In Search of a Modern Analog for Arthrophycus Ichnogenus

Arthrophycus ichnogenus describes a narrow, elongate burrow/trackway commonly preserved in full relief or convex hyporelief. A medial groove is present in some forms, imparting a bilobed symmetry and suggesting a trace maker with paired appendages, implicating arthropods. Other forms show regular annulations that evoke an annelid trace maker. Obliquely oriented, transverse ridges visible in some bilobed forms have been judged by some workers as too blunt for scratches made by arthropods. Specialists on modern annelids judged the trace as unlikely to have been made by that phylum, creating an identity crisis: if not arthropod or annelid, then what? Recent work converges on a “non-trilobite, long-bodied arthropod” as the likely Arthrophycus trace-maker. Given the breadth of morphological variation and range in size included in various ichnospecies of Arthrophycus there are likely multiple trace makers for this ichnogenus, and these trace makers may represent more than one phylum. There is also the possibility that these traces are the only physical record of an unknown or poorly fossilized group of organisms. We explore several lesser-known arthropod and non-arthropod groups as candidates for the Arthrophycus trace makers.