The Evolution of Scales (Part 1: Fish)

Scales are one of the three most common types of body coverings—the other two being fur and feathers, which I have addressed in my previous two blog posts. However, unlike them, scales have convergently evolved (that is, they appeared independently in different, unrelated species) many, many times.

For example, take fish: their scales are created from dermal tissue (the middle layer of skin) and have a soft inside called a pulp. Overall, they have a strikingly large similarity to teeth, but we’ll get to that later.

On the other hand, reptile scales are derived from the epidermis (which is the topmost layer of the skin). In addition, unlike those of fish, they are made out of keratin.

This list can be continued with the scales of mammals, such as the Pangolin, and birds, who have scales on their feet that seem to be derived from feathers.

So as you see, scales have evolved numerous times, each one following a new and different path. As a result, it is impossible to simply write a couple simple sentences that will retell the evolutionary history of all scales. Nonetheless, it is possible for us to go back to the very beginning—to the time and place where scales did not yet exist—and begin our journey from there.

The Conodonts were relatively primitive jawless fish. The first time they appeared was 495mya, and they managed to stay in existence for almost 300 million years before going extinct during the Lau event. These fish were filter feeders and were covered with nothing except for a thin layer of skin and, most likely, slime. They had no scales. They did, however, possess an excellent set of teeth that was passed onto them by their ancestors. Sharp, pointy teeth that were located in a ring shape inside their mouths.

Right now, you might be wondering what their beautiful teeth have to do with anything. Well, the thing is, one of the early ancestors of the Conodonts also passed on the gene for teeth to what became known as the Chondrichthyes (pronounced kan-DRIK-thee-ease). Due to some very fortunate mutations, these teeth (also called placoid scales or dermal denticles) began growing on the outside of the fish’s head.

(These same teeth earlier formed skeletons in the vertebrates, but that is a topic for another blog post.)

Over thousands of years, the placoid scales inched further down the body of the Chondrichthyes, completely covering them from head to tail.

Nevertheless, although these scales provided better protection than skin alone, they still had a couple major downsides. For one, placoid scales were incredibly small—about the size of a tenth of a millimeter—and they could not grow in size. As a fish aged, it simply grew more scales.

Placoid scale on the skin of a white shark under an electron microscope

Because they were unable to do a good job at protecting against predators, natural selection favored creatures that had larger plates covering their bodies. Thus, the cosmoid scale was born. These scales were derived from a fusion of placoid scales and could be found on a great number of organisms, such as lobe-finned fish.

The cosmoid scales on a Coelacanth fossil

From this point on, scales went through numerous minor changes; mainly, these were changes in size, shape, color, and chemical composition. However, previous “drafts” still remain in existence because they are better suited for some niches. For example, sharks are still covered by placoid scales (these are the miniature ones made out of a single tooth) because their benefits—high agility and maneuverability—outweigh their costs—low protection.

About 350mya (give or take 50 million years) the first amphibians began to appear, and all of a sudden it was no longer advantageous to have skin that was covered in scales. You see, amphibians get anywhere from 50 to 100 percent, depending on the species, of their oxygen intake by defusing it in through their skin. Scales, however, greatly impede cutaneous respiration, and, as a result, they were quickly replaced by slimy skin.

When scales were eventually reacquired by reptiles, they followed a different evolutionary path. Originally I planned to write about their development in this blog post (heck, I planned to talk about horns and nails as well), but I have decided that this one is already too long.

Expect part 2 in two weeks on Monday, and, once again, I await your comments and thoughts.

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