In a group as diverse as fishes, it’s no surprise that they developed differing kinds of scales. You recall the placoderms and ostracoderms, which had bony plates covering parts of their bodies. That model was unsuccessful – but flexible bodies, covered in scales, not only let fish survive for 400 million years, but might have led to the development of a distinct neck – an important anatomical feature for living on land.
|Devonian ganoid fish (Osteolepis)|
The ganoid fishes where bony fishes abundant during the Devonian, and they have many modern descendents including sturgeons, paddlefish, and gars. If you’ve ever gone fishing in the waters of Arkansas and caught a gar, you know how bony they are. One of my few clear memories from the time I was around 7 years old is fishing with my grandfather. I caught a gar that seemed to be as long as the boat – in reality it was probably 3 feet long – with a sharp nose, silvery scales, and lots of teeth. My grandfather fought with it for what seemed like an hour – probably 2 minutes – until he broke it in half. Fishermen in the backwaters of Arkansas in the 1950s didn’t like gars.
Scales are similar to human hair and fingernails. They’re composed of collagen, the structural protein that makes up lots of connective tissues in animals, combined with calcium phosphate, the mineral apatite, which is found in bones and teeth. Some scales also include calcium carbonate. In ganoid fishes, much of the mineral matter in the scales is ganoine, glassy, rod-like calcium phosphate. That’s what makes gars so shiny and silvery.
Other types of fish scales are somewhat more bony in the case of primitive fish like coelacanths. Most common modern fish have the overlapping flexible scales that you’re probably familiar with.
—Richard I. Gibson
Ganoid fish drawing from an old textbook (public domain)