The Evolution of Scales (Part 2: Reptiles)

For those of you who are new to my blog or have missed the previous installment of my “Evolution of a Trait” series, I suggest you check it out first (it is about the evolution of fish scales).

Personally, I find the evolution of scales in fish a lot more interesting than their respective evolution in reptiles; nonetheless, I have decided to write up this post because reptiles are very common, at least where I come from, and I think that you will enjoy reading about them.

To begin, I will try to quickly summarize everything I wrote about last time. If the prospect of reading about fish scales a second time does not interest you, just skip over the next two paragraphs and begin reading the sentence that starts with “I ended…”.

First, there were primitive, jawless fish. They did not have scales, but instead they had soft skin that was covered in slime. They also had teeth, and, over time, these teeth began growing all over their skin. These became the first scales.

As the ages went by, the placoid scales fused together into cosmoid scales to provide greater protection. Finally, they went through a couple minor chemical changes to become what we see today.

I ended the last blog post by mentioning how fish eventually lost their scales when they transitioned from water to land. Well, once you lose a trait or a process, evolution rarely brings it back. To bring a real life example, a long time ago primates lost their tapetum lucidum (you know, the thing that allows cats to see in the night) because it made their vision in the daytime blurry. However when tarsiers later decided to transition from diurnal to nocturnal creatures, they had to evolve a completely different mechanism to see in the dark. For tarsiers, this mechanism was simply bigger eyes.

Similarly, proto-reptiles needed a new way to protect their bodies from predators. Nevertheless, rather than doing something cool like spikes or coating their skin with oil and lighting it on fire, reptiles simply settled on scales, only a slightly different kind than before. I know; boring, right?

Following is how modern science believes these new scales evolved. About 320-330 million years ago—when the very first reptiles were appearing—the main issue affecting amphibians was their lack of desiccation resistance. Basically, because their skin had to be constantly kept wet (or else they would dry out and die of dehydration), amphibians were tied to bodies of water and were greatly limited in their abilities to walk inland. The dry land, however, was full of unfilled niches and free space.

Those organisms that had some sort of adaptation limiting their water loss were able to live deep inside the continent, practically without any predators. Consequently, the population of proto-reptiles boomed.

These pioneers managed to survive in the relatively dry climates by developing an extremely thick stratum corneum—which is the outermost layer of the skin and already existed in amphibians—and filling it with hydrophobic lipids. This adaptation created a sort of forcefield around reptiles that repelled all water, keeping it inside and stopping it from entering or exiting. In addition to making them waterproof, this new skin inadvertently also provided protection from UV radiation.

As proto-reptiles filled up the empty niches, space became limited and competition arose. Those organisms that could defend themselves from predators and not die from blood loss after narrowly managing to escape passed their genes on to their children.

The most viable and popular such method was the heightened production of keratin. Normal skin already produces keratin for strength and durability; however in reptiles this process was put on steroids (not literally).

A chameleon with “bumpy” scales.

To allow the animals to remain flexible—if their skin was made out of keratin 100%, they would be completely unable to move and would look similar to statues—two main methods were developed. In the first one, these keratinized regions were regularly spread about the body of the organism and produced protective bumps on its skin; this can be seen in chameleons and lizards as well as thousands of other species. Such a method does not produce scales in the traditional sense of the word since the bumps do not overlap, but rather are simply located near each other. In fact, in some countries one would use the word “skin” to describe this type of body covering in reptiles.

Overlapping snake scales.

The second method was to create folds of the skin, as can be seen in the image above. Although each scale looks as if it is completely separate from the others, they are actually all part of a single contiguous sheet. This can be seen when animals such as snakes shed their scales or in the diagram below.

The Development of Reptilian Scales
The Development of Reptilian Scales

Well, this concludes my last posts about scales, although I might be willing to write about the evolution of scales in birds and mammals if a lot of people ask for it. I will try to write my next post about the evolution of bones from teeth within the next two weeks; however I am also planning to be traveling for all 3 months of summer and am not sure if I will have internet access. 😦

Anyway, I look forward to your comments and have a nice day!

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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.

The Evolution of Fur and Mammals

There are many different kinds of views regarding why fur evolved in proto-mammals and all of them have their strengths and disadvantages. In this blog post I will be writing about the one that seems more probable and realistic to me.

The main problem with trying to trace back the evolution of fur is the overwhelming lack of evidence. Due to its keratin structure, fur remains in an excellent condition in fossils for the first 100-150 million years, sometimes being preserved even better than bones. However as the time goes by, fur tends to merge into one giant mess, and it is no longer easy to tell it apart for scales, feathers, or any other currently unknown body coverings. As such, it is hard to pinpoint the exact point in time when fur appeared in proto-mammals.

About 150 to 200 million years ago the very first mammals were confined to the underground. Although direct evidence of fur has not been preserved for this long, creatures such as the Morganucodons are still assumed to have had furry coats due to some of their other traits. For one, the Morganucodon spent its days sleeping in burrows and only came out at night to feed on insects. Such a lifestyle would be impossible for cold-blooded organisms, who are forced into a near-death state whenever there is a lack of sun and heat; therefore, by method of elimination, the Morganucodon was, in fact, an homeothermic creature. However, as mentioned in the previous blog post, scales are extremely poor thermal insulators (after all, they were developed to let in as much heat and warmth as possible). In addition to this, the fossils show that the Morganucodon had oil secretion ducts along its skin that are similar to those modern mammals and are presumed to have been used for grooming.

The proposed look of the Morganucodon.

But if you go slightly farther in time—to the realm of the early therapsids—you would be unable to find definite proof of a single furred creature (or at least one with some version of proto-fur). So what we have is not only a lack of direct evidence of fur, but also a lack of indirect evidence of the intermediary stages of its formation. It appears that over a short period of a million years (well… short it terms of evolution), fur spontaneously appeared—first in patches on the face of the early mammals and eventually covering their whole bodies.

If fur truly did evolve from scales as was once believed, we would find many different varieties of it in fossils. First we would see elongated scales with a different distribution of proteins. We would see these long scales becoming thinner and we would see them beginning to grow out of follicles. Furthermore, the first fur would not be growing in conjunction with scales, with single strands sticking out in between them, it would be a gradual transition. As such, I believe that fur was, just like feathers, an evolutionary novelty.

Its main purpose, however, was not to provide insulation or to attract mates. The first proto-mammals lived in very warm climates and thus could afford to live, sleep, and hide in dark burrows for short periods of times. Nevertheless, they faced a different problem: after living for hundreds of millions of years under the sun, their eyes were not well equipped for the limited light available in the burrows. They kept running into rocks and, like lemmings, falling off of cliffs (okay, the last one was an exaggeration… I love lemmings).

The solution was to develop some sort of tactile apparatus—in this case it was vibrissae, or their more common name, whiskers. The ancient mammals crawled about their burrows using their long and rigid hairs to feel their surroundings. This theory is supported by the indentations in the therapsids’ skulls that are akin to those of modern whiskered mammals. The Thrinaxodon, for example, lived 250mya and had pitted foramina—which are tiny openings that nerves pass through—on its skull. This species was also covered in scales for the most part. (Keep in mind that although this is strong evidence, it is still not proof: there are a couple species of modern lizards that have similar indentations, but do not have whiskers.)

Whiskers on the face of a Therapsid.

Originally these whiskers were located only on the head of the animals, protruding in between the scales, but eventually they spread to the rest of the body, giving the organism the ability to completely sense its environment. Over time, each hair became shorter, and the fur covered more and more area each generation. Finally evolution gave it another purpose—to protect the mammals from the cold weather and to help them retain heat.

Now that I’m done rambling about the evolution of fur, I will write about the one other defining features of mammals: the mammary gland. Since glands, in general, do not fossilize at all, theories about their evolution have to derive from their similarities to other existing organs. One popular theory states that they evolved from Apocrine sweat glands and were at first used to keep eggs moist (it also states that this organ evolved long before fur and other mammalian characteristics did). When the development of hard calcium shells replaced the previous rubbery ones, these sweat glands were repurposed—their new job was to provide nourishment to the newly hatched young, which, in turn, allowed the eggs they came from to become very small. The newborn hatchlings would then simply suck on the mother’s skin and the Apocrine sweat glands (now mammary gland) would provide them with lactose and other nutrients.

That’s it for today! Sorry for the shortage of pictures, I didn’t have much time to draw them, so I just found a couple on the internet. The next post will be about scales, nails, and horns and will be relatively short. Once again, I look forward to reading your comments.