81-6 New Ediacaran Transitional Fossil from Newfoundland Bridges Tubular and Frondose Body Plans

Abstract:

The Ediacara biota is the morphologically diverse product of early experiments with complex, large, multicellular eukaryotic lifeforms across a wide range of paleoenvironments over tens of millions of years. The oldest dated Ediacara biota in the fossil record are preserved in deep-water deposits in Newfoundland, Canada. However, the Ediacaran fossil record in Newfoundland shows a 1,000-meter stratigraphic and evolutionary gap between the pre-Gaskiers 580 Ma decimeter-to-meter-scale macrofossil Colinetia (i.e., a disc with an attached tube-in-tube structure) found in the Mall Bay Formation, and the post-Gaskiers <571 Ma more complex, centimeter- to meter-scale leaf-shaped fronds and other soft-bodied taxa preserved in the upper Drook Formation. The Mall Bay Formation fossils are large relative to the preceding record, but their simple morphology hinders interpretation of the evolutionary relationship (or lack thereof) between Colinetia and frondose fossils from the upper Drook Formation of the Avalon biota or tubular fossils from the Ediacaran White Sea biota. Thus, this gap in the fossil record poses questions about how biological complexity arose in the Ediacara biota. Our team recently discovered a surface containing more than 250 in situ specimens of a newly identified taxon that fills a stratigraphic, temporal, and morphological gap between Colinetia and Ediacaran fronds. Most specimens of the new taxon are centimeter-scale discs with eight lobes surrounding a central lobe. Several specimens display the same lobate disc attached to a conical extension composed of parallel unbranched tubular elements that are reconstructed as twisting clockwise into a cone. The tubular elements have millimeter-scale orthogonal architecture similar to the branching architecture of younger Ediacaran fronds. Each tube extends from a lobe in a communal discoidal holdfast. The twisting tubes may have helped the conical structure to maintain an upright position on the seafloor. The new taxon might be a colonial descendant of Colinetia, with its modules, orthogonal structure, and distribution in double-Thomas clusters potentially hinting at Ediacaran frond construction, architecture, and ecology. New U-Pb dates place the new taxon between 580 Ma and 571 Ma, supporting its intermediate position between Colinetia and later Ediacaran fronds. We interpret the new fossil taxon as representing the beginnings of biological complexity in the Newfoundland rock record and worldwide.

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