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A uranium-lead zircon age for a volcanic ash interstratified with fossil-bearing, shallow marine siliciclastic rocks in the Zimnie Gory section of the White Sea region indicates that a diverse assemblage of body and trace fossils occurred before 555.3 +/- 0.3 million years ago. This age is a minimum for the oldest well-documented triploblastic bilaterian Kimberella. It also makes co-occurring trace fossils the oldest that are reliably dated. This determination of age implies that there is no simple relation between Ediacaran diversity and the carbon isotopic composition of Neoproterozoic seawater.
The terminal Neoproterozoic interval is characterized by a period of supercontinent amalgamation and dispersal (1, 2), low-latitude glaciations (3, 4), chemical perturbations of seawater (5-7), and the first appearance and subsequent diversification of metazoans. Construction of a terminal Neoproterozoic biostratigraphy has been hampered by preservational, paleoenvironmental, and biogeographic factors (5, 8). Global biostratigraphic correlations within the Neoproterozoic are tenuous and rely on sparse numerical chronology, inferred stratigraphic diversity trends, and chemostratigraphy (8-13). As a result, the position of many late Neoproterozoic soft-bodied organisms in metazoan phylogeny remains controversial. Fundamental to a solution of this problem is an understanding of the temporal relationship between fossil occurrences with Ediacaran-type preservation and other body fossils, small shelly fossils, and trace fossils of burrowing organisms.
Although Ediacaran biotas are globally distributed, their temporal distribution is poorly known. Simple annuli and discs preserved below a glacial horizon in the Mackenzie Mountains of northwest Canada have been interpreted as possible metazoans and have been considered the oldest Ediacaran assemblage (14). Although their age is not constrained, they are commonly described as dating to 600 to 610 million years ago (Ma) (Fig. 1). The best-known diverse Ediacaran assemblages postdate the Varanger-Marinoan glaciation (5, 8). At Mistaken Point in eastern Newfoundland, fossils of soft-bodied organisms are dated at 565 +/- 3 Ma (15). In the Ukraine, fossiliferous Upper Vendian strata of the Redkino Stage overlie volcanic rocks in Poland that are dated at 551 +/- 4 Ma (16, 17). Ediacaran assemblages in the Nama Group in Namibia range from 548.8 +/- 1 to 543.3 +/- 1 Ma (8). Ediacaran-type fossils have been found together with Treptichnus pedum in what is described as Lower Cambrian sandstone of the Uratanna Formation in the Flinders Ranges,...