When saltwater fish adapted to freshwater environments millions of years ago, many developed a more advanced hearing system. This system, known as the Weberian apparatus, is present in about two-thirds of all freshwater fish species today. It allows these fish to detect higher-frequency sounds, with a range similar to that of humans.
A recent study led by Juan Liu, a paleontologist at the University of California, Berkeley, has revised the timeline for when these hearing adaptations emerged. Liu analyzed the structure of the Weberian apparatus in a newly discovered fossil fish and found evidence suggesting that otophysan fish—those with this specialized ear system—originated later than previously thought.
Earlier theories proposed that otophysan fish moved into freshwater about 180 million years ago during the time when Pangea was still intact. However, Liu’s research indicates they appeared around 154 million years ago in the late Jurassic Period, after Pangea began to break apart and modern oceans started forming.
Liu’s analysis combined fossil evidence and genomic data. The findings suggest that precursor bones for enhanced hearing evolved while these fish were still marine species. Full development of their sophisticated hearing occurred only after two separate lineages transitioned into freshwater: one lineage gave rise to catfish, knife fish, and tetras; the other led to carp, suckers, minnows, and zebrafish.
“The marine environment is the cradle of a lot of vertebrates,” said Liu. “A longtime consensus was that these bony fish had a single freshwater origin in the large continent Pangea and then dispersed with the separation of different continents. My team’s analysis of some fantastic fossils has shed new light on the evolutionary history of freshwater fish and found completely different results: the most recent common ancestor of otophysan fish was a marine lineage and there were at least two freshwater incursions after that lineage split up.”
She added: “These repeated incursions into freshwater at the early divergence stage likely accelerated speciation, and are key factors in explaining the extraordinary hyper-diversity of otophysans in modern freshwater faunas.”
The study describes Acronichthys maccagnoi, a 67 million-year-old fossil fish discovered in Alberta, Canada. The fossils were excavated over six field seasons starting in 2009 by ichthyologist Michael Newbrey from Columbus State University. Some specimens preserved details allowing researchers to identify clear Weberian ossicles—the small bones involved in hearing.
Technicians used 3D X-ray scans from facilities at both the University of Saskatchewan and McGill University to create digital models of these bones. Simulations indicated that even ancient otophysan fish had relatively sensitive hearing abilities comparable to those seen in modern zebrafish.
“We weren’t sure if this was a fully functional Weberian apparatus, but it turns out the simulation worked,” Liu said. “The Weberian apparatus has just a little bit lower output power, which means lower sensitivity, compared to a zebrafish. But the peak, the most sensitive frequency, is not too much lower than zebrafish — between 500 and 1,000 Hertz — which is not too bad at all and which means the higher frequency hearing should have been achieved in this old otophysan fish.”
Michael Newbrey commented on how this discovery changes previous assumptions: “For a long time, we presumed that the Otophysi probably had a freshwater origin because this group consisted almost exclusively of freshwater fishes,” he said. “The new species provides crucial information for a new interpretation of the evolutionary pathways of the Otophysi with a marine origin. It just makes so much more sense.”
Other contributors include Donald Brinkman (Royal Tyrrell Museum), Alison Murray (University of Alberta), Zehua Zhou (Michigan State University), Lisa Van Loon and Neil Banerjee (Western University). Funding came from an American Philosophical Society Franklin Research Grant.
More information can be found through Science magazine’s coverage on Marine Origins and Freshwater Radiations of Otophysan Fishes as well as resources provided by UC Museum of Paleontology.
