Who were the tracemakers of Protovirgularia – Molluscs, arthropods, or annelids?

The ichnogenus Protovirgularia is an old but valid ichnotaxon name with a convoluted taxonomic history and interpretation. Originally erected as body fossil, it is now recognized as a trace fossil with a global distribution throughout the Phanerozoic. A wide range of tracemakers from the phyla Annelida, Mollusca and Arthropoda can produce traces with the significant chevron-like ornamentation, which have been classified in several ichnospecies. Modern analogues have demonstrated that protobranch bivalves are able to make such traces, which led to the general assumption that Protovirgularia represents a characteristic bivalve trace fossil. Examples of three ichnospecies of Protovirgularia from the Middle Triassic of Germany are analysed and show that Arthropoda (e.g., Isopoda) and Annelida (e.g., Polychaeta) must equally be regarded as potential tracemakers of Protovirgularia. This interpretation comes from diverse lines of evidence having varying uncertainties, including functional morphology, modern analogues, and preserved producer. The fact that annelids and arthropods, in addition to bivalves, can be regarded as producers of Protovirgularia offers the possibility of utilizing this ichnogenus for enhanced palaeoenvironmental and evolutionary interpretations.