Skolithos linearis from the early Palaeozoic: The burrow of a priapulid?

The trace fossil Skolithos linearis is abundant in Cambrian sandstone of nearshore origin but its distribution declines throughout the early Palaeozoic. Known for almost three centuries and one of the most common trace fossils on earth, its producer remains unknown. Previous workers have favoured an interpretation as polychaete or phoronid burrow, based on modern analogues. In this study, Skolithos linearis was examined from Cambrian and Ordovician localities of the palaeocontinents Laurentia, Baltica and Gondwana. All occurrences confirm that populations included burrows with anomalous features and variations, such as varying orientation, branching, spreiten-like lateral displacement, a funnel-shaped aperture, stacked conical and funnel structures, bulbous expansions and annulation, a bulb-like basal termination, segmentation and striae. These characteristics support an interpretation of S. linearis as the burrow of priapulids, based on extant and fossil analogues. This not only solves a mystery debated for a long time but also opens a refined application of this trace fossil in palaeoenvironmental interpretations, as well as evolutionary and behavioural aspects of the phylum Priapulida. The focus on morphological variation to illuminate aspects of behaviour represented in a trace fossil is a promising approach for future attempts to identify tracemakers. From an ecological point of view, the interpretation of S. linearis as a priapulid burrow sheds light on the life mode of early Palaeozoic priapulids, which probably were abundant and deeply burrowing soft-bodied organisms. Sandstone beds with dense burrow accumulations (so-called pipe rock) are common around the globe and provide a glimpse of the endobenthic lifestyle of these worms. Moreover, gradual transitions into burrows with similar characteristics indicate that priapulids probably made a wide range of early Palaeozoic ichnotaxa.