Ediacaran Biota

Abstract

The Ediacaran biota (635–541Ma) is famous for posing a series of challenging paleontological problems. Foremost among these has been difficulty in discerning the phylogenetic relationships, if any, between Ediacarans and more familiar types of animals. The difficulty of assigning Ediacarans to animal higher taxa has resulted in the creation of informal categories for the various types of Ediacaran macrofossils. These categories include: rangeomorphs, spongiomorphs, ivesheadiomorphs, arboreomorphs, kimberellomorphs, dickinsoniomorphs, gehlingiomorphs, bilateralomorphs, cloudinids, triradialomorphs, tetraradialomorphs, and pentaradialomorphs. After the affinity problem, next in importance is the taphonomic problem, namely, how these large soft-bodied organisms came to be preserved in abrasive arenites. Sandstone is generally considered to be too coarse a sediment type to preserve soft-bodied fossils, as interstitial water flow through the sand would ordinarily lead to rapid decomposition of soft tissue. Two main hypotheses have been presented in attempts to resolve the taphonomic issues. The “Death Mask” hypothesis is one attempt at explanation. In this view, dead Ediacarans were rapidly overtopped by a fast-growing biofilm or microbial mat, which then generated a thin layer of carbonate biomineralization that hardened the upper surface of the macrofossil, allowing it to preserve in sandstone. Finally, the fossils themselves are ambiguous regarding their trophic strategies, rendering the Ediacaran mode of nutrition cryptic in many cases. In the “Garden of Ediacara” model, Ediacarans relied on osmotrophy, chemautotrophy and photosymbiosis in preference to strategies that became dominant later in the Phanerozoic, namely, filter feeding, deposit feeding, scavenging, and predation. New descriptions of Ediacarans showing convincing similarities to members of known animal phyla (Korifogrammia, Archaeaspinus, Palankiras, Clementechiton, Yilingia and others) are beginning to resolve the affinity problem in favor of an animalian interpretation for many if not all Ediacaran megafossils.