Skin covering has been a topic of heavy debate when it comes to dinosaurs since...well...the dinosaurs. In fact, it's something I've touched on in a previous post. However, I wanted to delve a little deeper into that today and take a look at all of types of skin coverings that dinosaurs had.
How does reptilian skin differ to mammalian?
Sounds kinda obvious, but a reptile's skin is quite different to a mammal's. The main difference is durability. A reptile's skin is actually very thin when compared to a mammal's, hence why it will often appear more wrinkly.
Not only that, but mammalian skin contains sebaceous glands that help to keep the skin hydrated. So how do reptiles keep their skin protected and hydrated?
That's where scales come in.
Scales are not actually the skin of a reptile. If you removed all of the scales of a reptile, there would still be skin underneath (I don't recommend you do that, just take my word for it). Unlike hair, scales will normally cover the entire body (except for the presence of feathers, but we'll get onto that), either forming a mosaic pattern or overlapping. One thing that scales do have in common with hair is what they are made of, which is keratin.
Keratin is a substance you've likely heard of before. It's the stuff that our hair and nails are made of, but comes in two types of molecular structure. Alpha keratin is the softer, more pliable stuff that hair is made from, while beta keratin is harder and more stiff and this is what scales are made from.
Given that a reptile's skin is thinner and lacks sebaceous glands, it is the scales that help keep the skin tough and hydrated through the use of a waxy material produced by the outermost layer of skin that is held by the scales. This is why many reptiles get the unfair label of 'slimy', since it helps give those scales that shine.
Scales covered dinosaurs in a variety of ways as seen by skin impressions that we have of many dinosaurs. I know that feathers on dinosaurs has been preached so heavily over the years, but an animal with feathers almost always have scales as well (just look at a bird's feet). As for the extent in which they were covered in feathers or scales, it varied from dinosaur to dinosaur. T.rex, long thought to have been feathered to some degree, was found with skin impressions distributed in a wide variety of places on its body, showing an all scaly beast (finally the debate settled and I owe someone £5). Dromaeosaurs such as as Velociraptor, on the other hand, were found to be covered in feathers to such a degree that they would have resembled giant ground hawks rather than bipedal lizards.
Which brings us neatly on to feathers. Long has there been confusion about how an animal can have both feathers and scales, so let's settle it: feathers are scales.
Despite how different they look, feathers are made up of the exact same keratin as scales and have actually been found to have evolved directly from scales. They might have very different textures, but remember that things get more pliable the thinner they are. Similar to how you can bend a solid steel paperclip but not a solid steel block, feathers are simply scales that are 'shredded', for lack of a better term.
Feathers do in fact come in many different forms known as 'stages'. The further the stage, the less and less it resembles a scale.
These stages refer to the development of the feather, depending on the dinosaur's age. At what stage development stops will actually depend on the placing on the body and the species. Stage 1 feathers are, more or less, stretched out scales and are we would commonly refer to as 'quills'. Stage 2 is when that quill becomes frayed and will give a 'fluffy' appearance, also known as down. Stage 3 is when the quill of the feather grows and has smaller quills coming out of the main stalk. These branches soon develop tiny, interlocking barbules that cling onto each other and this is known as stage 4.
After this, stage 5 feathers become bigger and develop an asymmetry. Feathers were likely developed originally for insulation and the aforementioned skin hydration (same function as scales), however, stage 5 feathers appear to serve an additional function, since this asymmetry is useful for aerodynamics, which really comes in handy for things like, oh I don't know...flight? It is thought that stage 5 feathers didn't necessarily evolve solely for flight though, since these feathers can be seen directly or indirectly in animals before birds. This stage of feathers can actually be seen on the forelimbs of dromaeosaurs and it is thought that as dinosaurs such as Velociraptor ran they would hold those aerodynamic wings out to their side and use them to assist in steering...it was a serious method used to achieve speed and control, they weren't just playing airplanes.
Further stages beyond this are normally an extension of the original stalk of the feather. The main purpose of this is showcasing another thing that feathers have over scales: they look pretty. Feathers can be a lot more varied in their structure, size, texture and colour, meaning they can be more heavily utilised in displays for mating or threatening. So these further stages are developed just to look impressive.
Earlier I mentioned that birds have both feathers and scales covering their skin, but the scales are not actually scales in their original form. In fact, it has been found that these scales are actually reverse engineered feathers, having been compressed back down to form scutes and scutella.
The evolution of feathers is also an interesting one. We still aren't too sure when and where feathers first came about, but what we do know is that feathers were present in the earliest known dinosaurs. On top of that that feathers were also present in all members of the sister taxon, pterosaurs (though they took on a slightly different appearance as very short 'fuzz' called pycnofibers). Since both groups have inherited them, this means that feathers are likely an ancestral trait, having evolved first in the ancestors of dinosaurs and pterosaurs.
Osteoderms are another type of skin covering that are ancient as well as having evolved several times across various groups. In fact they have evolved independently in several groups of reptiles, amphibians and even mammals.
Osteoderms are a type of skin covering that are often mistaken for scales, since they are very similar in appearance. While they are covered in a keratin sheath, osteoderms are actually lumps of bone that have grown within the skin. The most common example seen in extant animals would be crocodiles.
Osteoderms are another type of skin covering that has served several purposes. The most obvious one here is armour. This is showcased best by mammals such as armadillos (even the ginormous Glyptodon) and dinosaurs such as, of course, ankylosaurs.
However, another less obvious purpose has been explored, especially with dinosaurs. You see, sweating as a form of thermoregulation is...really quite weird. Not many animals do it, even amongst mammals. So how else would a reptile cool off if it needed to? Well, a variety of ways are adopted and osteoderms is one of them. When overheating, blood carrying that heat will rush to near the surface of those osteoderms to get rid of that heat. A give-away feature of this in fossilised remains is grooves that were once pathways for blood vessels, which in life would have been covered by the keratin sheath.
If the osteoderms are full of blood vessels, then it is likely it isn't for protection (you don't want something trying to bite into that!), instead, it is likely for heat shedding or even used for display, with colour rushing to the osteoderm when blood is pumped into it if the animal feels threatened or...um...excited. This idea has been explored with the back plates of stegosaurids, since they wouldn't really serve as very effective armour and are full of the vessel grooves.
Other notable dinosaurs with osteoderms were the pachycephalosaurs and many abeliasaurids such as Carnostaurus sastrei.
Are they all mutually exclusive?
This question has been brought up time and time again, especially in cases like when we found out that T.rex was covered in scales. Just because a dinosaur has scales and osteoderms, it doesn't mean that they can't have feathers too.
Just look at the basal ceratopsian, Psittacosaurus mongoliensis, for example. We know that this little guy was almost completely scaly, except for a set of long grass-like tail bristles.
The skin impressions found of T.rex were widely distributed, but who's to say it didn't have some tiny bristles running along its back? They may have been too tiny to even see at a glance, much a dolphin's hair, but it still technically had feathers.
The variations seen between the skin coverings that dinosaurs had is massive so, on one hand, we simply can't assume that we more or less know what they looked like...on the other hand, that's what makes it so exciting!
Until next time.