Weberian Ear Bones Rewrite Freshwater Fish Origins
For millions of years, freshwater fish have wowed scientists with their remarkable hearing abilities. Central to this capability is the Weberian apparatus, a set of small, bone-based connectors that transfers sound from an air-bladder system to the inner ear. In two-thirds of today’s freshwater species, including ubiquitous catfish and popular aquarium residents like tetras and zebrafish, this enhanced hearing allows them to detect higher-frequency sounds than most saltwater fish can perceive. New fossil discoveries and integrative analyses are now rewriting the origin story of these aquatic listeners.
UC Berkeley paleontologist Juan Liu and colleagues have used the preserved anatomy of the Weberian apparatus in a newly described fossil fish to challenge long-held beliefs about where otophysan fish began and how they diversified. The old consensus held that the otophysan lineage — the group that includes otophysan fishes such as catfish, knife fishes, tetras, carps, suckers, minnows and zebrafish — originated in freshwater during the late Paleozoic to early Mesozoic era, likely in the supercontinent Pangea before it shattered. Liu’s team argues instead that these fishes first developed the precursor bones of their sophisticated hearing in the ocean, with fully functional hearing evolving later after two distinct freshwater incursions as the lineage split and dispersed.
From Oceanic Origins to Freshwater Invasions
The researchers combined fossil data with modern genomic and morphological analyses to reconstruct a new timeline for otophysan evolution. They propose that the most recent common ancestor of otophysans was marine, and that at least two independent freshwater invasions occurred after that lineage diverged. One lineage gave rise to catfishes, knife fishes, and the African and South American tetra clades, while the other expanded into carps, suckers, minnows and zebrafish — the largest order of freshwater fishes by species richness.
According to Liu, the marine origin and subsequent freshwater incursions likely accelerated diversification. “Repeated habitat shifts between saltwater and freshwater may be a key driver of speciation, especially when combined with new sensory innovations,” she noted. This perspective challenges the traditional view that otophysans stemmed from a single freshwater migration and instead emphasizes the role of environmental opportunities in shaping modern biodiversity.
Acronichthys maccagnoi: A 67‑Million‑Year Clue
The study centers on a newly named fossil fish, Acronichthys maccagnoi, dating to about 67 million years ago. Measuring roughly 2 inches long, this specimen was recovered from Alberta, Canada, after years of fieldwork. The holotype and additional fossils preserve a clearly identifiable Weberian apparatus, making it the oldest known North American otophysan fossil with well-preserved hearing-related anatomy. While older otophysan fossils have been found elsewhere, none prior to this showed such intact Weberian bones, which is why it is pivotal to revising the fossil record and refining our understanding of early hearing in these fishes.
How the Weberian Apparatus Was Reconstructed
To verify functional hearing in the ancient fish, researchers used 3D scans of the fossil’s Weberian structure, created from imaging facilities at the Canadian Light Source in Saskatoon and at McGill University. In parallel, they analyzed the genomes and morphologies of living otophysans. Liu then built computational models of the ossicles — the bony elements bridging the air bladder and the inner ear — and simulated how they would respond to sound frequencies. The results suggest that even 67 million years ago, the Weberian system could deliver hearing sensitivity approaching that of modern zebrafish, with peak hearing in the lower thousands of Hertz range and substantial high-frequency capability retained despite its antiquity. While the oldest atmospheric tone of a fossil’s hearing is not an exact equivalence to today’s hearing, the simulations indicate a mature and functional innovation in an ancient marine fish that later enabled freshwater success.
Fieldwork, Institutions and Collaboration
Fieldwork spanning six seasons culminated in Alberta’s Royal Tyrrell Museum housing the specimens, with co-authors spanning the Royal Tyrrell Museum, the University of Alberta, Western University, and other institutions. The team included ichthyologist Michael Newbrey, who led the Canadian collection efforts, and a cadre of researchers who integrated paleontological data with genomic insights to reconstruct a more dynamic history of freshwater biogeography. The work underscores the value of interdisciplinary collaboration and advanced imaging in paleontology.
Implications for Evolutionary Biogeography
The new timeline reframes how scientists view the distribution of freshwater fishes across continents. If otophysans began in marine environments and repeatedly invaded freshwater after the breakup of Pangea, the current pattern of widespread diversity in rivers and lakes becomes the result of repeated ecological experiments rather than a single dispersal event. This insight may help explain why otophysan fishes occupy such diverse freshwater communities around the world and why sensory innovations like the Weberian apparatus have been so influential in their evolutionary success.
In sum, the discovery of Acronichthys maccagnoi and the refined interpretation of Weberian anatomy illuminate a nuanced history where marine origins meet repeated freshwater incursions. As Liu and her colleagues demonstrate, the skeletons of our aquatic neighbors can carry the signatures of deep-time journeys that continue to shape life in our rivers and lakes today.
