Quantitative methods in ichnology: spatial distribution among populations



Although patterns of dispersion are a fundamental attribute of modern populations, few attempts have been made to evaluate such spatial patterns in the rock record, evidently because of taphonomic uncertainties concerning in situ preservations of fossils. Unlike body fossils, however, ichnofossils almost inevitably remain in situ, and in some instances are indicative of contemporaneous populations of burrowers. Studies of dispersion among such ichnofossils, by means of the coefficient of dispersion and distance to nearest neighbour techniques, may reveal not only the spacing but also the specific trophic adaptations - and even the general identity - of the tracemakers. In this study, a population of the vertical shaft Skolithos linearis from the Lower Cambrian Bradorc Formation of Labrador exhibits an aggregated or clumped dispersal as a whole; yet individuals within the population exhibit random dispersion at low population densities and uniform dispersion at high population densities. These collective patterns, by analogy with modern tracemakers, suggest suspension feeders, either lophophorate phoronids or tentacular crowned polychaetes, as the ancient tracemaker. In contrast, a S. linearis population from the Middle Silurian Thorold Formation of Ontario exhibits random dispersion at all densities observed, suggesting that the ancient tracemaker was a polychaete that utilized either mucous nets or low, narrow tentacular crowns in suspension feeding. Populations of the vertical U-shaped burrow Diplocraterion sp., from the Thorold Formation, exhibit uniform to random dispersion, suggesting that it represents a dwelling burrow of a suspension-feeding polychaete. The uniform pattern resulted from a competition between individuals for living space. □Trace fossils, spatial distribution, Skolithos, Diplocraterion, trophic analysis. Palaeozoic.