Mating trackways of a fossil giant millipede

Three closely associated arthropleurid trackways, Diplichnites cuithensis, from the Lower Carboniferous of Fife, Scotland, exhibit signs of interaction between track-makers. An extra file of footprints is found on the downslope side of two trackways (A and C), the upslope side of another (B). These additional files of footprints suggest that either: each trackway resulted from two arthropleurids of different sizes walking in tandem, matching their footprints exactly on one side for some distance; or that one arthropleurid was partially mounted on the back of another producing the three parallel files. It is here argued that the latter is correct and that this represents evidence of mating behaviour in arthropleurids.