Abstract

STATISTICAL STUDIES OF THE DISTRIBUTION OF DIOPATRA CUPREA ACROSS MODERN TIDAL FLATS: IMPLICATIONS FOR THE AUTECOLOGY OF THE CAMBRIAN SKOLITHOS ORGANISM

SIMPSON, EDWARD L., Department of Physical Sciences, Kutztown University, Kutztown, PA 19530; VENN, CYNTHIA, Department of Geology, University of Pittsburgh, Pittsburgh, PA 15260; SKOOG, SONYA Y., Department of Physical Sciences, Kutztown University, Kutztown, PA 19530

Skolithos ichnofossils are the dominant sedimentary structure in Cambrian foreshore/shoreface deposits. Diopatra cuprea , a predaceous, tube-building polychaete, has been cited as a possible modern analog of the Skolithos organism. In this investigation we examine the spatial distribution of Diopatra across tidal flats, study their dispersion within a population and compare the results to Cambrian Skolithos ichnofossils.

A sampling network was constructed encompassing a tidal pool and channel on an ebb-tidal delta shoal (ETDS) in Chincoteague Inlet, VA, and traverses were surveyed from the high- to low-tide line in Toms Cove, VA. The number and position of worm tubes within a 1 x 1 meter grid were recorded for each station. Over all stations, the number of worms per grid varied from 0 to 83, the average being 9 worms. Plots of height above low tide-line and distance from the low-tide line versus number of worms demonstrate that fewer numbers of Diopatra are present in the higher intertidal reaches whereas lower intertidal areas have potentially greater numbers of organisms. Densities are at a maximum near the mean low-water line, and decrease into the subtidal zone. Results of the application of the Clark and Evans nearest neighbor technique to the worm distribution within each grid indicate that at high densities Diopatra are significantly clustered whereas at low densities their distribution is highly variable. R values (mean observed/mean expected for a random population) range from 0.20 to 1.73 for ETDS grids, with 69% less than 1; R values for transverses range from 0.31 to 1.67, with 72% less than 1. An R value of less than 1 indicates clustering. The coefficient of dispersion (5.1 to 24.6) indicates that the Diopatra population of each area is significantly aggregated.

In Cambrian-age, tidal-flat deposits in Virginia, the vertical change in Skolithos abundance parallels the change in numbers of Diopatra with height above low water on a modern tidal flat. Cambrian Skolithos shows a uniform distribution (R > 1), indicating a passive filter feeding habitat in contrast to the clustered distribution of Diopatra . Both Skolithos and Diopatra show significantly aggregated populations concentrated near the low-water line.

Kutztown University of Pennsylvania

A member of the State System of Higher Education

D.J. Moore
Oct. 2002
Last modified Sept. 2004
moore@kutztown.edu