Unique new fish fossil prompts rethinking of how Earth’s geology shapes evolution

Coelacanths are deep-sea fish that live off the coasts of southern Africa and Indonesia and can grow up to two meters long. For a long time, scientists believed they were extinct.

In new research published in Nature CommunicationsWe reveal the best-preserved coelacanth fossil ever discovered, dating back to the ancient period, hundreds of millions of years ago, when these ancient sea dwellers began to evolve. The fossil comes from the Gogo Formation in the Gooniyandi Shire in northern Western Australia.

We also studied the evolution of the hundreds of coelacanth species we know from fossils to discover what motivated the creation of new species over the eons.

The answer was surprising: it wasn’t ocean temperatures or oxygen levels that had the greatest influence on the coelacanth’s evolution, but tectonic activity. As the vast plates of the Earth’s crust moved more, new species were more likely to emerge.

“Living fossils”

Coelacanths are lobe-finned fish, meaning they have strong bones in their fins, much like the bones in our arms. Scientists think they are more closely related to tetrapods (animals with a backbone and four limbs, such as frogs, emus, and humans) than to most other fish.

Coelacanths have been around for a long time. The oldest known fossils are over 410 million years old. But because these fossils are mostly fragments, we don’t know much about what the first coelacanths looked like.

Later, during the time of the dinosaurs, about 250 million years ago, coelacanths diversified. In total, traces of more than 175 fossil species have been found from around the world.

Finally, at the end of the Cretaceous period, 66 million years ago, all traces of coelacanths mysteriously disappeared from the fossil record. For a long time, scientists assumed that the coelacanths were victims of a massive asteroid impact that also wiped out the dinosaurs (as well as about three-quarters of all life on Earth).

Everything changed in 1938, when fishermen in South Africa pulled from the depths of the ocean a large, enigmatic fish unlike anything they had seen before. Marjorie Courtenay-Latimer, a local museum employee with a passion for natural sciences, knew immediately that this fish was special.

Courtenay-Latimer called upon his friend JLB Smith, a renowned South African chemist who was interested in ichthyology (the study of fish). Smith identified and named Latimeria, the first living coelacanth known to science.

The discovery of this “Lazarus fish” was like finding a living Triceratops dinosaur that still roams the forests of North America today. Even today, coelacanths are often described as “living fossils.”

A new coelacanth fossil

Our team at Flinders University, working with colleagues from Australia, Canada and Europe, has discovered a new species of fossil coelacanth in the Gooniyandi Shire in northern Western Australia. Around 380 million years ago, the site was a tropical reef teeming with more than 50 species of fish.

Ngamugawi wirngarri, the new coelacanth fossil, is the first fish discovered in the area to bear a name given to us in the Gooniyandi language. The name means “ancient fish in honour of Wirngarri”, a respected elder in the community.

Ngamugawi is the best-preserved coelacanth in three dimensions from the Devonian period (359 to 419 million years ago). This fossil provides an excellent insight into the early anatomy of this lineage.

Plate tectonics drives coelacanth evolution

Studying this new species led us to analyze the evolutionary history of all known coelacanths. We calculated the rates of evolution over their 410 million years of history.

We found that coelacanths generally evolved slowly, with a few exceptions.

We also analyzed a series of environmental factors that we considered as potential candidates for influencing coelacanth evolution rates. These included plate tectonic activity, ocean temperatures, oxygen levels in the water, and carbon dioxide levels in the atmosphere.

Of all the variables we studied, the one that had the greatest influence on the rate of coelacanth evolution was plate tectonic activity. New coelacanth species were more likely to appear during periods of increased tectonic activity, when seismic movements transformed habitats.

Are coelacanths still evolving?

Alongside our analysis of all fossil coelacanths, we also took a close look at the two living species, Latimeria chalumnae and Latimeria menadoensis.

At first glance, these fish look almost identical to some of their counterparts from hundreds of millions of years ago. However, upon closer analysis, we were able to see that they were actually distinct from their extinct relatives.

Although Latimeria has mostly stopped developing new features, its body proportions and DNA details continue to change a bit, so it may not be a “living fossil” after all.

This article is republished from The Conversation under a Creative Commons license. Read the original article.