A fossil of a tiny fish found in southwestern Alberta is expected to help scientists learn more about how catfish and carp evolved. These fish are part of the second-largest superorder of fish called otophysans. This group contains 10,758 species, or about 28 per cent of known fish species and 68 per cent of freshwater species. 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. The fossil of the fish is about five centimetres long and dates from the Late Cretaceous period, about 70 million to 66 million years ago - the same time period as Tyrannosaurus Rex. Scientists, whose findings were published in Science, say this fossil is an entirely new kind of fish named Acronichthys maccognoi. The specimen is being studied by researchers at Western University, the Royal Tyrrell Museum of Palaeontology and international collaborators. “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. In order to learn more, the fossil underwent a Micro-CT scan at Canadian Light Source in Saskatoon. This is a non-destructive, high-resolution X-ray image that creates 3D virtual models of objects by taking a series of 2D X-ray projections. “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. 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 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. “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.” The study estimated a new divergence time for otophysans from marine to freshwater species at around 154 million years ago. This is after Pangea, the supercontinent, began to break apart about 200 million years ago. Researchers are left trying to understand how Acronichthys moved from continent to continent if they couldn’t swim across saltwater oceans.
