The déjà vu effect: Recurrent patterns in exploitation of ecospace, establishment of the mixed layer, and distribution of matgrounds

Abstract

Although the notion of recurrent trace-fossil assemblages through geologic time as a result of similar sets of environmental conditions has a long history in ichnology, the idea that recurrent ichnofaunas may have occurred in connection with macroevolutionary events has not yet been put forward. We refer to this phenomenon as “the Deja vu effect,” and suggest that understanding its signifi cance may shed light on how organisms colonize underutilized or empty ecospace, the establishment of the mixed layer, and the temporal and spatial distribution of matgrounds. These recurrent trace-fossil assemblages record the activity of the epifauna and a very shallow tier infauna, in the absence of mid- to deep-tier trace fossils and mottled textures. They typically consist of grazing trails, very shallow feeding burrows, and arthropod trackways. Shallow-tier burrows and trails with well-developed scratch marks may be present. These ichnofaunas suggest that the initial exploitation of underutilized or empty ecospace is linked to a set of recurrent behavioral strategies to obtain food from matgrounds and fi rmgrounds in the absence of a well-developed mixed layer. Preservation of these behaviors in the form of trace fossils is mediated by a set of taphonomic conditions. These taphonomic windows occur during the initial colonization of empty or underutilized ecospace, such as the Ediacaran-Cambrian transition or the subsequent colonization of the land, but also after the end-Permian mass extinction, which may have had a negative impact on the mixed layer in marine environments, producing a return to the ecologic conditions of the early Paleozoic.