Population structure of the oldest known macroscopic communities from Mistaken Point, Newfoundland

Abstract

The presumed affinities of the Terminal Neoproterozoic Ediacara biota have been much debated. However, even in the absence of concrete evidence for phylogenetic affinity, numerical paleoecological approaches can be effectively used to make inferences about organismal biology, the nature of biotic interactions, and life history. Here, we examine the population structure of three Ediacaran rangeomorph taxa (Fractofusus, Beothukis, and Pectinifrons), and one non-rangeomorph taxon (Thectardis) across five fossil surfaces around the Avalon Peninsula, Newfoundland, through analysis of size-frequency distributions using Bayesian Information Criterion (BIC). Best-supported models resolve communities of all studied Ediacaran taxa at Mistaken Point as single cohorts with wide variance. This result is best explained in terms of a “continuous reproduction” model, whereby Ediacaran organisms reproduce aseasonally, so that multiple size modes are absent from preserved communities. Modern benthic invertebrates (both as a whole and within specific taxonomic groups) in deeper-water settings reproduce both seasonally and aseasonally; distinguishing between biological (i.e., continuous reproductive strategies) and environmental (lack of a seasonal trigger) causes for this pattern is therefore difficult. However, we hypothesize that the observed population structure could reflect the lack of a trigger for reproduction in deepwater settings (i.e., seasonal flux of organic matter), until the explosive appearance of mesozooplankton near the base of the Cambrian.