pentadactyla

Pentadactyla, from Greek pente "five" and daktylos "finger," is a term used in anatomy and older zoological literature to denote a limb plan with five digits. In its descriptive sense, pentadactyl limbs follow the standard tetrapod pattern in which the limb is organized into three proximal segments—stylopod (femur or humerus), zeugopod (radius and ulna or tibia and fibula)—and the distal mesopod, ending in five digits, or autopod. The five-digited condition is the ancestral template for most land-dwelling vertebrates and is a hallmark of the pentadactyl limb.

Across living groups, the five-digit plan is highly conserved but not universal. In some lineages, digits are reduced, fused, or lost altogether, while in others additional digit variation can occur through polydactyly. Nevertheless, the five-digit arrangement remains a useful character in comparative anatomy and paleontology for tracing limb evolution.

In developmental biology, digit patterning is controlled by a network of genes and signaling centers. Key elements include a zone of polarizing activity (ZPA) and Sonic hedgehog (Shh) signaling, along with Hox gene regulation, which together influence digit identity and number. Disruptions can produce polydactyly or syndactyly.

Historically, Pentadactyla was used as a formal taxonomic grouping to include vertebrates with pentadactyl limbs; it is now largely obsolete in modern systematics, which classify tetrapods within broader clades such as Tetrapoda. Today, pentadactyly is primarily treated as a functional and developmental motif rather than a strict, named lineage.