The order Urodela includes the amphibians commonly known as salamanders and newts. This classification is one of the three extant orders within the subclass Lissamphibia, which also includes frogs, toads, and caecilians. Urodelans possess a body plan considered similar to ancestral amphibians, having branched off early in vertebrate evolution. Fossil evidence suggests their lineage, known as Caudata, originated around 237 million years ago.
Defining Characteristics of Salamanders and Newts
Salamanders are defined by a generalized, superficially lizard-like tetrapod body structure. They typically possess a slender trunk, a blunt snout, and short limbs that project outward at right angles to the body. A distinguishing feature is the presence of a tail that persists throughout all life stages, from larva to adult.
The limbs usually consist of four digits on the forefeet and five on the hindfeet, though some aquatic species may have reduced or absent hindlimbs. Their respiration relies heavily on their moist, glandular skin, a process known as cutaneous respiration. This permeable skin necessitates that they inhabit damp environments to prevent desiccation.
Salamanders range in size from minute species measuring just 27 millimeters to the Chinese giant salamander, which can reach up to 1.8 meters in length. Newts, belonging to the subfamily Pleurodelinae, are a type of salamander often differentiated by their rougher, sometimes warty skin in the terrestrial adult phase, known as the “eft.” While the distinction is mostly common usage, newts generally exhibit a more pronounced shift between aquatic and terrestrial adult phases.
The Remarkable Biology of Limb and Organ Regeneration
The unique biological capacity of Urodela is their ability to regenerate complex body parts without scarring. This capacity extends beyond simple wound healing, allowing them to fully regrow severed limbs, tails, portions of the spinal cord, jaw sections, and damaged parts of the heart and eye lens. In contrast, mammals respond to significant tissue loss by forming permanent scar tissue.
The process begins immediately following injury with the formation of the blastema, a specialized structure. This mass of undifferentiated cells accumulates at the site of amputation. These progenitor cells arise through dedifferentiation, where mature, specialized cells—including those from bone, muscle, and cartilage—revert to a less specialized state.
The blastema acts similarly to an embryonic limb bud, multiplying and redifferentiating into the precise tissues required to rebuild the missing structure. The nervous system plays a significant role, as the amputation site must be innervated for successful blastema formation and subsequent regeneration. Rebuilding a fully functional appendage is guided by complex molecular signaling pathways not fully understood in other vertebrates.
Life Cycle and Developmental Stages
Most urodeles exhibit a biphasic life cycle, which involves two distinct phases: an aquatic larval stage and a predominantly terrestrial adult form. Reproduction typically involves the female laying eggs in water, which then hatch into aquatic larvae. The larval stage is distinguished by the presence of external, feathery gills for underwater breathing and a prominent tail fin for swimming.
After a period of growth, the larva undergoes metamorphosis, a transformation mediated by thyroid hormones. During this process, the external gills are resorbed, the tail fin is lost, and the skin thickens, transitioning to a terrestrial form. The adult develops lungs and shifts its respiratory reliance to a combination of lung and cutaneous respiration, preparing for life on land.
However, a significant number of species exhibit neoteny, or paedomorphosis, retaining larval characteristics into adulthood while achieving sexual maturity. The Mexican Axolotl (Ambystoma mexicanum) is the most famous example, remaining permanently aquatic with external gills throughout its life. This retention of the larval state is often facultative, meaning it depends on environmental conditions, but in some species, it is obligate, preventing full metamorphosis.
Global Distribution and Major Family Groups
The majority of salamander diversity is concentrated in the Northern Hemisphere, largely within the Holarctic realm. North America is a major center of diversity, with the Appalachian Mountains region considered a global hotspot for salamander species. This geographic area provides the cool, moist, forested habitats necessary to support their permeable skin and complex life cycles.
The order Urodela is divided into several families, two of which illustrate evolutionary adaptations within the group. The Plethodontidae, or lungless salamanders, represent the largest family, accounting for nearly two-thirds of all known species. These amphibians rely solely on their moist skin and the lining of their mouth and throat for gas exchange.
The Ambystomatidae, known as mole salamanders, are another prominent family primarily distributed across North America. Many species are terrestrial as adults, spending time burrowed underground and only emerging to migrate to breeding ponds. This family contains the genus Ambystoma, which includes the Axolotl and other species that frequently display neoteny.