The fish supergroup earball, recognized for its superior hearing abilities, includes two-thirds of freshwater fish species. They were previously believed to have originated in freshwater prior to the breakup of the supercontinent Pangaea. This suggests an almost 80 million-year span from their inception to the earliest known fossils. However, the discovery of Acronichthys MacCognoi—a newly identified freshwater ear fish species from the late Cretaceous period—challenges this assumption.
Reconstruction of the Weberian apparatus in Acronichthys MacCognoi. The central golden bone arises from the rib bones (indicated in grey) that connect the fish’s air bladder (left) to the inner ear (right). Image credit: University of California, Berkeley Ken Chronicle.
Underwater ears require a distinct anatomy compared to those that sense airborne sounds.
Many terrestrial vertebrates have evolved eardrum-like structures that vibrate in response to sound waves.
These eardrums amplify sounds, moving bone arrays, similar to the malleus, incus, and stapes found in human middle ears, which then stimulate the fluid-filled inner ear.
In contrast, sound waves travel through fish, which have a density similar to that of the surrounding water.
Consequently, fish have developed an air-filled bladder that vibrates in response to sound passing through it.
These vibrations are then transferred to the fish’s inner ear in a basic manner in most saltwater species.
However, specialized fish have developed bony structures known as “bones” between the air bladder and inner ear (a system called Weberian apparatus), enhancing the amplification and range of frequencies detectable by their ears. For instance, zebrafish can detect frequencies of up to 15,000 Hz, nearing the human limit of 20,000 Hz.
The reason for these fish’s need to hear high frequencies remains unclear, though it may relate to their existence in varied and complex environments, from swift currents to still lakes.
“reason Acronichthys Maccagnoi Professor Neil Banerjee, a researcher at Western University, noted:
“This represents the oldest known North American member of the group and offers invaluable data for documenting the origins and early evolution of numerous freshwater fish species existing today.”
Acronichthys Maccagnoi thrived in the late Cretaceous period roughly 67 million years ago.
The authors utilized microCT scans of 4 cm long fossils to investigate their Weber structure.
They also studied the genomes and morphology of contemporary fish to refine the evolutionary lineage of freshwater species and simulate frequency responses of the middle ear structures of fossil fish.
Their model indicates that even 67 million years ago, Otophysan fish may have possessed hearing sensitivity on par with today’s zebrafish.
“We were uncertain if this constituted a fully functional Weberian device, but the simulation proved effective,” stated Dr. Juan Liu, a paleontologist at the University of California, Berkeley.
“The Weberian apparatus suggests it’s less sensitive than that of zebrafish.”
“However, the peak sensitivity frequency is not as low as that of zebrafish (500-1,000 Hz). This is not an insignificant finding, and it’s conceivable that this ancient Otophysan fish achieved a higher hearing frequency.”
This discovery indicates that at least two transitions from marine to freshwater species occurred during the evolution of otolaryngology.
Researchers estimate the divergence times for ear plants migrating from ocean to freshwater habitats occurred around 154 million years ago (late Jurassic period), following the fragmentation of Pangaea around 200 million years ago.
“Dinosaurs are fascinating, attracting significant attention, which means we know a lot about them. However, there’s still much to uncover regarding the diversity of prehistoric freshwater fish.”
“Many keys to understanding the origins of the groups that currently dominate rivers and lakes worldwide can be found in Canadian fossil sites.”
A paper detailing these findings was published in the journal Science on October 2nd.
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Juan Liu et al. 2025. Marine origin and freshwater radiation of ENT. Science 390 (6768): 65-69; doi: 10.1126/science.adr4494
Source: www.sci.news
