NEW PERSPECTIVES ON THE PROTEROZOIC TRACE FOSSIL RECORD

The early trace fossil record provides critical evidence of the timing of appearance of megascopic bilaterian animals. As such it also bears directly on the interpretation of the relatively rapid appearance of body fossils at the Precambrian-Cambrian boundary. In most cases the numerous trace fossils reported from strata preceding the later part of terminal Proterozoic can be immediately recognized as shrinkage cracks or some other inorganic structure. Other forms are problematic in terms of morphology or their toponomy being incompatible with a trace fossil interpretation. There appears to be a tendency to favor a biogenic origin in cases where no obvious explanation of inorganic origin is at hand. This approach is dubious as it makes the assumption that all inorganic sedimentary structures and their preservational variations are known, which is particularly unlikely for the Proterozoic. The correct identification of terminal Proterozoic trace fossils is also not unproblematic. Sedimentary structures of inorganic origin continue to be a source of confusion, and the matter is confounded by the appearance of diverse macroscopic organisms. Preservation in siliciclastic sediments of casts of parallel-sided or gently tapering tubular organisms, probably including metaphytes as well as metazoans, is virtually indistinguishable from simple horizontal trace fossils. Numerous examples of this error are available in the literature. Serial impressions such as Palaeopascichnus, long-ranging and widely distributed Ediacaran forms, traditionally interpreted as trace fossils are body fossils of unclear affinity. The geometry of the surface pattern of Harlaniella is not consistent with a spirally twisted trace fossil and a body fossil interpretation cannot be ruled out.