The Otophysi supergroup of fish, known for its enhanced hearing, comprises two-thirds of living freshwater fish species. Previously, they were thought to have originated in fresh water before the breakup of the supercontinent Pangea, implying a gap of almost 80 million years between their origin and the oldest known fossil. However, the discovery of Acronichthys maccognoi – a newly described species of freshwater otophysana that lived in the late Cretaceous – challenges this view.
Artistic reconstruction of the Weberian apparatus in Acronichthys maccognoi; the Weberian structure (golden bones in the center) comes from a rib (shown in gray attached to several bones on the back of the spine) and connects the fish’s air bladder (left) to the inner ear (right). Image credit: Ken Naganawa, University of California, Berkeley.
Ears that function underwater require a different anatomy than those that detect sounds traveling through the air.
Many land vertebrates have evolved an eardrum-like structure that vibrates in response to sound waves.
This eardrum moves a set of Rube Goldberg-like bones in the middle ear—in humans, the hammer, anvil, and stirrup—that amplify sound and prick the fluid-filled inner ear, which quivers and ultimately jostles the hairs that send signals to the brain.
But sound waves in water pass through a fish, which has a similar density to the surrounding water.
The fish therefore developed an air-filled bladder – essentially a bubble – that vibrates in response to sounds passing through it.
These vibrations are transferred to the fish’s inner ear in a rudimentary manner in most saltwater fish, limiting their hearing to low notes below about 200 Hz.
Otophysial fish, however, have evolved bony “ossicles” between the respiratory bladder and the inner ear – a system called the Weberian apparatus – to amplify and extend the range of frequencies that the ears can detect. Zebrafish, for example, can hear frequencies up to 15,000 Hz, which is not far from the 20,000 Hz limit of humans.
Why these fish need to hear high frequencies remains a mystery, although it may be because they live in diverse and complex environments, from rushing streams to static lakes.
“The reason Acronichthys maccagnoi is so exciting that it fills a gap in our record on the otophysial supergroup,” said Professor Neil Banerjee, a researcher at Western University.
“It is the oldest member of the group in North America and provides incredible data to help document the origin and early evolution of so many freshwater fish living today.”
Acronichthys maccagnoi lived at the end of the Cretaceous about 67 million years ago.
The authors captured micro-CT scans of the 4cm-long fossil and examined its Weberian structure.
They also analyzed the genome and morphology of modern fish to revise the genealogy of freshwater fish and simulate the frequency response of the fossil fish’s middle ear structure.
Their model suggests that 67 million years ago, otophyseal fish had hearing almost as sensitive as zebrafish today.
“We didn’t know if this was a fully functional Weberian device, but it turns out the simulation worked,” said Dr. Juan Liu, a paleontologist at the University of California, Berkeley.
“The Weberian device has just a little lower power output, which means lower sensitivity than a zebrafish.”
“But the peak, the most sensitive frequency, is not too much lower than that of the zebrafish – between 500 and 1,000 Hz – which is not too bad at all and which means that hearing at higher frequencies should have been achieved in this old otophysan fish.”
This finding suggests that the transition from marine to freshwater species occurred at least twice during otophysian evolution.
The researchers estimate that about 154 million years ago (Late Jurassic epoch) a new period of divergence for otophysians from marine to freshwater species occurred, after Pangea began to break apart about 200 million years ago.
“Dinosaurs are quite exciting, so a lot of time and effort has been spent on them, so we know a lot about what they looked like, but we have only scratched the surface when it comes to understanding the diversity of prehistoric freshwater fish,” said Dr Don Brinkman, emeritus curator at the Royal Tyrell Museum.
“There is still so much we don’t know, and a fossil site right here in Canada gives us the key to understanding the origins of the groups that now dominate rivers and lakes around the world. »
An article describing the results was published October 2 in the journal Science.
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Juan Liu and others. 2025. Marine origins and freshwater radiations of otophysan fishes. Science 390 (6768): 65-69; doi: 10.1126/science.adr4494
