Interpreting the autopodia of tetrapods: interphalangeal lines hinge on too many assumptions

Recently Peters proposed the concept of 'interphalangeal lines', defined as sub-parallel lines that could supposedly be drawn across the joints of the digits of all tetrapods. The lines were viewed as potential axes of rotation, and it was suggested that they could be used to determine the resting position of the digits, reconstruct missing digital elements of fossil tetrapods, and provide information on systematic relationships. Evidence was adduced from the skeletons of recent and fossil vertebrates and from footprints. However, detailed analysis shows that these claims are largely unfounded. Linear alignments of joints on neighbouring digits are not consistently present in tetrapods, especially across locomotor cycles. Even if present, interphalangeal (IP) lines would rarely be in an appropriate orientation to facilitate joint movements during locomotion. There is no reason to believe that IP lines would be homologous across different taxa, so they cannot be used to infer systematic relationships. Finally, the alleged support from the ichnological record is undermined by the uncertain relationship between the joint structure of the skeleton and the form of the print. We conclude that IP lines cannot be consistently constructed on tetrapod extremities, and would have minimal functional relevance or predictive power in any case.