The scales covering the skin of various animals are rigid or semi-rigid plates that provide a protective exterior and often help reduce friction during movement. Although they serve a similar purpose across different animal classes, the materials they are constructed from vary dramatically, reflecting distinct evolutionary pathways. The structure of these shields is tied to the specific environment and physical demands faced by the animal. Understanding the composition of these scales requires examining which layer of the skin they originate from and the primary structural material involved.
Dermal Scales The Composition of Fish Scales
Fish scales develop in the deeper layer of the skin, the dermis, and are considered dermal scales. Their composition is highly mineralized, often featuring materials similar to bone and teeth, which provides remarkable hardness and durability. The underlying structure usually includes collagen fibers, flexible proteins that form the framework for mineral deposition.
Most fish scales are complex composites, built from calcium phosphate minerals, primarily hydroxyapatite, the same mineral found in bone. For example, the scales of the Arapaima, a large freshwater fish, are multilayered composites consisting of mineralized collagen fibers. This design gives the scales a gradient structure, with a hard external layer backed by a more compliant inner layer for shock absorption.
The four main scale types—placoid, ganoid, cycloid, and ctenoid—have specific material components. Placoid scales, found on sharks and rays, are also called dermal denticles because they are structurally homologous to vertebrate teeth, featuring an outer layer of enameloid and an inner core of dentine. Ganoid scales, seen on the Alligator Gar, are exceptionally hard due to a highly mineralized outer layer of ganoine, an enamel-like substance composed of apatite crystallites.
Bony fish typically have cycloid or ctenoid scales, which are much thinner and composed of a surface layer of hydroxyapatite and calcium carbonate over a deeper layer of collagen. These scales grow throughout the fish’s life, adding concentric rings that can be used to estimate the animal’s age.
Epidermal Scales The Composition of Reptile Scales
In contrast to fish, the scales of lizards and snakes are epidermal scales, originating from the outer layer of the skin. The primary structural material is keratin, a fibrous structural protein that is also the main component of hair and fingernails in mammals. This keratin forms a tough, yet flexible, external covering that is highly effective at preventing water loss.
Reptile scales are essentially folds of the epidermis and are composed of two main types of keratin. The outer, rigid layer is made of beta-keratin, a hard and durable material unique to reptiles and birds. This layer is supported by a more pliable inner layer of alpha-keratin, which provides the necessary flexibility at the scale’s hinge region.
Because the outer beta-keratin layer is non-living, it does not grow continuously with the animal. For growth and renewal, the entire outer layer is periodically shed in a process known as ecdysis, or molting. This shedding allows the reptile to replace worn-out scales and accommodate an increase in body size.
Specialized Structures Scutes and Modified Scales
Some animals feature specialized structures that combine both dermal and epidermal elements, creating a composite armor. These structures are often called scutes, found on animals like crocodilians, turtles, and alligators. Scutes are distinct from the typical epidermal scales of snakes and lizards because they are anchored in the dermis.
The defining characteristic of a scute is the combination of a hard, keratinous outer layer overlaying a bony plate, or osteoderm, that grows from the dermis beneath. In turtles, the shell’s surface is covered by keratin scutes, similar in composition to human nails. These scutes lie over the carapace, a bony structure formed by fused vertebrae and ribs, creating an exceptionally strong protective shield.
Crocodilians also possess scutes, where the tough, epidermal keratin layer protects bony plates, or osteoderms, embedded in the skin. This structure provides significant mechanical protection. Snakes, while having purely keratinous scales, exhibit a specialization where the ventral scales are extra broad, interlocking folds of keratin that aid in locomotion.