Tetradactyla

Tetradactyla refers to the condition of having four digits on a limb. The term is used as a descriptive term in anatomy and paleontology to describe limb morphology, and it is often discussed in contrast to the more common pentadactyl condition found in many tetrapods as well as to various forms that show digit reduction or polydactyly.

Etymology and usage: The word derives from Greek tetra- meaning four and daktylos meaning finger. In scientific literature, tetradactyla is used to describe a limb with four digits, without implying a distinct taxonomic group. It can apply to observations in living species as well as to interpretations of fossil tetrapods where digit count appears reduced or specialized.

Occurrence in living groups: In extant vertebrates, forelimbs that display four digits are most commonly noted in many salamanders, where the forelimbs typically bear four digits while the hindlimbs have five. Other examples of tetradactyl limbs are rarer and often relate to specific evolutionary adaptations or developmental variations within a lineage. In many reptiles and amphibians, digit counts can vary through ontogeny or across species, with tetradactyly appearing as a derived or ancestral trait in particular clades, depending on the evolutionary context.

Development and significance: Digit number is controlled by developmental gene networks, including Hox genes and related pathways. Variations leading to tetradactyly are of interest in evo-devo because they shed light on the genetic and developmental mechanisms that shape limb morphology and the evolutionary history of tetrapods.

See also: pentadactyly, polydactyly, digit reduction, limb development.