In evolutionary terms, these are considered to be the most "recently evolved" family of salamanders.  This group of amphibians are referred to as "lung-less salamanders" as these terrestrial, aquatic, and arboreal salamanders have no lungs.  Remarkably respiration is carried out via cutaneous respiration as oxygen and carbon dioxide are transpired across the moist semi-permeable skin.
    Plethodontid salamanders contain more species diversity than all other groups of salamanders combined.  Approximately 28 genera are represented by 266 species.  Most of this family's diversity is found within the eastern United States.  Plethodontid salamanders have dispersed from their evolutionary origins in the United States to Mexico, Central America, and South America where plethodontid distribution extends from Columbia to eastern Brazil and central Bolivia.