Crowded Rosselia ichnofabric in the Early Devonian of Brazil: An example of strategic behavior

The occurrence of crowded Rosselia ichnofabric in Early Devonian deposits of ‘Transition Beds’ (Western Gondwana Paraná Basin, southern Brazil) and the paleobiology and paleoecology of its potential tracemakers are discussed in this paper. Crowded Rosselia ichnofabric has been recorded chiefly in Cenozoic storm beds associated with high sedimentation rates on the inner shelf, and is usually interpreted as resulting from opportunistic behavior of terebellid polychaetes. Rosselia is a common ichnotaxon in the archetypical Cruziana ichnofacies, developed preferentially in fair-weather deposits formed in inner shelf settings. During storm surges, however, this endobenthic community is strongly affected, decimating those that are unable to escape. New records presented herein reinforce the conclusion that short-term colonization represented by crowded Rosselia ichnofabric occurs essentially in erosive storm deposits of high sedimentation rates. Rather than being opportunistic, the crowded Rosselia ichnofabric appears to be the result of a strategic behavior developed by stress-tolerant terebellid polychaetes in inner shelf zones during or just after storm surges. This strategic behavior appeared during the early Paleozoic and was adopted by terebellids from their earliest appearance.