Paleoenvironmental controls on Skolithos serratus: A case study from the Lower Cretaceous of the Sanfranciscana Basin

Skolithos serratus differs from other simple, vertical, unbranched trace fossils by its distinctive horizontal scratch marks arranged in a screw-like pattern attributed to tiger beetle larvae (Cicindelidae). Despite its diagnostic morphology and established tracemaker, this ichnospecies remains poorly documented, with only sparse occurrences in the geological record. In this study, we present a detailed analysis of Skolithos serratus from floodplain deposits of the Quiricó Formation (Sanfranciscana Basin, Brazil). Our results demonstrate colonization in low-energy environments characterized by periods of non-deposition and subaerial exposure. This indicates a tracemaker preference for exposed, well-drained fine sandy substrates associated with lowered water tables, conditions necessary for the complete development of larval stages. Furthermore, Skolithos serratus colonization occurred predominantly during late-stage substrate drainage, facilitated by organisms with appendages adapted for burrowing into firm substrates developed between aggradation cycles driven by autocyclic processes. We also highlight the striking disparity in the documentation of S. serratus compared to other Skolithos ichnospecies in fluvial environments. This discrepancy reflects niche specialization by the S. serratus tracemaker and the strong control exerted by substrate properties, as all recorded occurrences are restricted to fine-grained heterolithic deposits composed predominantly of fine sand and mud. Finally, we expand the paleobiogeographic distribution of Cicindelidae in South America, expanding the record of their Mesozoic occurrences.