EDIACARAN FOSSIL MYTHS

Fossil assemblages of the Ediacara biota are widespread but still poorly understood, having been treated as curiosities rather than subjects for thorough paleobiological study. The description of new taxa alone will not clarify the affinities of the Ediacara biota. Interpretation of the impressed images of soft-bodied organisms requires attention to their sedimentary setting and taphonomy.

Studies of the best-preserved Ediacaran assemblages in South Australia, Newfoundland, Namibia and Russia are beginning to dispel many commonly held misconceptions.

The Ediacara biota is recorded in a least three environmental assemblages: “ash-fall”, “sand-cover” and “sand-smother”. Discoidal Ediacaran fossils are not stranded jellyfishes, but buried holdfasts of frond like organisms or clusters of anemones living between fair-weather and storm wave base. Many concentrically ornamented Ediacarian and Cambrian age discs are gas escape marks, load structures, or biological tool marks, formed by tethered tubular fossils. Three-dimensionally preserved Ediacarans represent the taphonomic bias of “sand-bag” burial of cup-shaped and frondose forms.

Dickinsonia and Kimberella are sometimes preserved in association with sets of resting traces and feeding traces, respectively. Trace fossils record simple feeding programs confined to mat surfaces on bed tops and the soles of thinly bedded sands. Trace fossil diversity is low; many forms are tubular body fossils. Many Ediacaran taxa are preserved in enormous numbers. The first assured trace fossil assemblages occur in the same facies as the first bilateral Ediacaran body fossils.

Although the biological affinities of many Ediacarans are still poorly understood, it is becoming clear that organisms of bilaterian-grade are present in Ediacaran assemblages 15 –20 million years before their oldest known skeletal descendants.