Hidden for 70 million years, a tiny fossil fish is rewriting freshwater evolution

Researchers from Western University, the Royal Tyrrell Museum of Palaeontology, and several international institutions analyzed the delicate specimen. Measuring just about 4 centimeters long, the fossil dates back to the Late Cretaceous period (the same era as the Tyrannosaurus Rex, roughly 100.5 million to 66 million years ago). Representing an entirely new genus and species, the fish has been named Acronichthys maccognoi.

Their findings were published on October 2 in the scientific journal Science.

A Missing Link in Fish Evolution

"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 worked alongside an international team that included Lisa Van Loon, an adjunct professor at Western; Don Brinkman, curator emeritus at the Royal Tyrrell Museum; Juan Liu of the University of California, Berkeley; and Alison Murray of the University of Alberta.

Unique Anatomy Reveals Early Adaptations

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.

From Ocean to River: Tracing an Evolutionary Journey

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."