New tiny prehistoric fish species unlocks origins of catfish and carp
Western collected micro-CT scans show saltwater species transitioned to freshwater
image:
Photograph of Acronichthys maccagnoi fossil (with scale), which was located well inland from the shoreline of the Western Interior Seaway
view moreCredit: Don Brinkman (Royal Tyrrell Museum)
The fossil of a tiny fish found in southwestern Alberta provides new insight into the origin and evolution of otophysans, the supergroup of fish that includes catfish, carp and tetras, which today account for two-thirds of all freshwater species.
The specimen, studied by researchers at Western University, the Royal Tyrrell Museum of Palaeontology and international collaborators, is a skeleton of a fish about 4 cm long from the Late Cretaceous period (the age of the iconic Tyrannosaurus Rex, about 100.5 million to 66 million years ago.) A new kind of fish entirely, it is now named Acronichthys maccognoi.
A study detailing the discovery was published today in the high impact journal, Science.
“The reason Acronichthys is so exciting is that it fills a gap in our record of the otophysans supergroup. It is the oldest North America member of the group and provides incredible data to help document the origin and early evolution of so many freshwater fish living today,” said Neil Banerjee, Earth sciences professor and author on the study.
Banerjee collaborated with an international team including Lisa Van Loon, adjunct Earth sciences professor at Western, Don Brinkman, curator emeritus at the Royal Tyrell Museum, Juan Liu from the University of California, Berkeley and Alison Murray from the University of Alberta.
Otophysans are distinctive in the way the first four vertebrae are modified to transmit vibrations to the ear from the swim bladder (a gas-filled internal organ that allows fish to maintain their position in the water without expending significant energy), basically functioning as a human ear. This is easily spotted in the skeleton of the found fossil of Acronichthys by the naked eye. Van Loon, using synchrotron beamlines at both the Canadian Light Source in Saskatoon, Saskatchewan, and the Advanced Photon Source in Lemont, Illinois, captured a more sophisticated, detailed look with computed tomography (micro-CT) scans.
Micro-CT scans are non-destructive (critical when studying prehistoric fossils), high-resolution X-ray images that create 3D virtual models of objects by taking a series of 2D X-ray projections as an object, in this case the Acronichthys, rotates.
“Many of the fossil specimens collected by the Royal Tyrrell Museum are incredibly fragile, and some are impossible to extract from the rock itself, so micro-CT scans provide not only the best method for acquiring detailed images of what’s inside, they’re also the safest way to avoid destroying the fossil all together,” said Van Loon.
One fish, two fish, red fish, blue fish
While the discovery of Acronichthys introduces a new species to paleontological records, it also provides critical data to trace the origins of otophysans, as the supergroup is understood to have started as a marine (saltwater) species before transitioning to a freshwater species. The discovery suggests the transition from marine to freshwater species happened at least twice during otophysans’ evolution.
The study estimated a new divergence time for otophysans from marine to freshwater species at around 154 million years ago (the Late Jurassic period) – after Pangea, the supercontinent, began to break apart about 200 million years ago. The researchers are left trying to understand how the tiny Acronichthys moved from continent to continent (as its freshwater ancestors now live on every continent except Antarctica) if they couldn’t swim across saltwater oceans.
“Dinosaurs are pretty exciting, so a lot of time and effort has been focused on them so we know a lot about what they were like, but we’ve only scratched the surface when it comes to understanding the diversity of prehistoric freshwater fish,” said Brinkman. “There’s still so much we don’t know, and a fossil site right here in Canada is giving us the key to understanding the origins of groups that now dominate rivers and lakes around the world.”
Journal
Science
Article Title
Marine origins and freshwater radiations of the otophysan fishes
Article Publication Date
2-Oct-2025
X-ray based CT image rendering of Acronichthys maccagnoi fossil.
Credit
Lisa Van Loon
(L-R) Lisa Van Loon, adjunct Earth sciences professor at Western University, Don Brinkman, curator emeritus at the Royal Tyrell Museum and Neil Banerjee, Earth sciences professor at Western University photographed at the Royal Tyrell Museum.
Credit
Neil Banerjee
The Acronichthys maccagnoi fossil was discovered in the Scollard Formation, which is a rock layer in Alberta, Canada that includes Dry Island Buffalo Jump Provincial Park. The area is known for its rich fossil record from the end of the Cretaceous and early Paleocene periods.
Credit
Don Brinkman (Royal Tyrrell Museum)
Fossilized ear bones rewrite the history of freshwater fish
New study concludes oceanic fish invaded fresh water multiple times, developing improved hearing along the way
University of California - Berkeley
image:
An artist's reconstruction of the Weberian apparatus in a 67 million-year-old fossil fish. The Weberian structure (gold-colored bones at center) arose from a rib (shown in gray attached to several back bones in the spine) and connect the fish's air bladder (left) with the inner ear (right). The bony structure endows the fish with more sensitive hearing and is still present today in two-thirds of all freshwater fish species. The background depicts the various fish lineages that evolved after the supercontinent Pangea broke up.
view moreCredit: Ken Naganawa for UC Berkeley
When saltwater fish long ago evolved to live in fresh water, many of them also evolved a more sophisticated hearing system, including middle ear bones similar to those in humans.
Two-thirds of all freshwater fish today — including more than 10,000 species, from catfish to popular aquarium fish like tetras and zebrafish — have this middle ear system, called the Weberian apparatus, which allows them to hear sounds at much higher frequencies than most ocean fish can, with a range close to that of humans.
University of California, Berkeley paleontologist Juan Liu has now used the structure of this Weberian apparatus in a newly discovered fossil fish to revise the origin story for the evolution of freshwater fish.
Fish with a Weberian ear system, referred to as otophysan fish, were thought to have moved into fresh water approximately 180 million years ago, before the supercontinent of Pangea had broken up into the continents we see today. Based on Liu's new timeline, they now appear to have arisen much later — about 154 million years ago, during the late Jurassic Period — after the beginning of Pangea's breakup and coinciding with the appearance of today’s oceans.
Liu's analysis of fossil and genomic data implies that the fish originally developed precursor bones of their superb hearing while still in the ocean. Only later did they develop fully functional enhanced hearing, after the two separate lineages moved into fresh water: one evolving into today's catfish, knife fish and African and South American tetras; the other evolving into the largest order of freshwater fish, the carp, suckers, minnows and zebrafish.
"The marine environment is the cradle of a lot of vertebrates," said Liu, an assistant adjunct professor of integrative biology and an assistant curator in the UC Museum of Paleontology. "A long time 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 that 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."
This finding reshapes our understanding of the evolutionary history and intricate biogeography of the world's most successful group of freshwater fish, 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."
Liu and her colleagues describe and name the 67 million-year-old fossil fish, Acronichthys maccagnoi, in a paper that will be published Oct. 2 in the journal Science. In that paper, the researchers analyze 3D scans of the fossil's Weberian structure and the genomes and morphology of modern fish to revise the genealogy of freshwater fish, and also simulate the frequency response of the fossil fish's middle ear structure.
A Rube Goldberg-like structure in the middle ear
Ears that work underwater require a different anatomy than ears that detect sound traveling through the air. Many land vertebrates evolved an eardrum-like structure that vibrates in response to sound waves. That eardrum moves a Rube Goldberg-like array of bones in the middle ear — in humans, the malleus, incus and stapes — that amplify the sound and poke the fluid-filled inner ear, which jiggles and eventually jostles hairs that send signals to the brain.
But sound waves in water go right through a fish, which has a similar density to the surrounding water. So fish developed a bladder filled with air — essentially a bubble — that vibrates in response to sounds passing through the fish. Those vibrations are transferred to the fish's inner ear in a rudimentary way in most saltwater fish, which limits their hearing to bass notes below about 200 Hertz.
Otophysan fish, however, developed bony "ossicles" between the air bladder — often inaccurately referred to as the swim bladder — and the inner ear to amplify and extend the frequency range the ears can detect. Zebrafish, for example, can hear frequencies up to 15,000 Hz, not far from the 20,000 Hz limit of humans.
Why these fish need to hear high frequencies is a mystery, though it may be because they live in diverse and complicated environments, from rushing streams to static lakes.
Liu studies the Weberian apparatus in living and fossil fish, and last year published a computational simulation of how the apparatus works. That simulation allows her to predict the frequency response of the bony ossicles, and thus the hearing sensitivity of fish.
Numerous specimens of the newly named fossil fish, a mere 2 inches long, were excavated and collected in Alberta, Canada, over six field seasons starting in 2009 by ichthyologist and co-author Michael Newbrey of Columbus State University in Georgia. The fossils are housed in the Royal Tyrrell Museum in Drumheller, Alberta. A couple of specimens were so well preserved that the bones in the middle ear were clearly Weberian. The fish is the oldest known North American fossil of an otophysan fish, or Otophysi, dating from the late Cretaceous Period, only a short time before the non-avian dinosaurs disappeared. Older specimens have been found elsewhere in the world, but none had a well-preserved Weberian apparatus, Liu said.
Technicians with the Canadian Light Source at the University of Saskatchewan in Saskatoon and at McGill University in Montreal captured 3D X-ray scans of the fish, and Liu modeled the ossicles of the Weberian apparatus in her laboratory. The model suggests that, even 67 million years ago, otophysan fish had nearly as sensitive hearing as zebrafish do today.
"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."
She noted that the findings highlight a general pattern in evolution: sudden increases in new species can arise from repeated incursions into new habitat rather than a single dispersal event, especially when coupled with new innovations, such as more sensitive hearing.
“For a long time, we presumed that the Otophysi probably had a freshwater origin because this group consisted almost exclusively of freshwater fishes,” Newbrey 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 coauthors of the paper are Donald Brinkman of the Royal Tyrrell Museum, Alison Murray of the University of Alberta, former UC Berkeley undergraduate Zehua Zhou, now a graduate student at Michigan State University, and Lisa Van Loon and Neil Banerjee of Western University in London, Ontario. Liu was funded by a Franklin Research Grant from the American Philosophical Society.
CT scan of fossil fish Acronichthys maccagnoi [VIDEO]
A 3D model of the head of the newly named 67 million-year-old fossil fish, Acronichthys maccagnoi, based on CT scans. The skull bones are brightly colored while the ribs and backbones of the spine are in gray. The small, bright red bones at the junction between the spine and head are ossicles of the Weberian apparatus.
Credit
Juan Liu, UC Museum of Paleontology & Don Brinkman, Royal Tyrrell Museum
Vibrations of the Weberian apparatus [VIDEO]
Juan Liu and her student used finite element analysis to create a computer model of the vibrational response of the Weberian ossicles of fish. This simulation shows the amplitude and vibrations of zebrafish ossicles at a frequency of 1,012 Hertz. The large, triangular ossicle is called the tripus and is a modification of the rib and third vertebra to amplify sound vibrations from the air bladder.
Credit
Juan Liu & Zehua Zhou, UC Berkeley and UCMP
Journal
Science
Article Title
Marine origins and freshwater radiations of the otophysan fishes
Article Publication Date
2-Oct-2025
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