Ediacara Nama Assemblage

The youngest Ediacara fossils are known from Namibia, where they range through the last 7 million years of the Ediacaran to just below the base of the Cambrian. The Nama Assemblage occurs primarily in very shallow marine delta plain to distributary mouth bar environments (Saylor et al, 1995; Grazhdankin and Seilacher, 2002; Grazhdankin, 2004) and is preserved, often transported, as three-dimensional casts within event beds (typical "Nama-style" preservation; Narbonne, 2005). Compared to the earlier Avalon and White Sea Assemblages, the Nama Assemblage in its type area in Namibia has low diversity, represented primarily by 4 genera, Pteridinium (Fig. 5A), Rangea, Swartpuntia (Fig. 5B), and Ernietta. Examples of Nama-type fossil assemblages found in Russia, preserved in distributary mouth-bar sediments in the middle Verkhova Formation and lower Yorga Formation (Grazhdankin, 2004), are slightly more diverse, including the probable anemone-like Nemiana in addition to unusual forms such as Ausia and Ventogyrus. Although trace fossils are present, bilaterian body fossils are absent. The major evolutionary innovation found in the Nama Assemblage is the earliest calcified fossils, including Cloudina and Namacalathus (Fig. 5C).

The transported nature of Nama Assemblage fossil occurrences hinders ecological studies; however, in Namibia most fossil localities contain monospecific or nearly monospecific occurrences, for example, of Pteridinium (Grazhdankin and Seilacher, 2002) or Swartpuntia (Narbonne et al., 1997). In contrast, Russian localities contain 4-6 taxa in typical Nama Assemblage occurrences (Grazhdankin, 2004). The tiering diorama presented in Fig. 6C is based on the qualitative data presented by Grazhdankin (2004) and shows a prominent upper tier of fronds such as Rangea and Swartpuntia with a lower tier possibly containing Nemiana (if it represents an anemone-like organism, not a holdfast like Aspidella). Based on presence-absence data, frondose fossils dominate the Nama Assemblage, implying a similar two-level tiering structure to the White Sea but with a dense upper tier of fronds and only a few species present near the sediment-water interface.

Namacalathus Fossil
Figure 5. Representative members of the Ediacara Nama Assemblage. (A) Pteridinium from Namibia. Coin is 22.5 mm diameter. (B) Swartpuntia frond from Namibia. Scale bar 2 cm. From Narbonne etal. (1997). (C) Namacalathus, one of the earliest known skeletal fossils. From Grotzinger et al. (2000).
Namacalathus

Figure 6. Tiering dioramas for the Avalon, White Sea, and Nama Assemblages. The dioramas are based on quantitative data from Clapham and Narbonne (2002) for the Avalon assemblage and presence-absence data from Grazhdankin (2004) for the White Sea and Nama assemblages but are composites of a number of individual communities and do not intend to show the proportion of organisms in each tier. (A) Avalon Assemblage. Fossils depicted: a. "Spindle" rangeomorph, b. "ostrich feather" rangeomorph, c. Charniodiscus, d. "feather duster" rangeomorph, e. Charnia rangeomorph, f. Bradgatia rangeomorph, g. "pectinate" rangeomorph, h. Thectardis, i. "xmas tree." After Clapham and Narbonne (2002). (B) White Sea Assemblage. Fossils depicted: a. Discoidal fossil (probable holdfast), b. Parvancorina, c. Tribrachidium, d. Dickinsonia, e. Yorgia with grazing impression, f. Kimberella with Radulichnus trace, g. Charniodiscus, h. Vendomiid (e.g., Vendia), i. Eoporpita. (C) Nama Assemblage. Fossils depicted: a. Rangea, b. Nemiana, c. Swartpuntia.

Figure 6. Tiering dioramas for the Avalon, White Sea, and Nama Assemblages. The dioramas are based on quantitative data from Clapham and Narbonne (2002) for the Avalon assemblage and presence-absence data from Grazhdankin (2004) for the White Sea and Nama assemblages but are composites of a number of individual communities and do not intend to show the proportion of organisms in each tier. (A) Avalon Assemblage. Fossils depicted: a. "Spindle" rangeomorph, b. "ostrich feather" rangeomorph, c. Charniodiscus, d. "feather duster" rangeomorph, e. Charnia rangeomorph, f. Bradgatia rangeomorph, g. "pectinate" rangeomorph, h. Thectardis, i. "xmas tree." After Clapham and Narbonne (2002). (B) White Sea Assemblage. Fossils depicted: a. Discoidal fossil (probable holdfast), b. Parvancorina, c. Tribrachidium, d. Dickinsonia, e. Yorgia with grazing impression, f. Kimberella with Radulichnus trace, g. Charniodiscus, h. Vendomiid (e.g., Vendia), i. Eoporpita. (C) Nama Assemblage. Fossils depicted: a. Rangea, b. Nemiana, c. Swartpuntia.

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