81-5 Ediacara-type Macrofossils in the Prelude to the Gaskiers Glaciation (580 Ma) in Newfoundland, Canada

Abstract:

The Ediacara biota records the appearance and diversification of Earth’s first large, soft-bodied eukaryotes, including interpreted early animals, yet their evolutionary origins remain unresolved. Previously, the oldest-known large (centimeter- to meter-scale) Ediacara-type macrofossils known anywhere were <571 Ma leaf-shaped fronds from the upper Drook Formation at Mistaken Point in the Avalon Peninsula of Newfoundland. The Drook fossils are large, diverse, and architecturally complex, implying that they may have had macroscopic precursors, a view consistent with molecular clock estimates of a Tonian or Cryogenian origin of animals. Here we describe macrofossil remains from deep-water strata of the Mall Bay Formation at two sites on the Colinet Islands offshore of the Avalon Peninsula of Newfoundland. These remains, radioisotopically dated at 580 Ma, occur in the cold-water prelude to the overlying Gaskiers diamictite. Both new fossil sites are more than a kilometer stratigraphically lower and probably 8-10 million years older than the oldest previously described Ediacara-type macrofossils from Mistaken Point or anywhere else worldwide. The Colinet Islands structures include simple cm-scale discoidal remains similar to the Ediacaran form-genus Aspidella, and the newly named bipartite fossil Colinetia which comprises a centimeter-scale disc attached to an unbranched, clay-lined, decimeter- to meter-long, cylindrical tube that is preserved horizontally at the top of the bed. The tube contains an inner tube (tube-in-tube morphology) and maintains a consistent diameter along its length and among all specimens in the assemblage. Colinetia specimens exhibit features consistent with a biological reconstruction as open cylindrical tubes that were at least partly infaunal, with post-mortem fill by sediment injection guided by the horizontally oriented tubes. They range up to 2 m in length and thus represent the first abundant occurrence of large lifeforms in Ediacaran evolution. Morphologically, they strongly resemble the basal parts of younger Ediacaran fronds and tubular organisms, suggesting a possible evolutionary affinity, but they lacked the morphologic complexity of many younger Ediacaran taxa. Discovery of two mid-Ediacaran fossil sites in a deep-water glaciomarine environment near the beginning of large size and biological complexity supports the hypothesis that the Ediacara biota first emerged under cold conditions, and adds weight to the hypothesis that increased oceanic ventilation during glacial onset ~580 Ma may have played a role in the origin of the Ediacara biota.

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