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 5 cm long from the Late Cretaceous period (the same time period of the iconic Tyrannosaurus Rex, about 70 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 Oct. 2 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.
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. (Neil Banerjee)
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.
X-ray based CT image rendering of Acronichthys maccagnoi fossil. (Lisa Van Loon)
"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."
