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.