Doushantuo Fauna

The Doushantuo phosphorites of southwestern China, constrained between 635 and 551 Ma (Condon et al, 2005) but probably 600-570 Ma in age (Barfod et al., 2002; Condon et al, 2005), are famous for phosphatized eggs and embryos (Fig. 2A) (Xiao et al., 1998; Xiao and Knoll, 2000). Other reported animal fossils include tiny adult sponges (Fig. 2C) (Li et al, 1998), stem cnidarians (Fig. 2D) (Xiao et al, 2000; Chen et al, 2002) and a tiny bilaterian (Fig. 2B) (Chen et al, 2004; Bottjer, 2005). These fossil occurrences are consistent with molecular clock analysis (Peterson et al, 2004; Peterson and Butterfield, 2005) that predicts the presence of cnidarians, sponges, and stem-group bilaterians. This assemblage of animal fossils was likely primarily microscopic—no macroscopic fauna has been found for this time in other rocks, although there may be an age overlap with the earliest Ediacara biota. Adult animals in this assemblage were at most several millimeters tall. Thus, there would not have been any tiering on the macroscopic scale as currently defined. However, the number of forms indicates that they utilized a variety of food sources, and likely lived in an ecosystem of some complexity. These tiny adult sponges and stem cnidarians may have lived partially inserted into the seafloor as mat stickers (Seilacher, 1999). In sufficient densities, these meadows of tiny sponges and stem cnidarians would have caused the microbial mat to assume a "fuzzy" appearance (Chen et al, 2002).

Embryon Doushantuo

Figure 2. Representative members of the Doushantuo fauna. (A) Phosphatized embryo. Scale 100 ^m. From Xiao and Knoll (2000). (B) Vernanimalcula, a putative adult bilaterian. Field of view approximately 120 ^m. From Chen et al. (2004). (C) Microscopic sponge, arrows denote spicules. Scale bar 100 ^m. From Li et al. (1998). (D) Sinocyclocyclicus, a putative adult stem-group cnidarian. Scale bar 140 ^m. From Xiao et al. (2000).

Figure 2. Representative members of the Doushantuo fauna. (A) Phosphatized embryo. Scale 100 ^m. From Xiao and Knoll (2000). (B) Vernanimalcula, a putative adult bilaterian. Field of view approximately 120 ^m. From Chen et al. (2004). (C) Microscopic sponge, arrows denote spicules. Scale bar 100 ^m. From Li et al. (1998). (D) Sinocyclocyclicus, a putative adult stem-group cnidarian. Scale bar 140 ^m. From Xiao et al. (2000).

Figure 3. Representative members of the Ediacara Avalon Assemblage. (A) Charnia wardi frond from the Drook Formation, Pigeon Cove. Coin diameter 24 mm. (B) "Spindle" rangeomorph form from the Mistaken Point Formation, Mistaken Point. (C) Bradgatia rangeomorph from the Mistaken Point Formation, Green Head, Spaniard's Bay. (D) Charniodiscus spinosus from the Mistaken Point Formation, Mistaken Point.

Figure 3. Representative members of the Ediacara Avalon Assemblage. (A) Charnia wardi frond from the Drook Formation, Pigeon Cove. Coin diameter 24 mm. (B) "Spindle" rangeomorph form from the Mistaken Point Formation, Mistaken Point. (C) Bradgatia rangeomorph from the Mistaken Point Formation, Green Head, Spaniard's Bay. (D) Charniodiscus spinosus from the Mistaken Point Formation, Mistaken Point.

4.2 Ediacara Avalon Assemblage (575-560 Mya)

The Avalon assemblage contains the oldest-known Ediacara fossils (indeed the oldest-known megascopic metazoans), is the only diverse deep-water locality, and is the only region where Ediacaran fossils are preserved beneath volcanic ash layers. The assemblage is numerically dominated by enigmatic fractally-organized organisms such as Charnia, Bradgatia, and "spindles" (Fig. 3), grouped into a biological clade called "Rangeomorpha" (Narbonne, 2004). The average numerical abundance of rangeomorphs within communities at Mistaken Point is 75%, with many communities containing greater than 90% rangeomorphs (Clapham et al., 2003). For example, rangeomorphs comprise 99.7% of the 1488 specimens found on the "D surface" at Mistaken Point (Clapham et al., 2003). The only important non-rangeomorph taxa in the Avalonian assemblage are the frond Charniodiscus (Fig. 3D) (Laflamme et al., 2004), the pustular, but possibly non-metazoan, discoidal fossil Ivesheadia (Boynton and Ford, 1995), and the rare conical Thectardis (Clapham et al., 2004). The preserved fossil communities contain between 3-12 forms (approximately equivalent to genera) (Clapham et al., 2003), although recent taxonomic studies indicate that the classic "E Surface" may contain as many as 15-18 genera (M. Laflamme, pers comm., 2005). Macroscopic bilaterian body fossils and trace fossils are absent from these oldest Ediacara assemblages, either because large bilaterians had not yet evolved or were restricted to shallow settings.

Because of the great morphological diversity displayed by rangeomorphs, ranging from recumbent sheets, to bush-like forms, to tall frondose shapes (Narbonne, 2004), Avalonian assemblages have a well developed tiering structure (Fig. 6A) (Clapham and Narbonne, 2002). As in highly tiered Phanerozoic assemblages, greater than 90% of the individuals occupy the lowest tier at less than 8 cm above the seafloor. The characteristic taxon of this tier at Mistaken Point is the "spindle" rangeomorph form (Fig. 3B), in conjunction with small specimens of the "pectinate" or "comb" rangeomorph form and Bradgatia (Fig. 3C), and very small fronds (e.g., Charniodiscus, Charnia, "duster" rangeomorphs). The intermediate tier (8-22 cm) is numerically dominated by the bush-like rangeomorph Bradgatia, the "pectinate" form, and small frondose specimens, whereas the upper tier (2235 cm) exclusively contains frondose forms such as Charnia and Charniodiscus (Fig. 6A). Rare taxa, such as Charnia wardi (Fig. 3A) and the "Xmas tree" fossil (Fig. 6A), demonstrate that the maximum height attained by these earliest Ediacara fossils (1 m or even greater) was similar to that of the tallest Phanerozoic marine invertebrates.

4.3 Ediacara White Sea Assemblage (560-550 Mya)

The White Sea Assemblage, represented by fossils from the White Sea in Russia and from the Flinders Ranges in South Australia, includes many of the archetypal Ediacara fossils (e.g., Dickinsonia; Fig. 4A). Radiometric dating of ash layers from Russia constrains the age of this assemblage to ca. 560-550 Ma (Martin et al, 2000), and sedimentological investigation has shown that the fossils lived in a variety of shallow marine environments, from relatively distal lower shoreface to onshore distributary mouth bars (Grazhdankin and Ivantsov, 1996; Gehling, 2000; Grazhdankin, 2004).

Some fossil localities display Nama-style preservation, but the dominant type of preservation is Flinders-style (Narbonne, 2005), preserved beneath storm deposits. The White Sea Assemblage represents the peak of Ediacara diversity, including some frondose forms known from the earlier Avalon Assemblage (Charniodiscus, Charnia), abundant bilaterian fossils [Dickinsonia, Kimberella (Fig. 4C), Yorgia, and other putative bilaterians such as Spriggina (Fig. 4B) and "vendomiids"], and enigmatic forms such as Parvancorina and Tribrachidium (Fig. 4D). In addition to bilaterian body fossils, trace fossils appear for the first time in the White Sea Assemblage. In contrast to the Avalon Assemblage, rangeomorphs are significantly less abundant in the White Sea Assemblage, represented only by rare specimens of Charnia and Rangea (Grazhdankin, 2004).

Charnia Wardi

Figure 4. Representative members of the Ediacara White Sea Assemblage. (A) Dickinsonia from Ediacara, South Australia. Length of largest specimen is 13 cm. Photo courtesy of B. Runnegar, from Bottjer (2002). (B) Spriggina from Ediacara, South Australia. Length of specimen is 4 cm. Photo courtesy of B. Runnegar, from Bottjer (2002). (C) Kimberella from the White Sea, Russia. Length of specimen is 8 cm. From Fedonkin and Waggoner (1997). (D) Tribrachidium from Ediacara, South Australia. Specimen is about 20 mm across. From Selden and Nudds (2004).

Figure 4. Representative members of the Ediacara White Sea Assemblage. (A) Dickinsonia from Ediacara, South Australia. Length of largest specimen is 13 cm. Photo courtesy of B. Runnegar, from Bottjer (2002). (B) Spriggina from Ediacara, South Australia. Length of specimen is 4 cm. Photo courtesy of B. Runnegar, from Bottjer (2002). (C) Kimberella from the White Sea, Russia. Length of specimen is 8 cm. From Fedonkin and Waggoner (1997). (D) Tribrachidium from Ediacara, South Australia. Specimen is about 20 mm across. From Selden and Nudds (2004).

Quantitative counts of bedding-plane assemblages reveal the presence of several different community types in the White Sea Assemblage from Russia (Grazhdankin and Ivantsov, 1996). Two small bedding planes are strongly dominated by discoidal holdfasts (Aspidella), comprising 92.5-100% of the specimens. Other taxa present include Dickinsonia, Parvancorina, and

Kimberella. Another assemblage contains only Dickinsonia (76%) and Tribrachidium (24%). The final assemblage studied by Grazhdankin and Ivantsov (1996) is a 6.6 m2 bedding plane containing a diverse community dominated by Kimberella (35.4%) and Tribrachidium (14.6%). Dickinsonia, Parvancorina, and Aspidella are also present but each comprise less than 6% of the assemblage. Presence-absence data collected by Grazhdankin (2004) indicates that the community diversity of White Sea fossil assemblages ranges from 1-9 forms, comparable to the diversity of Mistaken Point communities. Quantitative community paleoecology of the Australian White Sea assemblages also documents a wide range of paleocommunity types with taxonomic richness of 2-11 forms (Droser etal, 2006). Some bedding planes were extremely dominated by Aspidella frond holdfasts (99.3% of bed MM b1) whereas others contained abundant Dickinsonia (84.6% of bed CG db), Arkarua (76.2% of bed CH 29.14) or Palaeophragmodictya (90.2% of bed BathT3) (Droser et al., 2006).

The tiering structure of the White Sea assemblage appears to be less developed than that of Mistaken Point, although the taphonomic bias against frondose fossils obscures the exact tiering relationship (Droser et al, 2006). The abundant discoidal fossils present in certain White Sea communities likely represent holdfasts of frondose organisms (such as Charniodiscus) but, although there is a positive relationship between frond height and disc size, it is not possible to estimate the frond height from the size of the basal disc alone. Nevertheless, specimens of Charniodiscus arboreus from Australia have a frond size of approximately 30 cm (Glaessner and Wade, 1966), indicating that the upper tiers of Ediacaran communities were present in some assemblages. A qualitatitve tiering diorama, compiled from quantitative data in Grazhdankin and Ivantsov (1996) and Droser et al. (2006) and qualitative data in Grazhdankin (2004), is presented in Fig. 6B. Two broad tiers are present: a lower tier of benthic grazers and attached presumed suspension-feeders, and an upper tier of frondose taxa. The lower tier includes probable suspension feeders such as Tribrachidium and other taxa such as Parvancorina but is dominated by putative bilaterians such as Kimberella and Dickinsonia. Fronds present in the upper tier include Charnia and Charniodiscus. The relative abundance of organisms in these two tiers varies among communities, but the overall presence-absence data presented by Grazhdankin (2004) implies that upper tier frondose organisms are generally rare in relation to the lower tier bilaterians.

Quantitative data presented by Grazhdankin and Ivantsov (1996) and Droser et al. (2006) document an apparent exclusionary relationship between frondose taxa and bilaterians, with some communities strongly to exclusively dominated by discoidal holdfasts and others strongly to exclusively dominated by bilaterians and other non-frondose taxa. Some

Australian communities tend to have superdominant frondose forms (99.3% of bed MM b1) or putative grazers such as Dickinsonia (84.6% of bed CG db). However, Charniodiscus and Dickinsonia do co-occur in abundance in bed BuG A (comprising 52.5% and 26.2%, respectively). In Russia, communities that contain both frond holdfasts and bilaterians tend to be dominated by smaller diameter holdfast discs, implying smaller fronds (Grazhdankin and Ivantsov, 1996).

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