Abstract
Trace fossils of sediment bulldozers are documented from terminal Ediacaran strata of the Nama Group in Namibia, where they occur in the Spitskop Member of the Urusis Formation (Schwarzrand Subgroup). They consist of unilobate to bilobate horizontal to subhorizontal trace fossils describing scribbles, circles and, more rarely, open spirals and meanders, and displaying an internal structure indicative of active fill. Their presence suggests that exploitation of the shallow infaunal ecospace by relatively large bilaterians was already well underway at the dawn of the Phanerozoic. Efficient burrowing suggests coelom development most likely linked to metazoan body-size increase. These trace fossils are the earliest clear representatives so far recorded of sediment bulldozing, an activity that may have had a negative impact on suspension-feeding and/or osmotroph communities, as well as on matgrounds, representing early examples of ecosystem engineering and trophic-group amensalism. The occurrence of sediment bulldozers may have promoted the establishment of gradients in horizontal and vertical distribution of organic material in connection with spatially heterogeneous environments on the sea floor at a critical time in Earth evolution.
Highlights
Ediacaran benthic marine ecosystems were dominated by microbial mats that sealed the sediment from the water column essentially in the absence of bioturbation, and which have been referred to as matgrounds[1]
It is generally believed that these bulldozers are absent in Ediacaran and lowermost Cambrian rocks, represented by the Treptichnus pedum Zone, and that they first appear in the overlying Rusophycus avalonensis Zone, where they are accompanied by bilobed arthropod trace fossils that may have had a significant effect on incipient sediment-mixing
In this paper we document the presence of moderately abundant, almost 1 cm wide, trace fossils produced by sediment bulldozers in shallow-marine deposits of terminal Ediacaran age in the Spitskop Member of the Urusis Formation (Schwarzrand Subgroup, Nama Group) (Fig. 1) exposed in the Fish River Canyon region of southern Namibia (SI Appendix, Fig. S1)
Summary
There are no radiometric dates for the level of the basal Cambrian GSSP on Newfoundland, but it is currently considered to be 541 ± 1 Ma31. The bilaterian trace-fossil nature of these structures is indicated by: (1) heterogeneous (i.e. patchy) distribution on stratigraphic surfaces reflecting the ability to detect and imperfectly exploit an area enriched in organics; (2) active backfill and sediment displacement as illustrated by an envelope zone; (3) consistent width along the same toponomic level; (4) course characterised by scribbles, nested loops, pretzel-like forms and common self-overcrossing; (5) lack of preferential orientation (which would otherwise indicate current alignment of body fossils or tool marks made by entrained organisms); and (6) absence of frayed or angular terminations[51]. In any case, disregarding detailed morphological differences, affinities with these ichnotaxa are based on a similar way of exploiting the sediment and exploring substrate heterogeneities, clearly supporting a bilaterian origin and continuity of the “Psammichnites evolutionary lineage” across the Ediacaran-Cambrian transition[55]
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