Arthrophycus Hall 1852 is a long-studied ichnogenus reported from localities worldwide, including all seven continents and twenty-eight countries. It is most abundant in Ordovician and Silurian strata and is regarded by some ichnologists as having biostratigraphic utility, although other occurrences of the ichnogenus have been reported from the Proterozoic to the Miocene. Imprecise or overly-brief descriptions and ambiguous or unclear illustrations, drawings, and photographs of specimens regarded as Arthrophycus have resulted in a taxonomic "wastebasket" and have led to confusion over the biostratigraphic utility of the ichnogenus. A review of the primary Arthrophycus literature converged on the following diagnostic characters for the ichnogenus: annulations, simple form, branches, bundles, shape of cross-section, median groove, and internal structures. Characters such as annulations, branching, and median groove encompass variability not previously discussed in reviews of the ichnogenus. Refining the definition of these characters produced sixteen characters that could be coded for a numerical phenetic analysis of Arthrophycus ichnospecies. Cluster analyses revealed two main clusters: a cluster of six (A. alleghaniensis, A. brongniartii, A. linearis, A. lateralis, A. minimus, A. parallelus) considered to be valid Arthrophycus and a second cluster of discredited iclulospecies, members of which probably belong in other ichnogenera. The cluster analyses were supported by PCO and cladistic analyses. Only Paleozoic occurrences of Arthrophycus are considered valid, including specimens from the Cambrian, Devonian, and Carboniferous, challenging previous conclusions that Arthrophycus is confined to the Ordovician and Silurian. This analysis also confirms an hypothesis that Arthrophycus originated in the southern continents in the Cambrian, persisted there in the Ordovician, and then expanded to the northern continents during the late Ordovician and Silurian.
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