ICHNODIVERSITY VS. SPECIES DIVERSITY: VARIABILITY IN TRACE FOSSILS OF THE PENNINGTON FORMATION (LOWER CARBONIFEROUS), NORTHWEST GEORGIA

In some instances, observed ichnodiversity correlates with species diversity in modern environments, and thus can reflect overall biodiversity of organisms recorded in a sedimentary rock. Trace fossils from the Pennington Formation (Lower Carboniferous) in northwest Georgia have a relatively high ichnodiversity, consisting of Lockeia, Hillichnus, and Protovirgularia; these trace fossils, however, were probably made by behaviors from a single species of bivalve in different growth stages in delta-front subtidal muds. Our interpretation is based on differently sized but morphologically similar burrows and resting traces. To test similarities between trace fossils on different samples from the Pennington, we conducted ANOVA on burrow size measurements taken from different rock samples. Each sample had statistically significant differences in sizes, indicating high levels of variability in burrow sizes. This conclusion is based on 725 measurements taken of positive-relief hypichnia on five slabs of silty sandstone, each of varying size; 138 horizontal burrows and 9 resting traces were observed on these slabs. Multiple width measurements were taken along the lengths of burrows, which were used in the ANOVA test with a 95% confidence interval. These differences in shape, size, and pattern of trace fossils can be attributed to a range of tracemaker sizes, as well as variations in behaviors and substrates. The overall effect was to produce ichnodiverse beds, but made by only one species.