producer: China Science Expo

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Kunene Province is located in the northwest of Namibia, adjacent to the Atlantic Ocean. It is a remote area covered by mountains, wasteland and sparse bushes. It is dry and hot here, with annual rainfall often less than 100 mm.

Standing in this ochre-red desert, it is often hard for people to imagine that in the early Permian period 280 million years ago, when the Atlantic Ocean and Africa had not yet formed, Kunene was a world full of rivers, lakes and swamps. At that time, Namibia was still further south on the earth. On the eve of the complete unification of the supercontinent, as a corner of the southern Gondwana continent, it stretched across the high-latitude area of ​​about 55° south latitude.

Here, Claudia Mascano from the University of Buenos Aires, Argentina, Jason Pardo from the Field Museum in Chicago and their colleagues unexpectedly discovered a type of giant prehistoric salamander that had never been reported before - they named it Gaias jennyae (tentative translation, binomial method: Gaiasia jennyae). According to estimates, Gaias jennyae was about 2-4 meters long and was the top predator of the freshwater system of that era.

The holotype of Gaiasaurus, including a complete skull and most of the vertebrae

(Image source: Reference 1)

The genus name Gaiasia comes from the Gai-As formation, where the fossils were found, and the specific epithet is in honor of the late famous paleontologist Jenny Clack . As a pioneer and outstanding figure in the study of early tetrapod evolution, Jenny Clack was the first to notice that the first tetrapods to step onto land often had a peculiar "multi-fingered" phenomenon: Ichthyostega in the Carboniferous period had seven fingers, while Acanthophodon had eight.

The widespread phenomenon of polydactyly suggests that the "five-fingered" structure of modern terrestrial vertebrates is not a common ancestral feature of tetrapods. In other words, Ichthyostega and Acanthostega are not our direct ancestors; humans and other modern terrestrial vertebrates evolved from a type of early tetrapod with five fingers.

The evolution of early tetrapods

(Image source: translated from reference 3)

Although Gaiasaurus is not as ancient as Ichthyostega, its primitive skull structure and vertebral characteristics indicate that it also belongs to a long-standing group: through phylogenetic comparative analysis, paleontologists believe that Gaiasaurus should be a sister group of the Colosteidae, and the two together constitute an outgroup of modern crown-group tetrapods.

It is worth noting that the species of the family Cynodontidae mainly lived in the Carboniferous period, at least 27 million years away from the era of Gaiassima. In previous studies, scientists once believed that early tetrapods including Cynodontidae had been replaced by emerging early amniotes and slithering amphibians in the late Carboniferous period.

A reconstruction of Gaiassaur, noting its large square head and tiny limbs.

(Photo credit: Gabriel Lioh)

However, the discovery of the Gaiassaur fossils completely changed our understanding of this process - in the south of Gondwana, there was also a group of giant primitive tetrapods that dominated the ecosystem at that time.

Phylogenetic position of Gaiasaurus

(Image source: Reference 1)

Many evidences indicate that Gaiasaurus was a powerful predator, with a huge skull that could reach 60 cm in length, comparable to modern large crocodiles. The square snout indicates that it mainly swallowed its prey by suction, but the huge fangs on the upper and lower jaws indicate that it also had the ability to bite large prey.

The preserved sturdy vertebrae indicate that it had a very sturdy trunk. The strong physique indicates that Gaiassima was at the top of the ecological chain at that time. They might live between mudflats and swamps like crocodiles, ambushing unwary prey.

However, the lack of appendicular skeletons has led paleontologists to suspect that the limbs may have degenerated or been lost entirely, as happened to its relatives Crassigyrinus or Aistopoda.

A reconstruction of the thick frog newt, which lived in the Carboniferous period and was smaller than the Gaias newt.

(Image source: Wikipedia)

What makes Gaias salamander unique is not only its kinship with ancient taxa, but also its habitat location that fascinates scientists - this large salamander lives in high latitudes and faces a harsh cold climate that most creatures cannot adapt to.

According to today's climate zone classification, 55° south latitude is already in the subarctic zone, which is not suitable for amphibians to survive. As cold-blooded animals, amphibians often need to absorb enough solar radiation to maintain physiological activity. Therefore, in such a cold environment, large salamanders such as Gaias newts find it difficult to survive.

So, was the era in which Gaiassima lived warmer than it is now? The answer is shocking: the Earth in the early Permian was not only not warmer than it is today, but it may have been colder. Starting from the Late Devonian 360 million years ago, the Earth entered the most severe ice age in the Phanerozoic Eon. This long ice age, which lasted for 100 million years and ended in the late Permian, is called the Late Paleozoic Ice Age (LPIA).

Well-preserved moraines and glacial scratches have been found in the Karoo basin of South Africa near Namibia, indicating that during this period, intense glacial activity formed a large ice sheet over the southern Gondwana continent.

Because of this, the fossil record of early tetrapods is largely confined to the relatively warm equatorial regions of modern-day Europe and North America, and scientists suspect that tetrapods did not have the opportunity to spread to other parts of the world until the climate gradually warmed in the late Permian period.

Reconstructed curve of global average temperature in the Phanerozoic. The Earth entered a continuous ice age starting around 360 million years ago.

(Image source: Reference 5)

However, after the discovery of Gaiassima, this inherent cognition was also challenged: the primitive characteristics of Gaiassima showed that it was more closely related to early tetrapods living near the equator, which ruled out the possibility that Gaiassima was of local origin or evolved from other emerging tetrapods in the Permian.

This suggests that Gaias theraps had to face a high-latitude climate that was no weaker than today's, or that there were a series of intermittent short warm periods during the long ice age of hundreds of millions of years that allowed Gaias theraps and their ancestors to survive.

But in any case, this high-latitude fossil site clearly shows that the distribution and radiation of early tetrapods were far more extensive than we thought. The limitation of early fossils may be an illusion caused by biased sampling - after all, there are still a lot of unstudied strata in Africa, South America and Antarctica corresponding to Gondwana, where there are probably still treasures buried that we have never heard of.

Distribution and dispersal of tetrapods during the Late Paleozoic (the red star indicates where Gaiasaurus was found)

(Image source: adapted from reference 1)

Although Gaiastris may have lived in a relatively warm interglacial period, it also showed adaptations to low temperatures. In 1847, German scholar Carl C. Bergmann discovered that organisms in polar regions tend to be larger than those in equatorial regions, a phenomenon that later became known as Bergmann's law.

According to this law, a larger organism has a smaller surface area-to-volume ratio, which reduces heat consumption and better maintains body temperature. The huge size of Gaiastris may be a response to the low temperature climate, which provides an important reference for us to understand the physiological characteristics of early tetrapods.

Although Gaiassaur is still shrouded in many mysteries, its discovery has clearly provided paleontologists with a new perspective. Our ancient ancestors may be more resilient than expected, and their ability to adapt to the environment is far beyond expectations. 300 million years ago, there may be more creatures like Gaiassaur wandering in unexplored lands. Under the shadows of the stone pine and reed forests, there are countless secret worlds hidden that we have never imagined. They are now sleeping under the rocks, waiting for us to wake them up.

Regardless, Gaiasaurus successfully conquered southern Gondwana and became the top predator in freshwater ecosystems.

(Image source: Reference 1)

References:

1. Marsicano, CA et al. Giant stem tetrapod was apex predator in Gondwanan late Palaeozoic ice age. Nature (2024).

2. Coates, MI & Clack, JA Polydactyly in the earliest known tetrapod limbs. Nature 347, 66-69 (1990).

3. Ahlberg, PE Follow the footprints and mind the gaps: a new look at the origin of tetrapods. Earth and Environmental Science Transactions of the Royal Society of Edinburgh 109, 115-137 (2018).

4. Olroyd, SL & Sidor, CA A review of the Guadalupian (middle Permian) global tetrapod fossil record. Earth-Sci. Rev. 171, 583-597 (2017).

5. Scotese, CR, Song, H., Mills, BJW & van der Meer, DG Phanerozoic paleotemperatures: The earth's changing climate during the last 540 million years. Earth-Sci. Rev. 215 (2021).

(Note: Latin parts in the text should be italicized)