Both amphibians and reptiles are classes of vertebrates that share the characteristic of being ectotherms, meaning they regulate their body temperature using external sources like the sun. Despite this shared trait, their evolutionary paths diverged significantly over 330 million years ago from a common tetrapod ancestor, leading to fundamental differences in how they manage life on land. The class Amphibia includes frogs, toads, salamanders, newts, and caecilians, while the class Reptilia encompasses snakes, lizards, turtles, crocodiles, and alligators. These differences primarily concern water conservation and life cycle.
Skin Structure and Respiratory Methods
The most immediate difference is their integument, or outer covering. Amphibians possess thin, smooth skin that lacks scales, making it highly permeable to both gases and water. This structure allows for cutaneous respiration, where oxygen is absorbed directly through the skin, supplementing lung function. Because this skin must remain constantly moist to facilitate gas exchange, amphibians are susceptible to dehydration, restricting most species to humid environments or aquatic habitats.
Reptiles, conversely, exhibit skin that is thick, dry, and covered in protective scales or scutes composed of the tough protein keratin. This keratinized armor creates a nearly watertight barrier, preventing desiccation and allowing survival in arid habitats. The dense, impermeable nature of reptilian skin means it cannot be used for gas exchange, so reptiles rely entirely on their lungs for respiration. Reptilian lungs are generally more complex and efficient than those found in amphibians, providing the necessary oxygen supply for a fully terrestrial life.
Developmental Stages and Metamorphosis
The life cycle is a defining feature, showcasing the amphibian’s dual existence compared to the reptile’s terrestrial independence. Amphibians undergo metamorphosis, a transformation involving a shift from an aquatic larval stage to a semi-terrestrial adult form. A typical amphibian, such as a frog, begins life as a gill-breathing tadpole with a tail, spending its early development strictly in water.
During metamorphosis, thyroid hormones trigger a series of profound changes, including the resorption of the tail and gills, the development of limbs, and the maturation of lungs for air breathing. This process effectively requires two different body plans and two distinct habitats to complete the life cycle. Reptiles, however, experience direct development, where the young hatch or are born as miniature versions of the adult. There is no intermediate larval stage, no gills, and no major structural transformation, allowing the juvenile to immediately occupy the same terrestrial niche as its parents.
Reproductive Strategies and Egg Types
Reproductive methods solidify the different evolutionary strategies for surviving on land. Amphibians typically lay non-amniotic eggs encased in a gelatinous coating. Because this coating provides minimal protection against water loss, the eggs must be deposited in water or a very damp location to prevent desiccation. Fertilization in many amphibians, such as frogs, is external, occurring when the male releases sperm onto the eggs as the female lays them in the water.
Reptiles, in contrast, utilize internal fertilization and lay the amniotic egg. This egg possesses a shell, which can be leathery or calcified, and contains specialized internal membranes like the amnion, chorion, and allantois. The amnion creates a private, fluid-filled environment for the embryo, effectively acting as an internal pond. This self-contained system allows reptiles to reproduce far from standing water, enabling them to colonize dry, terrestrial ecosystems.