MINUTE BURROW-LIKE FOSSILS FROM THE EDIACARAN DOUSHANTUO FORMATION, YANGTZE GORGES, CHINA

The appearance of the hydrostatic skeleton conferred new opportunities on animals. It permitted the anchoring of complex muscles that facilitated active movement, including burrowing. The first animal with a hydrostatic skeleton would likely have been soft-bodied and difficult to fossilize. Its existence might be inferred from any trace fossils produced by its activity. Chert in limestone of the Ediacaran Doushantuo Formation in the Yangtze Gorges, Hubei, China, contains solitary, slightly curved, unbranched cylindrical structures, 200 to 400 μm in diameter. Cylinders contain internal laminae of paraboloid-like shape; laminae are ~10 mm thick at the center (axis); they thin and merge as they near cylinder walls. Laminae do not touch cylinder walls; they are usually separated from the walls by a thin, clear boundary. Cylindrical structures occur with long axis parallel to bedding or at angles of up to 35^o to bedding. These structures resemble minute meniscus-backfilled burrows produced by peristaltic locomotion. Other possible interpretations include: fluid escape structures, microstromatolites, re-deposited microstromatolites, and multilaminate cyanobacterial sheaths. Because the structures cross-cut (disturb) the sediment and their internal laminae are cup-shaped (not cylindrical), merge near the cylinder's wall, and composed of fined-grained particulate material, cyanobacterial sheaths are ruled out. Their cross-cut nature also rules out re-deposited microstromatolites. Their sub-horizontal or slightly concave-downward orientation and the sediment disturbance preclude them from being stromatolites that grew in situ. The cylinders' orientation argues against a fluid-escape origin. The most likely interpretation is that these structures are trace fossils (burrows). However, the small size challenges the burrow interpretation. But, the sediment is so fine-grained (~1 μm) that the size requirements for burrowing can be scaled to allow for minute size. Such a burrow could only have been made by an animal possessing a hydrostatic skeleton. Thus, this structure was likely produced by a coelomate or pseudocoelomate, although non-coelomates cannot be ruled out. These fossils represent the oldest evidence for the use of a hydrostatic skeleton by an animal.