Variation of population and community ecology over large spatial scales in Ediacaran early animal communities

Abstract

The Ediacaran strata of Newfoundland, Canada (580–560 Ma) record some of the first animal communities. The in-situ preservation of these sessile organisms means that the positions and sizes of specimens on the bedding planes encapsulate their life-histories, enabling spatial analyses to reconstruct their ecological dynamics. However, it is not known how these Ediacaran ecological dynamics vary across large spatial scales. Fortunately, the E and G surfaces at Mistaken Point, Newfoundland (∼565 Ma) crop out at multiple locations, providing the opportunity to compare ecological dynamics between communities separated by large spatial scales (∼800 m). In this study, we collected data from two outcrops each of E and G surfaces using a combination of laser-line probe, LiDAR and photogrammetry. We mapped out Mistaken Point G surface over 7.22 m², finding 9 frondose taxa across 158 specimens Watern Cove East G surface across 154 m² (93 m² when fractures are excluded) finding 19 frondose taxa across 1320 specimens; and Watern Cove West E Surface across 20.02 m², finding 11 frondose taxa across 734 specimens. We compared the Watern Cove West E surface with previously collected data from Mistaken Point E Surface, which had 2977 specimens over 85.42 m². The two G outcrops exhibited remarkably similar community compositions, both dominated by Bradgatia with high proportions of frondose rangeomorphs and arboreomorphs. In contrast, the compositions between the E surfaces were notably different, with Watern Cove West E surface showing relatively higher proportions of Fractofusus and Bradgatia and lower proportions of frondose taxa. For comparisons of population ecology between the outcrops, only Bradgatia (G) and Fractofusus (E) populations occurred in sufficient numbers to enable spatial analyses. We quantified the spatial distributions using spatial point process analyses, finding for E surfaces that the Fractofusus populations showed significantly similar spatial patterns, which indicated reproductive events. In contrast, on G surface, the Bradgatia populations show remarkably different underlying processes, with the Watern Cove East population showing reproductive clusters on a background environmental heterogeneity, whereas the Mistaken Point population shows spatial segregation, indicating intra-specific competition. The Watern Cove East Bradgatia have a much lower density than those on Mistaken Point, suggesting that the increased density leads to competition due to insufficient resources to maintain this higher density. In modern deep-sea benthic communities, increased population densities occur within communities when different areas are subject to different flow regimes, and so different nutrient flux. Our results suggest that the Ediacaran communities of Mistaken Point show variability in ecological dynamics, even while composition of these communities remain remarkably similar.