Evidence for an amphibian sixth digit.

Introduction: Despite the great diversity in digit morphology reflecting the adaptation of tetrapods to their lifestyle, the number of digits in extant tetrapod species is conservatively stabilized at five or less, which is known as the pentadactyl constraint.

Results: We found that an anuran amphibian species, Xenopus tropicalis (western clawed frog), has a clawed protrusion anteroventral to digit I on the foot. To identify the nature of the anterior-most clawed protrusion, we examined its morphology, tissue composition, development, and gene expression. We demonstrated that the protrusion in the X. tropicalis hindlimb is the sixth digit, as is evident from anatomical features, development, and molecular marker expression.

Conclusion: Identification of the sixth digit in the X. tropicalis hindlimb strongly suggests that the prehallux in other Xenopus species with similar morphology and at the same position as the sixth digit is also a vestigial digit. We propose here that the prehallux seen in various species of amphibians generally represents a rudimentary sixth digit.