Fossils reveal new origin of keen-eared freshwater fishes

Two-thirds of all freshwater fish today can hear sounds in a range close to humans.

The supergroup of more than 11,500 species, called the ‘otophysans’, includes the well-known zebrafishes, carps and catfishes.

Their superb hearing is possible thanks to a middle ear system called the ‘Weberian apparatus’. The tiny bones transmit sound vibrations from the fishes’ swim bladder to the inner ear, allowing them to hear much higher frequency sounds than most fish in the ocean.

All but a few species detect sounds with frequencies of more than 3,000 Hz (humans can detect up to 20,000 Hz).

“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,” says Juan Liu, a palaeontologist at the University of California, Berkely and an assistant curator in the UC Museum of Paleontology, USA.

The supercontinent Pangea broke up about 181 million years ago (mya) into Laurasia (now Europe, Asia, North America and Greenland) and Gondwanaland (Antarctica, Africa, South America, India, Madagascar and Australia).

“My team’s analysis of some fantastic fossils shed new light on the evolutionary history of freshwater fish and found completely different results,” says Liu.

Liu and her colleagues analysed specimens of the 67-million-year-old fossil fish Acronichthys maccagnoi excavated from Alberta, Canada. Dating from the late Cretaceous Period, it is the oldest known North American fossil of an otophysan fish.

The new study, published in the journal Science, estimates marine species diverged to freshwater octophysans in the Late Jurassic about 154 mya.

“The most recent common ancestor of otophysan fish was a marine lineage and there were at least 2 freshwater incursions after that lineage split up,” explains Liu.

“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.”

One lineage evolved into today’s catfish, knife fish and African and South American tetras, the other evolved into the carp, suckers, minnows and zebrafish.

The 4cm-long fossil skeleton retained a well-preserved Weberian apparatus which the researchers imaged in 3-dimensions using micro-CT scans.

They used computer simulations to understand just how sensitive this ancient fish’s hearing would have been.

“The Weberian apparatus has just a little bit lower output power, which means lower sensitivity, compared to a zebrafish,” says Liu.

“But the peak, the most sensitive frequency, is not too much lower than zebrafish – between 500 and 1,000 Hz – which is not too bad at all, and which means the higher frequency hearing should have been achieved in this old otophysan fish.”