The Paleoecological and Environmental Significance of Trace Fossils

Trace fossils have great paleoecologic utility because they are (1) widespread in space and time, (2) found in place, and (3) largely the record of animal behavior and response, making them ideal indicators of environmental conditions. Traces may be used together with body fossils to increase knowledge of taxonomic richness in an ancient biotic assemblage. The morphology of traces may be used to reconstruct modes of feeding by many trace-producing organisms. The orientation of traces within sediments is sensitive to such depth-related variables as salinity, temperature, and food supply. Adaptation of many shallow-water benthos for vertical burrowing reflects the effectiveness of sedimentary cover as a temperature and salinity “buffer.” Gradients in dissolved oxygen may be detected by size gradients in burrow diameter. Low-oxygen marine environments are characterized by small-diameter horizontal feeding traces. Primary deformational features of traces may be used to reconstruct the original water content of the sea floor (bottom hardness). In addition, the presence or absence of shallow burrows, the preserved evidence of vertical movement reflected by specific types of traces, and gradients in intensity of bioturbation may be used to reconstruct relative and, in some cases, absolute rates of sedimentation or erosion. The greatest value of trace fossils in paleoecologic reconstruction is realized when they are used as independent evidence brought together with other sources of paleoecologic data. An interpretive, environmental model is proposed here in which information gained from the relative abundance of body and trace fossils is integrated; in this model, eight biolithofacies are recognized.