Australian scientist helps identify ‘evolutionary missing link’ and solve century-old mystery

By Karen McGhee, Science and Environment Editor

May 27, 2022

A 390 million year old fossil that has baffled scientists for more than a century has been identified as a creature that lived between water and land.

The transition from water to terrestrial habitat was a monumental event in the history of life on Earth.

Thus, any piece of evidence that helps elucidate how and when this happened is essential to understanding the evolution of land animals, including humans.

Now a team of researchers from Japan and Australia have identified one of the first animals that may have been involved in this important transition.

And that means it’s a long-lost ancestor of man… albeit from our very, very distant past!

A diagram of the head bones of Palaeospondylus. (Hirasawa et al., Nature, 2022)

ancient vertebrate

The resemblance is certainly beyond skin-deep as the creature is eel-shaped and just over 6cm long.

It bears the scientific name of Palaeospondylus gunni and is known from a range of fractured fossils first found in a quarry in Scotland in the late 1800s.

Since its discovery, fossil experts have wondered what it could be.

It was always known to be some vertebrate of some sort, but opinions varied on the exact type.

The most recent breakthrough on its identity involved a collaboration between experts from the Australian National University (ANU), the University of Tokyo and the RIKEN Evolutionary Morphology Laboratory, Tokyo,

ANU evolutionary biologist Dr Daisy Hu says there is now good reason to believe Palaeospondylus gunni was possibly a lobe-finned fish and possibly even a “stem tetrapod” – a potential ancestor of the first four-legged creatures.

Lobe-finned fish are an ancient lineage of vertebrates, most of which became extinct millions of years ago. But their morphology includes many primitive features still seen in modern vertebrates that have come to live on land, including ourselves.

The reconstructed skull of Palaeospondylus. (RIKEN)

A relative from Queensland

Only eight species of lobe-finned fish still survive. They look like living fossils, and one of the best known is the Queensland lungfish, which is thought to have existed as a species for around 400 million years.

Daisy agrees that Palaeospondylus gunni would have been “comparable” to the modern Queensland lungfish, which, unlike most fish, can breathe through the lungs to survive on land in pools of evaporating water during times of drought.

What made you understand Palaeospondylus gunniThe place of the possible in our evolutionary tree was the application of relatively new technology to the fossils of the creature. But before that could happen, Daisy and her colleagues had to sift through all the fossil material linked to the unknown creature to identify specimens that could be used for this new analysis.

“Morphological comparisons of this animal have always been extremely difficult for scientists,” Daisy said.

But recent improvements in “high resolution 3D segmentation and visualization” have made this possible.

This meant first finding fossils that were sufficiently adapted and intact to be subjected to the new techniques. Daisy and her colleague spent nearly five years sifting through thousands of pieces of rock to find suitable fossil specimens.

Daisy said that finding the perfect few specimens for analysis, “like winning the lottery, or even better!”

“It was an important step in identifying exactly what this animal was,” Daisy said.

So where to go now for this research?

One direction the research can now take is to compare it more closely with living species that may be close relatives, and that includes the Queensland lungfish.