Fossil fish rewrites otophysan evolution history
A tiny fossil fish discovered in southwestern Alberta is the oldest North American otophysan, offering new insights into the evolution of freshwater fish species. Named Acronichthys maccognoi, the 4 cm specimen from the Late Cretaceous period suggests otophysans transitioned from marine to freshwater environments at least twice. The find, detailed in a study published on October 2 in Science, challenges previous understandings of their global spread.
The fossil of Acronichthys maccognoi, measuring about 4 cm long, was unearthed in southwestern Alberta and dates to the Late Cretaceous period, spanning 100.5 million to 66 million years ago—the era of Tyrannosaurus Rex. This specimen represents a new species within the otophysan supergroup, which includes catfish, carp, and tetras, accounting for two-thirds of today's freshwater fish species.
Researchers from Western University, the Royal Tyrrell Museum of Palaeontology, the University of California, Berkeley, and the University of Alberta collaborated on the analysis. They used micro-CT scans at the Canadian Light Source in Saskatoon, Saskatchewan, and the Advanced Photon Source in Lemont, Illinois, to create detailed 3D models without damaging the fragile fossil. These scans revealed modifications in the first four vertebrae, which transmit vibrations from the swim bladder to the ear, a key otophysan adaptation visible even to the naked eye.
"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 at Western University and study author.
The discovery indicates otophysans originated in marine environments before shifting to freshwater at least twice, with a divergence estimated around 154 million years ago during the Late Jurassic, after the supercontinent Pangea began breaking apart about 200 million years ago. This raises questions about how these freshwater ancestors dispersed to every continent except Antarctica without crossing oceans.
"Dinosaurs are pretty exciting, so a lot of time and effort has been focused on them... but we've only scratched the surface when it comes to understanding the diversity of prehistoric freshwater fish," said Don Brinkman, curator emeritus at the Royal Tyrrell Museum. "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."
Lisa Van Loon, adjunct Earth sciences professor at Western, emphasized the value of micro-CT: "Many of the fossil specimens collected by the Royal Tyrrell Museum are incredibly fragile... 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 altogether."
The study, titled "Marine origins and freshwater radiations of the otophysan fishes," was published in Science on October 2, 2025.