Arthrophycus is an iconic ichnogenus known from lower Paleozoic clastic strata worldwide, yet its origin remains controversial. A medial groove imparts a bilobed symmetry in some forms, implicating arthropods as the tracemaker. Other forms have regular annulae that evoke an annelid body plan. Transverse ridges in some bilobed forms of Arthrophycus, however, were deemed by some as too blunt to have been made by arthropods, and the annulation is unlikely to have anything but a superficial resemblance to “worms.” Recent work has converged on a nontrilobite, long-bodied arthropod as the likely Arthrophycus tracemaker. Given the breadth of morphological variation and range in size included in various ichnospecies of Arthrophycus, there are likely multiple tracemakers for this ichnogenus, and the tracemakers may belong to more than one phylum. Some ichnospecies of Arthrophycus may even represent the only physical record of an unknown or poorly fossilized group of organisms. Observations of the physiology and neoichnology of modern Arthrophycus tracemaker analogs support the conclusion that the maker of the Carboniferous ichnospecies Arthrophycus parallelus is neither an annelid nor a previously described arthropod but a yet unknown member of the Ecdysozoa.

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