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How do the scales of snakes and lizards differ from those of crocodiles and turtles?

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A painted turtle (Chrysemys picta) with a dark shell and red and yellow markings rests on a rock near water, surrounded by green aquatic plants. — reptile Painted turtle (*Chrysemys picta*).

The scales of snakes and lizards differ from those of crocodiles and turtles primarily in their composition, arrangement, and shedding process.

While all reptilian scales are composed of keratin, the structural organization of these proteins varies:

Snakes and Lizards: These animals possess scales where alpha and beta keratins form continuous layers, with the alpha keratin located beneath the beta keratin. Their growth cycle involves the formation of a complete new epidermal surface before the old, cornified skin is sloughed off, either in portions or as a single sheet.1
Crocodiles and Turtles: In these reptiles, the outer scale surface consists exclusively of beta keratin, while the hinge region contains only alpha keratin. Unlike snakes and lizards, they undergo continuous cell division and gradual exfoliation of cells at the skin surface.1

Furthermore, while some reptiles—particularly crocodiles—feature bony plates known as osteoderms in the dermis beneath their scales, turtles possess a unique, rigid shell composed of dermal and endochondral bone that they cannot shed.234