Amphibian skin is a particularly complex organ that is primarily responsible for respiration, osmoregulation, thermoregulation, defense, water absorption, and communication. The skin, as well as many other organs in the amphibian body, has undergone the most extensive rearrangement in the adaptation from water to land. Structural and physiological features of skin in amphibians are presented within this review. We aim to procure extensive and updated information on the evolutionary history of amphibians and their transition from water to land—that is, the changes seen in their skin from the larval stages to adulthood from the points of morphology, physiology, and immunology.

The reptile skin is a barrier against water loss and pathogens and an armor for mechanical damages. The integument of reptiles consists of two main layers: the epidermis and the dermis. The epidermis, the hard cover of the body which has an armor-like role, varies among extant reptiles in terms of structural aspects such as thickness, hardness or the kinds of appendages it constitutes. The reptile epithelial cells of the epidermis (keratinocytes) are composed of two main proteins: intermediate filament keratins (IFKs) and corneous beta proteins (CBPs). The outer horny layer of the epidermis, stratum corneum, is constituted of keratinocytes by means of terminal differentiation or cornification which is a result of the protein interactions where CBPs associate with and coat the initial scaffold of IFKs. Reptiles were able to colonize the terrestrial environment due to the changes in these epidermal structures, which led to various cornified epidermal appendages such as scales and scutes, a beak, claws or setae. Developmental and structural aspects of the epidermal CBPs as well as their shared chromosomal locus (EDC) indicate an ancestral origin that gave rise to the finest armor of reptilians.

Rain‐harvesting behaviour represents an adaptation for water collection that has evolved in some species of iguanian reptiles inhabiting arid environments. To date, such behaviour has been observed only in members of the families Agamidae and Phrynosomatidae. A common set of integumentary features characterizes these rain‐harvesting species, including scale surface microstructures (SSMs), capillary channels and hinge joints. The influence of variations in these features on rain‐harvesting has been a subject of discussion for many decades. Nevertheless, a comprehensive comparative study of similarities and differences between harvesting and non‐harvesting species on a broader scale remains lacking. In this study, we classify scale surface microstructures into three categories: large hexagons (SSM1), smaller nested hexagons (SSM2) and hinge pits (SSM2H). As the first two SSM types are widespread, they do not appear to be directly linked to the adaptation for rain‐harvesting. Conversely, the presence or absence of hinge pits distinguishes harvesters from non‐harvesters. Additionally, channel hierarchy, width and structure determine the effectiveness of the rain‐harvesting architecture. Only Moloch horridus exhibits distinct integumentary features in comparison to other agamids and rain‐harvesting species. Ancestral character state reconstruction suggests that rain‐harvesting behaviour was likely absent in the ancestor of Iguanians, even though overlapping scales and SSM1 were present. Our findings illustrate that rain‐harvesting species have independently converged upon similar structural solutions to address their water acquisition challenges, building upon shared pre‐existing features.