MOBILE ORGANISMS IN AN ORGANIC MAT DOMINATED EDIACARAN WORLD

The Ediacara Biota contains the oldest fossil evidence for diverse animals capable of movement in an environment covered by an organic substrate. The trace fossil Helminthoidichnites represents evidence of small mobile Ediacaran bilaterians. Scratch traces associated with the proboscis bearing Kimberella indicate that it moved over and fed on the organic mat. While these well-documented examples reveal that members of the Ediacara Biota were mobile, the Ediacara Member of South Australia contains at least four other taxa with characters indicative of movement.

Dickinsonia and Yorgia are interpreted as mobile based on associations with similarly sized depressions, or footprints, left in the organic mat. The rarity and left skewed size distribution of footprints suggests that they were ephemeral structures overprinted quickly by mat regrowth, indicating that movement occurred on short, ecological timescales. Footprint trackways of both Dickinsonia and Yorgia span more than 2 meters and represent mobility over large areas of the Ediacaran seafloor. The slightly larger size of footprints relative to rare associated body fossils and detailed replication of morphology in footprints supports formation as a result of active feeding on the mat by Dickinsonia and Yorgia. Specimens of Yorgia are significantly smaller than associated body fossils and are commonly irregular in shape, demonstrating a more pliable body construction compared with that of Dickinsonia. Wrinkle marks and the direction of displacement imply movement via peristalsis.

Two other taxa have morphological features indicative of body manipulation related to movement. Specimens of Andiva are rarely lifted off the Ediacaran seafloor, similar to that observed in Dickinsonia, suggesting that it was a free-living organism. Andiva shares a similar morphology to that of Dickinsonia and Yorgia and specimens are commonly asymmetrical about the midline indicating possible movement by pedal locomotion. Spriggina commonly exhibits a curled or bent morphology even in the most immaculately preserved specimens. A lack of consistent orientation demonstrates that this is not a taphonomic effect but evidence of body manipulation suggesting that Spriggina was capable of mobility.