Elongate surface trails that abound in late Ediacaran and early Paleozoic sandstone and siltstone are often attributed to early animals. These trails commonly exist on the same beds as wrinkle structures (millimeter- to centimeter-size ridges and pits that are interpreted as evidence of the former presence of microbial mats). Here we show that interactions between oscillatory flow and centimetric microbial aggregates produce elongate trails on the surface of a sediment bed. Trails left by moving microbial aggregates share a number of characteristics with some presumed trace fossils of the earliest animals: elevated edges, zig-zag patterns, smooth curves, reversals, intersections with other trails, series of pits, and paths that terminate abruptly and restart nearby. Under the same flow conditions, millimetric microbial aggregates generate wrinkle structures. Thus, the interaction between flow and microbial aggregates on a sediment bed can produce a number of structures that are commonly interpreted as evidence of early animal locomotion.

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