Synopsis Of Proterozoic Macroalgal Fossils

Most Proterozoic macroalgae are preserved as carbonaceous compressions. Relatively few macroalgae are preserved in the permineralization windows (i.e., silicification and phosphatization), which are widely open for Proterozoic microorganisms (Schopf, 1968; Knoll, 1985); it is worth mentioning in passing that the contrast between the compression and permineralization windows may represent some major taphonomic biases or environmental heterogeneity. Recently, it has been recognized that some Ediacaran macroalgae may have been preserved as casts and molds, in a way similar to the preservation of classical Ediacara fossils (Droser et al, 2004), but the diversity of these macroalgal fossils awaits systematic documentation.

Hofmann (1994) compiled a comprehensive database of Proterozoic carbonaceous compressions and he classified them into thirteen formally defined families. Several new reports of Proterozoic carbonaceous fossils have been published since 1994 (Chen et al., 1994a; Chen et al., 1994b; Steiner, 1994; Ding et al, 1996; Gnilovskaya et al, 2000; Xiao et al, 2002); however, most of these new fossils can be classified into one of the thirteen families. Because these families were defined on morphological basis, it is likely that some of these families may be polyphyletic. However, as long as we can ascertain that these families represent macroalgae, these morphologically defined families may be to some degree analogous to macroalgal functional-form groups (Littler and Littler, 1980), and they should have ecological if not phylogenetic significance. Four of the thirteen families were considered as likely (Saarinidae and Sabelliditidae) or possible (Sinosabelliditidae and Protoarenicolidae) metazoans, and their nomenclature followed the ICZN rules (Hofmann, 1994). These family names are preserved here for convenience, even though we believe that the sinosabelliditids and protoarenicolids are probably macroalgae. Below we briefly consider the algal affinity of these groups.

Chuariaceae: This group includes the circular compressions Chuaria (millimetric diameters; Fig. 1A) and Beltanelliformis (centimetric diameters). Both often have concentric wrinkles and sometimes simple splits (Butterfield et al., 1994; Steiner, 1997; Xiao et al., 2002), indicating that in life they were spherical vesicles. Three-dimensionally preserved casts and molds confirm their spherical morphology (Hofmann, 1985; Narbonne and Hofmann, 1987; Yuan et al., 2001). Thus, both genera can be reconstructed as spherical fluid-filled vesicles with a flexible organic wall. This morphological reconstruction is inconsistent with an affinity with cyanobacterial colonies such as Nostoc balls (Sun, 1987; Steiner, 1997), where filaments are held in a mucilaginous matrix (Graham and Wilcox, 2000). More likely, both Chuaria and Beltanelliformis are structurally similar to acritarchs with a coherent and resistant organic wall (Ford and Breed, 1973; Vidal and Ford, 1985; Butterfield et al., 1994). In fact, some Chuaria-like compressions have been interpreted as benthic organic vesicles (Butterfield et al., 1994; Butterfield, 1997, 2001), or as planktonic propagules of Tawuia-like thalli that are considered as benthic chlorophytes or xanthophytes (Kumar, 2001). Likewise, Beltanelliformis has been compared to spherical gametophytes of the benthic coenocytic green alga Derbesia (Xiao et al., 2002). Parachuaria simplicis, another Chuaria-like fossil, has a millimetric circular compression with a subtending filament (Yan et al., 1992; Tang et al., 1997), which may well represent a stipe-like structure that tethered the spherical vesicle to a benthic substrate, in a way similar to Longfengshania (Hofmann, 1985; Du and Tian, 1986). Thus, Chuaria and Beltanelliformis are best considered as benthic or having a benthic stage in their life cycle. It is also probable that they may have been photosynthetic eukaryotes, given that their spherical vesicles have morphological analogues among modern coenocytic algae (e.g., Derbesia and Valonia), but not among animals or fungi. Thus we tentatively regard chuariaceans as macroalgae. It should be noted, however, that the major patterns of macroalgal morphological history would probably stay even if we had removed chuariaceans from our analysis, because chuariaceans are ubiquitous throughout the entire Proterozoic.

Tawuiaceae: Tawuia, the eponymous genus of this group, is reconstructed as a tubular structure with closed and round termini (Hofmann and Aitken, 1979; Hofmann, 1985). Like Chuaria, it can be preserved as two-dimensional compressions or three-dimensional molds (Hofmann and Aitken, 1979; Hofmann, 1985). Because all reported populations co-occur with Chuaria, Tawuia is generally considered ontogenetically or phylogenetically related to Chuaria (Duan, 1982; Hofmann, 1985). Recently, Kumar (2001) reported a population of carbonaceous compressions from the Suket Shale of the lower Vindhyan Supergroup in the Rampura-Chittorgarh area, central India. The Suket population, probably between 1600 and 1140 Ma (Kumar, 2001; Rasmussen et al., 2002; Ray et al., 2002; Ray et al., 2003; Sarangi et al., 2004), includes Chuaria- and Tawuia-like fossils. The termini of several Suket Tawuia-like specimens bear circular (Chuaria-like) or trapezoidal structures, which Kumar interpreted as compressed spherical cysts and holdfasts, respectively. Kumar gave different taxonomic names to the different parts of the same specimen; the trapezoidal holdfast was described as Tilsoia or Suketea depending on how it is preserved, the cylindrical stem as Tawuia, the spherical cyst as Chuaria, and the complete organism was named Radhakrishnania. While the identification of the Suket tubular fossils as Tawuia dalensis is debatable and the taxonomic practice of Kumar is undesirable, the Suket population does provide a general model by which Chuaria and Tawuia may be ontogenetically related. This model implies 1) Tawuia represents only the benthic stage of a biphasic alga and 2) Chuaria and Tawuia should have similar geographic, environmental, and stratigraphic distribution. However, these implications are difficult to test, because Chuaria is almost certainly a polyphyletic taxon and also because planktonic cysts (i.e., Chuaria) can be preserved beyond the geographic and environmental distribution of their benthic vegetative parents (i.e., Tawuia). Given that Tawuia populations from the type locality (Hofmann and Aitken, 1979; Hofmann, 1985) and elsewhere (Zhang et al., 1991) also contain individuals, including some U-shaped individuals, with a terminal disk at one end, it is probable that Tawuia and Chuaria may indeed be organ taxa of the same organism. Other Tawuia-like fossils, for example Bipatinella (Fig. 1B) from the early Neoproterozoic Liulaobei Formation and Shijia Formation in northern Anhui of North China (Zheng et al., 1994) also appear to have terminal swellings. If Tawuia and Chuaria are indeed organ taxa of the same organism, the combination of characters (a planktonic stage and a benthic stage with holdfast) is most consistent with a macroalgal interpretation for Tawuia. Thus, in our compilation, we follow the traditional view that Tawuia represents a benthic, tubular macroalga.

Ellipsophysaceae: Ellipsophysa (Fig. 1D) and related genera from the Liulaobei, Jiuliqiao, Xiamaling, and Changlongshan formations in North China, are elliptical to oval compressions with a maximum/minimum axis ratio between 1.4 and 2 (Zheng, 1980; Du and Tian, 1986). It is uncertain whether these compression fossils should be classified in the Chuariaceae or in a separate family. Nonetheless, their elliptical/oval morphology is intermediate between Chuaria and Tawuia, and by analogy they may also be interpreted as macroalgae.

Longfengshaniaceae: Longfengshania (Fig. 1C) and Paralongfengshania can be reconstructed as algal thalli with an ellipsoidal, ovoidal, or panduroidal vesicle and a subtending stipe (Hofmann, 1985; Du and Tian, 1986). Some specimens preserve a simple discoidal holdfast (for example,

Du and Tian, 1986, plate X, Figs. 2, 8A, 9; plate XI, Figs. 9-11), suggesting a benthic habit. Longfengshania was once interpreted as a bryophyte (Zhang, 1988), but this interpretation was disputed because it lacks any diagnostic bryophyte features (Liu and Du, 1991). The simple morphology and marine habitat of Longfengshania and Paralongfengshania is more consistent with a macroalgal interpretation. Indeed, several modern algae such as Botrydium (a xanthophyte), Botryocladia (a rhodophyte), and Valonia (a chlorophyte), all of which have a balloon-like vesicle tethered to a holdfast or a branch (Abbott, 1999; Graham and Wilcox, 2000), are good interpretive analogues for Longfengshania and Paralongfengshania.

Grypaniaceae: Grypania is a spiral ribbon-like compression fossil that occurr in Paleoproterozoic and Mesoproterozoic rocks (Walter et al, 1976; Du et al, 1986; Walter et al, 1990; Han and Runnegar, 1992). It is reconstructed as a spiral cylindrical organism, probably a photosynthetic alga (Walter et al, 1990; Han and Runnegar, 1992).

Neoproterozoic Fossil Photos

Figure 1. (A) Chuaria circularis from the early Neoproterozoic Huaibei Group, North China. (B) Bipatinella cervicalis (a Tawuia-like fossil,) from the early Neoproterozoic Huaibei Group, North China. (C) Longfengshania stipitata from the early Neoproterozoic Little Dal Group, northwestern Canada. Photo courtesy of Hans Hofmann. (D) Ellipsophysa axicula from the early Neoproterozoic Jiuliqiao Formation, North China. (E) Seirisphaera zhangii from the Ediacaran Lantian Formation, South China. Photo courtesy of Chen Meng'e. Scale bar represents 1 mm if not otherwise indicated.

Figure 1. (A) Chuaria circularis from the early Neoproterozoic Huaibei Group, North China. (B) Bipatinella cervicalis (a Tawuia-like fossil,) from the early Neoproterozoic Huaibei Group, North China. (C) Longfengshania stipitata from the early Neoproterozoic Little Dal Group, northwestern Canada. Photo courtesy of Hans Hofmann. (D) Ellipsophysa axicula from the early Neoproterozoic Jiuliqiao Formation, North China. (E) Seirisphaera zhangii from the Ediacaran Lantian Formation, South China. Photo courtesy of Chen Meng'e. Scale bar represents 1 mm if not otherwise indicated.

Chuaria Tawuia

Figure 2. (A-C) Specimens that can be identified as Protoarenicola baiguashanensis from the early Neoproterozoic Huaibei Group, North China. Transverse annulations not well preserved in (A) Note discoidal holdfast-like structures (arrows). (C) Courtesy of Xunlai Yuan. (D) Doushantuophyton lineare from the Ediacaran Doushantuo Formation, South China. (E) Baculiphyca taeniata from the Ediacaran Doushantuo Formation, South China. (F) Phosphatized algal thallus (possibly Thallophyca ramosa) from the Ediacaran Doushantuo Formation, South China. Scale bars represent 1 mm.

Figure 2. (A-C) Specimens that can be identified as Protoarenicola baiguashanensis from the early Neoproterozoic Huaibei Group, North China. Transverse annulations not well preserved in (A) Note discoidal holdfast-like structures (arrows). (C) Courtesy of Xunlai Yuan. (D) Doushantuophyton lineare from the Ediacaran Doushantuo Formation, South China. (E) Baculiphyca taeniata from the Ediacaran Doushantuo Formation, South China. (F) Phosphatized algal thallus (possibly Thallophyca ramosa) from the Ediacaran Doushantuo Formation, South China. Scale bars represent 1 mm.

Eoholyniaceae: Hofmann (1994) created this family to accommodate all branching forms. Some fine filaments, such as Daltaenia (Hofmann, 1985) and Chambalia (Kumar, 2001), appear to have branches and would be included in this family. However, the junctions of these branching filaments tend to be T-shaped rather than Y-shaped; they could be cyanobacterial branches (e.g. Fischerella) and are thus excluded from our analysis. Instead, we focus on carbonaceous fossils with dichotomous, monopodial, or helical branches, because these are more likely eukaryotic algae. A number of carbonaceous compressions from the Ediacaran Doushantuo and Lantian formations, including Anomalophyton, Doushantuophyton (Fig. 2D), Enteromorphites, Konglingiphyton, Longifuniculum, and Miaohephyton

(Chen and Xiao, 1992; Steiner, 1994; Ding et al, 1996; Yuan et al, 1999; Xiao et al, 2002; Yuan et al., 2002), are considered members of this group. Some fan-shaped thalli, such as Anhuiphyton, Flabellophyton, and Huangshanophyton from the Lantian Formation, may also contain rare dichotomously branching filaments (Yan et al., 1992; Chen et al., 1994a; Steiner, 1994; Yuan et al., 1999), but this is difficult to verify because of dense compaction of fine filaments. Nonetheless, the macroscopic thallus size, morphological complexity, and the presence of a holdfast structure (in Flabellophyton at least) independently suggest their macroalgal affinity and benthic habit. Thus, these Lantian forms are also considered members of this family.

Sinosabelliditidae and Protoarenicolidae: These two groups are characterized by ribbon-shaped compressions with transverse annulations (Fig. 2A-C). They occur in early Neoproterozoic rocks in North China (Sun et al, 1986), and similar forms have been reported from late Riphean rocks in southern Timan (Gnilovskaya et al., 2000). Some specimens are three-dimensionally preserved with a circular transverse cross section (Zheng, 1980; Wang, 1982; Wang and Zhang, 1984; Xing et al., 1985; Sun et al., 1986; Chen, 1988; Qian et al., 2000), suggesting that they were originally cylindrical tubes. Representative genera are Sinosabellidites, Pararenicola, Protoarenicola, Parmia, and many other synonyms (Wang and Zhang, 1984; Xing et al, 1985; Gnilovskaya et al., 2000). Pararenicola and Protoarenicola appear to bear a proboscis-like structure or a terminal opening in their presumed anterior end. The proboscis-like structure and transverse annulations led some to interpret Pararenicola and Protoarenicola as possible worm-like animals (Sun et al, 1986; Chen, 1988). Sinosabellidites has similar transverse annulations but no terminal opening or proboscis-like structure, and it was considered less likely to be an animal (Sun et al, 1986). It is interesting to note that a number of protoarenicolid specimens (for example, Wang and Zhang, 1984, plate 7, Fig. 2; Xing et al., 1985, plate 39, Fig. 1; Qian et al., 2000) appear to have holdfast-like structures. In fact, several transversely annulated or corrugated tubular fossils from the Doushantuo Formation, including Cucullus and Sinospongia (Xiao et al, 2002), can be considered members of the Protoarenicolidae (Hofmann, 1994) and they also have holdfast-like structures. Our own observations of protoarenicolids suggest that some of them have a discoidal holdfast structure (Fig. 2A-C). Thus, it is possible that the proboscis-like structures present in a small number of specimens of protoarenicolids (Sun et al, 1986) may be poorly preserved holdfasts or artifacts due to physical tearing of the discoidal holdfast. If confirmed, these observations and interpretations would indicate that protoarenicolids are similar to tawuiaceans described from the Suket Shale in the Vindhyan

Supergroup (Kumar, 2001) in having a holdfast structure. The only major difference is the presence or absence of transverse annulations, which is not a diagnostic animal feature (Sun et al., 1986; Chen, 1988). Thus, the animal interpretation of sinosabelliditids and protoarenicolids is poorly supported. A more likely interpretation is that they were siphonous macroalgae analogous to modern dasycladaleans (Berger and Kaever, 1992).

Moraniaceae, Beltinaceae, Vendotaeniaceae, Saarinidae, and Sabelliditidae: These groups are not included in the current study because their macroalgal affinity is problematic. Moraniaceans, beltinaceans, and vendotaeniaceans may represent bacterial colonies (Walcott, 1919; Vidal, 1989; Hofmann, 1994), although vendotaeniaceans have been interpreted as brown or red algae (Gnilovskaya, 1990; Gnilovskaya, 2003). In addition, beltinaceans and vendotaeniaceans are often fragmented and folded, making it difficult to reconstruct their morphology and paleoecology. Saarinids and sabelliditids have been interpreted as pogonophoran tubes (Sokolov, 1967; Hofmann, 1994); certainly, ultrastructures of Sabellidites cambriensis tubes, which consist of interwoven filaments with a diameter of 0.2-0.3 ^m (Urbanek and Mierzejewska, 1977; Ivantsov, 1990; Moczydlowska, 2003), have no analogues among modern macroalgae.

Other Macroalgae: Baculiphyca (Fig. 2E) from the Doushantuo and Lantian formations in South China was questionably placed in the Protoarenicolidae (Hofmann, 1994). Baculiphyca was undoubtedly a benthic macroalga with clavate or blade-like thallus and rhizoidal holdfast but no transverse annulations (Xiao et al., 2002). Thus Baculiphyca does not belong to the same family (or functional-form group) as protoarenicolids that are characterized by cylindrical thallus, transverse annulations, and possible discoidal holdfast. Another taxon that was not classified in any of the formally defined families is Orbisiana from Vendian rocks in Russia (Sokolov, 1976). Orbisiana consists of serial or biserial rings or spheres 0.20.9 mm in diameter, and it is probably an algal fossil (Jensen, 2003). Similar fossils (Fig. 1E), preserved as carbonaceous compressions and described as Catenasphaerophyton (Yan et al, 1992) or Seirisphaera (Chen et al, 1994a), have been known from the Ediacaran Lantian Formation in South China.

Permineralized Macroalgae: In addition to carbonaceous compressions, some permineralized algal fossils can also reach macroscopic size (Fig. 2F). Phosphatized and silicified algae in the Doushantuo Formation (Xiao, 2004; Xiao et al, 2004), for example, can be millimetric in size. However, the overall diversity and abundance of permineralized macroalgae is much lower than carbonaceous ones.

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