FUNCTIONAL MORPHOLOGY OF LOCOMOTION IN THE GIANT PALEOZOIC MILLIPEDE ARTHROPLEURA: INSIGHTS FROM TRACE FOSSILS AND KINEMATICS OF LOCOMOTION IN EXTANT MILLIPEDES

Large Diplichnites trace fossils attributable to the giant Paleozoic millipede Arthropleura are known from the Carboniferous of North America and Europe. These trackways were surveyed and found to fall into two distinct groups: those consisting of a continuous series of footfalls and those with footfalls grouped into crescentic clusters. Surface traces and undertracks were also distinguished. In order to gain insight into the possible functional morphology of locomotion in Arthropleura, the kinematics of walking in the penicillate millipede Polyxenus anacapensis and representative chilognath millipedes were analyzed using high speed video and digitization software and trackways were plotted. The parameters measured included speed, stepping frequency, stride length, angle of appendage swing, period, ratio of protraction to retraction, and phase lag. When Polyxenus locomotes contralateral legs step alternately. It was found that at relatively low speeds, footfalls plot in a continuous series. In contrast, at relatively high speeds, footfalls plot in discrete clusters. This difference in trackway morphology was determined to be due largely to an increase in stride length generated through a stretching of the body at faster speeds. Chilognath millipedes locomote with contralateral legs stepping synchronously at all speeds and their skeletomuscular anatomy does not allow for significant trunk elongation. The trunk ring architecture of Arthropleura was reconstructed and determined to be most similar to the Polyxenida, among extant millipedes. Given the morphological similarities between Polyxenus and Arthropleura, and the similarities in trackway morphology, it seems reasonable to hypothesize that Arthropleura also utilized distinct gaits at different speeds, with the clustered trackways representing locomotion at relatively faster speeds.