FEDONKIN, M.A.: Megascopic trace and body fossils of the oldest metazoans

Bilateral body fossils Dickinsonia, Ovatoscutum, Chondroplon, Yorgia and Andiva from the Vendian 555 Ma old deposits (White Sea) have the shield-like form, bipolarity, complete and homonomous metamerism combined with the symmetry of gliding reflection. Modes of deformation, growth pattern and the regeneration marks indicate that the fossils represent a convex, thin and flexible dorsal carapace composed of an organic and slowly degradable substance. Interpretation of the fossils as the triploblastic invertebrates is based on some features of their internal anatomy and their locomotion trails and tracks that are found in the direct association with the body fossils. Crawling has been realized by the peristaltic wave of the ventral surface over the sediment. Yorgia (up to 25 cm long) produced the trails and the chain of the resting tracks. Short impulses of locomotion alternated with the longer resting phases during which more mucus was produced and exact imprint of the ventral side was fixed in the sediment. Mollusk-like Kimberella produced the trails by the dorsal side of the shell while it was trying to escape the sand, and the fan-shape scratch marks while it was grazing (the scratches reflect the work of the proboscis with the bilateral hook-like organ on its end).