RETRODEFORMATION AND DISTRIBUTION ANALYSIS OF AN EDIACARAN FOSSIL FROM THE BOSTON BAY GROUP, EASTERN MASSACHUSETTS

Populations of numerous individuals of a ring shaped Ediacaran taxon occur as positive epireliefs on beds mapped as the Cambridge Argilllite of the Neoproterozoic Boston Bay Group. Fossils analyzed in this study occur on Grape and Slate Islands and in the Hewitts Cove area of the southern Boston basin. The same fossils also occur in more northerly outcrops of the Cambridge Argillite. Fossils occur as sediment rings 2-3 mm high in thinly laminated sandstones to virtually flat, dark pyritic rings in gray-black, very thinly laminated mudstones. Densities of clustered specimens range from 5-20 to 100-200 individuals per 400 square cm. Specimens are nearly circular to strongly elliptical. Long axes of elliptical specimens are generally co-linear and are closely aligned to a foliation. Thin section analysis of fabric of one outcrop reveals a mild, spaced, rough disjunctive foliation expressed as sub-parallel alignment of phyllosilicates and reorientation and solution elongation of quartz and plagioclase grains. The foliation dips steeply (80 to 85 degrees) and has a strike nearly parallel to bed dip direction. Thus the tectonic compression that created the foliation deformed the nearly circular fossils to ellipses. Earlier interpretations of ellipticity as a result of paleocurrent or soft sediment downslope deformation seem unlikely. Retrodeformation of digital images of bed surfaces and overlays removing tectonic distortion permitted a more accurate evaluation of size distributions of populations and contact relationships among individuals. Most populations of these Ediacaran ring fossils are unimodal with moderate standard deviations and likely represent a single cohort. A few bedding surfaces have marked bimodal or polymodal distributions and higher standard deviations and can be interpreted as multiple cohorts caused by repeated reproductive events. Despite having very high densities on a small bed surface area ring fossils almost never touch or overlap.