Full-body impressions: a category of trace fossils unique to exceptionally preserved bedding planes and indicators of true substrates

Certain biogenic sedimentary structures can be considered to be reliable indicators of true substrates. Full-body impressions, or FBIs, are trace fossils that are shallow, highly detailed impressions of the ventral axial anatomy of low body mass animals made while living . Their mode of preservation makes FBIs distinct from undertracks, body moulds or passive tool marks. FBIs are made at the air– or water–substrate interface during sedimentary stasis. Associated trace fossils, like those that exhibit excavations with preserved waste mounds and very near-surface burrow roofs, located on the same bedding plane, can support the interpretation of true substrate preservation. Integration of the biogenic and sedimentological data from both Carboniferous and Pleistocene sites with well-preserved bedding-plane trace fossil assemblages has confirmed the conditions under which true substrates are preserved. These include periods (as brief as days to weeks) of sedimentary stasis, followed by gentle deposition without erosion, and without the need to invoke biofilms or biomats as consolidants. The potential for these taphonomic conditions exists in both alluvial and lacustrine environments, as well as intertidal-estuarine valleys, as described by other published examples of FBI preservation. Future FBI discoveries will prove useful in identifying true substrates across a range of time periods and environments.