The multifaceted ichnogenus Protovirgularia: Taxonomy, producers and environments

Protovirgularia is an old name for prominent trace fossils common throughout the Phanerozoic and fascinating geologists for about 175 years. Repeatedly described under different names, taxonomic work from the 1990's argued for the inclusion of several morphologically similar ichnogenera in Protovirgularia. A critical review of the type specimens of the included ichnospecies and their synonyms confirms the validity of five ichnospecies by applying uniform ichnotaxobases. A morphometric analysis of key parameters assists in the distinction of these ichnospecies. The type ichnospecies Protovirgularia dichotoma, generally regarded as a trail, is better qualified as a trackway, in contrast to the remaining ichnospecies, which represent shallow burrows. Beside the habitus of the burrow, the shape, density and orientation of its sediment pads (or chevrons) and their ornamentation allow the distinction of P. pennata, P. rugosa, P. longespicata and P. bifurcata. Compound forms resulting from locomotion and resting are rejected as an ichnotaxobase. The enigmatic nature of Protovirgularia led to speculations about its origin, including various body-fossil interpretations (e.g., sea pens and annelids). Now widely accepted as a trace fossil, the interpretation of its producer varied and became oversimplified based on an experiment made with protobranch bivalves. Several characteristics support an interpretation of many Protovirgularia specimens as arthropod-produced (e.g., malacostracan crustaceans) instead of molluscs, which is in alignment with suggestions by older work. A literature review of verifiable illustrations of trace fossils conformable with one of the five ichnospecies not only confirms their recurrence and validity as endmembers, but also highlights stratigraphical and palaeoenvironmental trends.