Morphological, ethological, and environmental significance of the ichnogenera Scoyenia and Ancorichnus

Abstract

The monospecific ichnogenus Scoyenia White is characterized by longitudinally striat- ed wall linings surrounding meniscate burrow fills. Ancorichnus Heinberg is similar but has an unornamented exterior. Ichnospecies of Ancorichnus are based upon fabricational differences in menisci; those of A. coronus (n. ichnosp.) are continuous with the wall lining, whereas those of A. ancorichnus Heinberg are not. Scoyenia and Ancorichnus are interpreted as feeding structures, although certain details of the feeding-burrowing mechanism remain conjectural. The Scoyenia organism is thought to be an arthropod (other than insects or decapods); Ancorichnus ancorichnus may have been produced by priapulids, sipunculids, or ethologically equivalent worms, whereas A. coronus seems to have been made by arthropods somewhat akin to the Scoyenia animal. These last two forms also may have more or less similar environmental significance. By definition, Scoyenia gracilis is diagnostic of the Scoyenia ichnofacies. However, various different interpretations or applications of this ichnofacies concept have rendered it virtually mean- ingless beyond a general nonmarine connotation. Recent work suggests that 1) nonmarine ichno- facies are equally as numerous and different as marine ichnofacies, 2) the Scoyenia ichnofacies is only one among many nonmarine assemblages, and yet 3) even the Scoyenia ichnofacies remains largely undefined and poorly understood. As a test of the distinctiveness and validity of the concept, we suggest that future designations of this ichnofacies be restricted to trace fossil assemblages in which Scoyenia gracilis, Ancorichnus coronus, or ethologically and ecologically equivalent burrows predominate. Known occurrences of such trace fossils indicate that the tracemakers preferred moist or wet nonmarine substrates, whether shallow aquatic deposits periodically exposed to air or low- lying subaerial deposits periodically inundated by water.