Cylindrichnus concentricus Toots in Howard, 1966 (trace fossil) in its type locality, Upper Cretaceous, Wyoming

Cylindrichnus concentricus is a wide spread trace fossil in shallow marine sedimentary rocks of Mesozoic and Cenozoic age. This paper clarifies the ichnotaxonomy of C. concentricus by offering an emended diagnosis of the ichnogenus and a new diagnosis of the ichnospecies. The broad, bow-shaped architecture of C. concentricus with two openings at the sediment surface suggests that the trace maker was either a filter-feeding animal that captured suspended food particles from the water column or else a surface deposit feeder that employed tentacles or an eversible pharynx to collect nutritious sediment from the surface around one or both of the burrow openings. Although C. concentricus has been reported in a variety of different sedimentary environments around the world, in its type loca ity in the Cretaceous of Wyoming the trace fossilis found in low-diversity or monoichnospecific beds that exhibit hummocky cross stratification. This occurrence suggests the burrower’s tolerance (or perhaps preference) for an episodic high-energy hydrodynamic environment related to major storms.