Amphibians (Class Amphibia) and reptiles (Class Reptilia) are distinct branches of vertebrate life, yet they share fundamental biological characteristics that differentiate them from mammals and birds. Both are vertebrates, possessing a backbone, and are classified as tetrapods, indicating a four-limbed body plan, even if some species have secondarily lost their limbs. Understanding the commonalities between a frog and a lizard requires examining their physiological strategies, shared evolutionary history, and internal anatomy.
Shared Physiological Strategy Ectothermy
The most defining shared trait is ectothermy, a physiological strategy where they primarily use external heat sources to regulate body temperature. This contrasts sharply with endotherms, like mammals, which generate most of their body heat internally. Due to their low metabolic rate, amphibians and reptiles require significantly less energy and food intake than a similarly sized endotherm.
This dependence necessitates behavioral thermoregulation. Both groups must actively seek optimal thermal conditions, such as basking in direct sunlight or lying on warm rocks, to raise body temperature. Conversely, they seek shade, burrow, or enter water when they need to cool down.
Maintaining a preferred body temperature is essential for internal functions. Digestive enzymes, muscle performance, and nerve activity operate best within a narrow, elevated temperature range. This behavioral control allows them to conserve energy that would otherwise be spent on internal heat generation.
Shared Evolutionary Heritage
Amphibians and reptiles are descendants of the earliest four-limbed vertebrates, known as stem-tetrapods, which emerged from lobe-finned fish ancestors approximately 390 million years ago. This shared lineage represents the initial transition of vertebrate life from aquatic to terrestrial environments. Both classes trace back to this common ancestral group that first developed limbs and lungs.
The evolutionary path diverged when a group of these early tetrapods developed the amniotic egg, a self-contained reproductive structure with protective membranes. Reptiles, birds, and mammals belong to the Amniota, an adaptation allowing reproduction away from water. Amphibians retained the ancestral requirement of laying eggs in water or moist environments, classifying them as non-amniotes.
Despite this reproductive divergence, the two classes share a common organization of their musculoskeletal and nervous systems inherited from their early terrestrial ancestors. Their placement on the tree of life confirms a closer relationship to each other than to mammals or birds. They represent successive stages in the exploration of land, with reptiles developing greater independence through the amniotic egg.
Shared Anatomical Features
A significant internal similarity is the structure of their circulatory system, specifically the heart. Most amphibians and reptiles possess a three-chambered heart composed of two atria and a single ventricle. The two atria receive blood separately: one deoxygenated from the body and one oxygenated from the lungs.
In the single ventricle, the oxygenated and deoxygenated blood mixes before being pumped out. This mixing is less efficient than the four-chambered hearts of mammals and birds, but it is tolerable due to their lower metabolic needs. Crocodilians are the notable exception among reptiles, having evolved a four-chambered heart.
Regarding respiration, both groups utilize lungs for breathing. While reptiles rely almost exclusively on their lungs, most amphibians supplement this by absorbing oxygen directly through their skin, a process called cutaneous respiration. Neither amphibians nor reptiles possess insulating features like hair or feathers, reinforcing their reliance on external heat sources.