A limb digit is one of the terminal rays of a tetrapod limb, such as a finger or toe. A digit includes a series of phalanges and surrounding tissues; it is not synonymous with one phalanx. Several bones can therefore belong to one digit, with a joint connecting successive elements.
Counting digits and counting their component bones answer different questions. If a hypothetical limb has five digits and each has three phalanges, it has five digital rays but fifteen phalangeal bones. This deliberately simplified example shows why an isolated row of bones must first be reconstructed into its anatomical arrangement before it is used to infer digit number.
Nor is five digits the defining ancestral condition of every early tetrapod limb. In 1990, fossil descriptions documented eight digits in the forelimb of Acanthostega, while other early specimens had different counts. These observations challenged reconstructions that began by assuming an originally fixed five-element pattern. The specimens are evidence about the early diversity of limbs, rather than abnormal individuals judged against the human hand.[ref]
Polydactyly in the earliest known tetrapod limbs by Michael I. Coates and Jennifer A. Clack. Abstract: eight-digit Acanthostega forelimb, other Devonian counts, and challenge to primitive pentadactyly
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