Not only do fish have ears, they can be identified by them. Some of the foremost experts in fossil fish, Christoph Gierl and Bettina Reichenbacher of Ludwig-Maximilians University in Munich, Germany and their associates have published a paper in the journal PLOS ONE, exploring what fish ears can tell us about the history of an order.
Like mammals, bony fish use their ears to sense balance, orientation, acceleration and sound. The tiny bones of the inner ear that make this possible are called otoliths. In fish, otoliths are relatively large and lie behind their brains, “floating” in their ear canals, unattached to the skull.
When otoliths fossilize, they turn into rock-like crystals, formed of a mineral known as aragonite, and are even more durable than bone. This type of fossil is important because each species of fish produces such uniquely shaped otoliths that they are as reliable as DNA when identifying the fish that left the fossil. They can even tell the age of the individual fish – if you can tell the otolith from the rock. Otoliths are only millimeters in size and are almost never found neatly nestled within a fish skeleton.
Much of the information on ancient fish paleontologists have collected has actually come from skeleton-less otoliths found all over the world, but this was not the case last year when Gierl and Reichenbacher and their associates studied a fossilized fish found in Southern France.
This 23 million-year-old fossilized skeleton was strikingly complete. Its delicately preserved bones left paleontologists sure that it belonged to the sub-order Gobioidei, but this still left a lot of room for conjecture. The sub-order Gobioidei is one of the largest of bony fish in that it contains around 2,000 species—ranging from the mudskippers of the Black Sea to the bumblebee gobies you might have in your fish tank—and that is just to number the living ones.
Gierl and Reichenbacher announced in their paper that they have identified the species largely based on – you guessed it – the ear crystals it left behind. The fossil was so complete that it was one of the few that still contained its tiny otoliths.
This particular fossil was the remains of a species belonging to the small sub-genera Butidae. Species of this sub-genera are relatives to the true gobies and sleeper gobies that make up the rest of Gobioidei. They now live in Asia, Australia, Madagascar and West Africa. Until Gierl and Reichenbacher’s discovery, not a single fossil specimen of Butidae was known to exist.
The fish that donated this fossil lived in the shallow, tropical ocean that encompassed today’s Mediterranean Sea and much of Europe. Its fossil is one of the most complete Gobioidei fossils ever discovered—even its stomach contents are still preserved in stone.
Read the May 15, 2013 study in the journal PLOS ONE.
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