A scute is a bony plate or shield-like scale found on the skin of some reptiles, birds, and mammals. Although similar in appearance to scales, scutes have different origins and properties. Some scientists speculate that the structures may provide clues to the early evolution of feathers, since many dinosaurs, widely considered as closely related to birds, also had scutes.
Reptiles with prominent scutes include crocodiles, alligators, and turtles. Scutes on alligators and crocodiles are the bony, ridged plates that make up the animal’s tough skin, protecting it from predators and possibly helping it regulate its temperature. These scutes have a bony base and are known as osteoderms.
In turtles and tortoises, the entire carapace, or upper shell, is a structure of fused scutes. The scutes running down the center of the shell along the spine are called central scutes. The next rows of scutes on either side of the spine are called costal scutes, and the outer scutes are known as marginal scutes. The plastron or underside of the turtle’s shell is also made up of scutes.
Each scute forms in a lower layer of the skin, the dermis, under the epidermis. It may contain bone at its base, in which case it is known as an osteoderm. The rest of the scute consists of a fibrous protein, also found in horns, known as keratin. Unlike snakes and lizards that shed their entire outer layer of skin, animals with these structures shed only the outer layer of keratin. Scales are distinct from scutes in that they develop out of the epidermis rather than the dermis.
Birds and mammals, like reptiles, can have scutes. In birds, they are dermal structures on the feet. Mammals such as armadillos and pangolins have plate-like osteoderms that form a layer of armor. As in crocodilians, scutes in mammals serve a protective function.
Researchers investigating the ancestry of birds have used scutes as a possible lead in finding the origin of feathers. It has been theorized that feathers evolved from reptilian scales, but testing has revealed that feathers and scales are genetically and chemically different. Scutes, on the other hand, may be genetically more closely linked with feathers. It is even possible that scutes evolved from feathers, rather than vice versa.
Fossil discoveries have indicated that many dinosaurs likely had feathers, and that feathers themselves may be a more primitive characteristic than previously thought. If this is true, it is possible that birds and dinosaurs evolved from a common feathered ancestor. Some scientists speculate that the scute could have developed from an early feather structure on an ancestral organism of this type.