After the evolutionary loss of dermal scales of fishes, amniotes developed an entirely new type of scale derived from the epidermis. The skin of vertebrates is rarely naked, it is usually provided with protective scales, bony plates, feathers or hairs. There are no epidermal scales in fishes and amphibians, they appear for the first time in reptiles. They are cornified derivatives of the Malphigian layer and are generally shed and replaced.
As keratin is evolved, it begins to form epidermal scales which form a protective covering of the body by forming continuous armour (a protective comvering). The scales overlap each other, in snakes and lizards they are continuous and they undergo ecdysis (shedding off) periodically, the entire corneal layer of scales is shed as whole and in snakes it is turned inside out. The scales on the ventral side in most snakes differ from their other scales in being long and transversely arranged, they aid in locomotion. Turtles and crocodiles have different kinds of scales which do not overlap, nor undergo periodic ecdysis, but the scales are gradually worn off (exhausted) and replaced. Large epidermal scales, such as those on the shell of turtles and on the head of snakes, are generally called scutes. In birds the scales are confined to the shanks and feet and some at the base of the beak. They gradually overlap as in snakes and lizards. In mammals, epidermal scales are found on the tail and paws of rats, mice, shrews. These scales are not much cornified, nor do they undergo ecdysis; hairs project from beneath the scales. In armadillos there are large scales which fuse to form plates on the head, shoulders and hips; in the middle of the body, except mid-ventrally, the scales fuse to form ring-like bands; these scales do not undergo ecdysis but are gradually worn off and replaced.
Corneal structures– Some epidermal scales of the tail of a rattle snake are modified to form a rattle, it consists of a series of old dried scales. During ecdysis the scale at the tip of the tail is not shed and it forms a ring, thus after several ecdyses a series of rings form a rattle, each new ring being larger than its predecessor. The rattle may wear off and break at the tip. In turtles, tortoises and modern birds there are no teeth, each jaw bone being covered with modified epidermal scales which form a beak. In monotremes there is a soft bill which differs from the beak of birds in not being covered with modified epidermal scales.