Unusual occurrence of Cochlichnus in aeolian sandstones: evidence for a stiletto fly larval origin

Abstract Cochlichnus anguineus is an ichnotaxon recognized for its distinctive regular wavy morphology. It is a characteristic trace fossil typically associated with aquatic and marginal aquatic environments. However, in this study we examine its unusual occurrence in the aeolian sandstones of the Botucatu Formation (Lower Cretaceous, Paraná Basin, Brazil). Drawing on neoichnological evidence from modern dune environments in Brazil and Namibia, we propose that stiletto fly larvae (Diptera, Therevidae) are the most likely tracemakers of these fossils. This represents the earliest known occurrence of Therevidae in the fossil record, supporting evidence from molecular analyses. Furthermore, we compare these traces with similar ones from other aeolian units, discussing ichnotaxonomic implications. A key focus is the nuanced distinction between trails and burrows. In light of this, we propose an emended diagnosis for ichnogenus Cochlichnus , restricting it to shallow trails made in the substrate and on its surface, reflecting a behavioural pattern distinct from Cymataulus . While Cymataulus also has a similar wavy pattern, it represents dwelling burrows rather than surface trails.