pentadactyl

Pentadactyl refers to the condition of having five digits on a limb, typically the forelimb or hindlimb of tetrapods. The term is derived from Greek penta meaning five and daktylos meaning finger. In anatomy it contrasts with more- or fewer-digit configurations.

Most modern tetrapods exhibit a pentadactyl limb design, organized into proximal stylopod, middle zeugopod, and distal autopod with five digits. Fossil evidence shows that some early tetrapods possessed more than five digits, but during the late Paleozoic the five-digit arrangement became widespread and largely conserved, though numerous lineages later modified digit number through loss or fusion.

Developmentally, pentadactyly arises from conserved limb-patterning networks. HOX genes regulate limb segments, while sonic hedgehog (SHH) signaling from the zone of polarizing activity establishes digit identity and spacing. Changes in the timing, intensity, or area of SHH signaling can produce variations such as polydactyly (extra digits) or oligodactyly (fewer digits).

Variations: In humans and most mammals the five-digit pattern is typical. Some mammals have reduced digits in the forelimb or hindlimb, such as horses (one main weight-bearing digit) and many ungulates. In birds the forelimbs are wings with a reduced number of digits, usually three, reflecting their pentadactyl ancestry but modified during evolution. Amphibians and many reptiles retain five digits in both limbs, though limb morphology varies widely among lineages.

Pentadactyly is a common reference point in discussions of tetrapod limb evolution and comparative anatomy, illustrating how a basic plan can be conserved while diversified by changes in development and function.