A new timeline for the origin of otophysan fish
Scientists have long noted that two‑thirds of all freshwater fishes possess a sophisticated hearing system known as the Weberian apparatus, a set of middle-ear bones that lets these fish hear much higher frequencies than most ocean species. In a stunning revision of this history, UC Berkeley paleontologist Juan Liu and colleagues used the fossil record and modern genomics to rethink where and when otophysan fish first arose.
Previously, researchers believed otophysans—fishes with the Weberian apparatus—date back to about 180 million years ago, when freshwater habitats were largely tied to the supercontinent Pangea. New analyses place their origin later, around 154 million years ago in the late Jurassic, a time that overlaps with the breakup of Pangea and the broad expansion of today’s oceans. This new timeline suggests that the lineage originated in the oceans and only later made its way into fresh water in at least two waves of colonization.
From there, one lineage diversified into today’s catfish, knife fish and the African and South American tetras, while another lineage led to the carp, suckers, minnows and zebrafish—the largest order of freshwater fishes. “A marine origin with multiple freshwater incursions after divergence explains the extraordinary hyper‑diversity of otophysans we see in modern freshwater faunas,” Liu said.
The Weberian apparatus and the evolution of hearing
In water, sound travels differently than through air, so many land animals evolved eardrum‑like structures and a simple middle‑ear chain. Otophysan fishes solved the challenge by developing bony ossicles that bridge the air bladder (often called the swim bladder) and the inner ear. This Weberian apparatus amplifies and narrows the frequency range, letting these fish detect higher pitches that other fishes cannot. For example, zebrafish can hear up to about 15,000 Hz, close to the human limit, a capability that opens up a world of acoustic cues in their water habitats.
What makes the 67‑million‑year‑old fossil interesting is not just the anatomy but its implications for hearing across time. Liu and co‑authors used a combination of 3D scanning, computational modeling and genomic comparisons to predict the frequency response of the fossil’s Weberian bones, showing a level of hearing sensitivity remarkably close to that of modern representative otophysan species despite its age.
The 67‑million‑year‑old fossil that reshapes the story
The named fossil, Acronichthys maccagnoi, is a tiny ~2‑inch specimen found in Alberta, Canada, over six field seasons starting in 2009. The Royal Tyrrell Museum houses multiple well‑preserved specimens that reveal a clearly Weberian middle ear, making it the oldest known North American otophysan fossil with such a structure. Although older Weberian fossils exist elsewhere, none prior to this discovery preserved the apparatus so well, providing a unique window into early hearing evolution in freshwater fishes.
Technicians used 3D X‑ray scans from facilities in Saskatchewan and Montreal, and Liu then modeled the ossicles in the lab. The results indicate the fossil’s hearing remained highly functional, with peak sensitivity in the lower end of the higher frequency range—roughly 500 to 1,000 Hz—only modestly reduced from that of modern relatives. This points to an already capable auditory system well before the full transition into freshwater ecosystems.
From ocean to freshwater: implications for evolution and biogeography
The study underscores a broader evolutionary pattern: rapid diversification can follow repeated habitat shifts, especially when paired with novel innovations like an enhanced ear. If otophysan fishes originated in the marine realm and repeatedly invaded freshwater after their lineage split, then dramatic ecological and geographic reshuffling likely accelerated speciation and helped create today’s hyper-diverse freshwater communities.
Newbrey and colleagues emphasize that the fossil record, when integrated with modern genomic data, can overturn long‑standing assumptions about vertebrate history. The finding also reframes our understanding of the Otophysi’s biogeography, highlighting multiple freshwater incursions rather than a single, continent‑centered dispersal event.
Looking ahead
As imaging technologies and comparative genomics advance, researchers expect to refine this timeline further and explore how hearing and other sensory systems influenced the ecological trajectories of major fish groups. The Alberta fossils will remain a touchstone for discussions about marine origins, freshwater colonization, and the remarkable evolutionary experiments that gave rise to one of the planet’s most successful aquatic lineages.
Authors on the Science paper include Liu, Newbrey and colleagues from several institutions, with funding support from the American Philosophical Society. The work not only rewrites a chapter of fish history but also highlights the enduring value of integrating fossil evidence with cutting‑edge digital methods.
