A new discovery of tiny fossilized fish bones has forced scientists to rethink the origins of some of the world’s most successful freshwater fish.
These fish, which include catfish, minnows, tetras, and even popular aquarium zebrafish, share a special ear structure that lets them hear much better than most ocean fish.
Until now, researchers believed these fish developed this hearing system after moving into rivers and lakes about 180 million years ago, when the giant supercontinent Pangea was still intact.
But new evidence suggests the story is much more complex—and more fascinating.
The key to this story is a structure called the Weberian apparatus.
Found in about two-thirds of all freshwater fish today, it works a bit like the middle ear bones in humans.
In people, sound waves vibrate the eardrum and are carried through a chain of small bones to the inner ear. In fish, sound passes straight through their bodies, which are about the same density as water, making hearing difficult.
To solve this, otophysan fish (the group that includes catfish, carps, minnows, and tetras) evolved a clever system.
Their air bladder vibrates in response to sound, and a set of tiny bones transfers those vibrations to the inner ear. This allows them to hear a wide range of frequencies—sometimes as high as 15,000 Hertz, close to the limit of human hearing. By contrast, most ocean fish can only pick up very low sounds below about 200 Hertz.
The new research was led by Juan Liu, a paleontologist at the University of California, Berkeley. Liu and her colleagues studied fossils of a small fish, Acronichthys maccagnoi, discovered in Alberta, Canada.
At only two inches long, this fossil may seem unremarkable, but it contained something rare: a well-preserved Weberian apparatus. Using 3D X-ray scans and computer models, the team showed that this fish, which lived about 67 million years ago, had hearing sensitivity not far off from modern zebrafish.
This finding matters because it shows that otophysan fish may not have originated in freshwater as long thought. Instead, Liu’s team suggests that their ancestors began developing the building blocks of the Weberian system while still living in the ocean.
Only later, after splitting into different lineages, did some groups invade rivers and lakes, completing the evolution of their remarkable hearing system.
According to the new timeline, this shift into freshwater happened around 154 million years ago, during the late Jurassic Period, after Pangea had already begun breaking apart and new oceans were forming.
The idea of multiple freshwater invasions, rather than a single ancient one, helps explain why otophysan fish are so incredibly diverse today.
By repeatedly entering new environments, they may have found more opportunities to adapt and evolve into the 10,000 species we now see worldwide. Liu argues that this pattern fits a broader rule in evolution: bursts of new species often follow repeated moves into new habitats, especially when coupled with a new biological advantage, like better hearing.
The fossil evidence was collected over six field seasons starting in 2009 and is now housed in the Royal Tyrrell Museum in Alberta. Scientists at Canadian Light Source and McGill University captured detailed scans, which allowed Liu’s team to model how the fossil fish would have heard.
Their analysis suggests the fossil fish’s hearing was tuned between 500 and 1,000 Hertz, not too far from the human conversational range. This shows that even tens of millions of years ago, these fish already had a sophisticated way of perceiving sound.
The discovery not only revises the timeline of freshwater fish evolution but also highlights the role of the ocean as a cradle of innovation for vertebrates.
“For a long time, we thought these fish had a single freshwater origin,” said ichthyologist Michael Newbrey, one of the study’s co-authors. “But now we see a marine origin followed by at least two moves into fresh water. It just makes so much more sense.”
By connecting fossil evidence with modern genetics and computer simulations, Liu’s team has revealed how a tiny set of ear bones reshaped the evolutionary path of freshwater fish.
Their work shows that sometimes, the smallest bones can tell the biggest stories about life’s history.
Source: UC Berkeley.
