SEARCH

SEARCH BY CITATION

Keywords:

  • Proterozoic;
  • phytoplankton;
  • acritarchs;
  • algae;
  • microfossils;
  • phylogeny

Abstract

  1. Top of page
  2. Abstract
  3. Aspects of morphology and biochemistry
  4. Representative microfossils
  5. Phylogeny of Chlorophytes
  6. Conclusions
  7. References

Abstract:  Morphological and reproductive features and cell wall ultrastructure and biochemistry of Proterozoic acritarchs are used to determine their affinity to modern algae. The first appearance datum of these microbiota is traced to infer a minimum age of the divergence of the algal classes to which they may belong. The chronological appearance of microfossils that represent phycoma-like and zygotic cysts and vegetative cells and/or aplanospores, respectively, interpreted as prasinophyceaen and chlorophyceaen microalgae is related to the Viridiplantae phylogeny. An inferred minimum age of the Chlorophyte origin is before c. 1800 Ma, the Prasinophyceae at c. 1650 Ma and the Chlorophyceae at c. 1450 Ma. These divergence times differ from molecular clock estimates, and the palaeontological evidence suggests that they are older.

T he best examples of unicellular, organic-walled microfossils of uncertain origins (acritarchs) from the Palaeoproterozoic to Early Ordovician are reviewed herein to demonstrate their morphological and reproductive structures (Text-figs 1–3), which are indicative of their affinities to photosynthetic microalgae. New records of Cambrian and Ordovician acritarchs representing cysts and preserved at the moment of release of the endocyst through the excystment structure provide evidence of their algal affinities (Moczydłowska 2010; Text-fig. 2G–I). Zygotic cyst morphology, excystment structure and the endocyst are diagnostic features of microalgae (green microalgae and dinoflagellates), which are not known in other protists, or fungi or any life stage of metazoans. Acritarchs with similar phenotypic characters are recognized in the Proterozoic and allow the origin of the green microalgae to be traced to the Palaeoproterozoic (Text-fig. 4). By definition, acritarchs are organic-walled microfossils of uncertain affinities but mostly phytoplankton (Tappan 1980; Traverse 2007). After their biological affinities are recognized, they are withdrawn from the group of acritarchs.

image

Figure TEXT-FIG. 1..  Neoproterozoic microfossils. A, B. Leiosphaeridia sp. A, with pylome. Chuar Group, the Great Canyon, northern Arizona (Cryogenian). A, Specimen LO 5658. B, Specimen LO 5659. C, Appendisphaera grandis. Khamaka Fromation, eastern Siberia (Ediacaran). Specimen PMU-Sib.1-L/27/1. D, Trachyhystrichosphaera vidalii. Khajpakh Foramtion, eastern Siberia (Tonian). Specimen PMU-Sib.6-N/43/3. Scale bars represent 10 μm for A, 7 μm for B, 38 μm for C and 46 μm for D. All are light photomicrographs.

Download figure to PowerPoint

image

Figure TEXT-FIG. 2..  Neoproterozoic to early Ordovician microfossils. A–B, Appendisphaera grandis from the Khamaka Formation, eastern Siberia (Ediacaran). A, Specimen in immature stage; VNIGRI.3758/2-G/42. B, Specimen with circular opening; NVIGRI.2128/2-X/35. C, Trachysphaeridium laufeldi with operculate pylome. Visingsö Group, Sweden (Cryogenian). Specimen BV/50.80-2.T/30. D–F, Developmental stages in Skiagia ornata from the Lower Cambrian. D, Immature stage; specimen GGU 274795-1A. E, Specimen with ruptured opening, GGU 274571.2. D–E, Buen Formation, North Greenland. F, Specimen with endocyst; 7036RS137-3, D56/4, Arrowie Basin, South Australia. G–I, Process of releasing the endocyst. Stelliferidium timofeevii from the Lower Ordovician chalcedony in the Holy Cross Mountains, Poland. G, Endocyst at a moment of escape, specimen PMU-Pl.2009.1-H/15/1. H, Empty cyst, specimen PMU-Pl.2009.2-T/27. I, Rimmed pylome of the cyst, specimen PMU-Pl.2009.3.V/32. Scale bars represent 10 μm for all specimens. Scale bars represent 35 μm for A, 20 μm for B, 15 μm for C–F and 10 μm for G–I.

Download figure to PowerPoint

image

Figure TEXT-FIG. 3..  Cambrian microfossils. A–C. Timofeevia lancare. A, Immature stage, specimen SP-2009.10.A/10. B, Specimen with ontogenetically defined pylome, ZL05-13b-L/43. C, Specimen with opened pylome, ZL05-13-G/20/4. D–E, Cristallinium cambriense. D, Specimen in immature stage, SP-2009.11.C/15/2. E, Specimen with two open pylomes, SP-2009-12.V/16. F, Polygonium varium containing the endocyst, specimen PO106-11N2-A/15/4. Specimens A, D–F, Oville Formation, Cantabrian Mountains, northern Spain; specimens B, C, Playón Formation, Ossa-Morena Zone, central Spain. Scale bars represent 15 μm for all specimens.

Download figure to PowerPoint

image

Figure TEXT-FIG. 4..  Phylogeny of the Viridiplantae (modified from O’Kelly 2007 and Moustafa et al. 2009). The chronologically arranged, earliest appearance of microfossils interpreted to be green microalgae constrains the origin of the classes Prasinophyceae, Chlorophyceae and Ulvophyceae. The first appearance data of the microfossils are compiled from sources cited in the text. Terrestrial expansion of biota at c. 850 Ma is according to Knauth and Kennedy (2009). The time axis is drawn not in scale and shows the time of the origins of the Chlorophytes prior to c. 1800 Ma, Prasinophyceae c. 1650 Ma, Chlorophyceae c. 1450 Ma, and major radiations of phytoplankton at c. 650, 580 and 520 Ma.

Download figure to PowerPoint

The first indication that a microfossil may be algal is a decay- and acid-resistant cell wall, which reflects its biochemistry and ultrastructure, and probably indicates the ability to protect a resting/reproductive cyst. Although some Proterozoic heterotrophic protists, like amoebozoans and rhizarians, produced resistant cell walls (Porter 2006), their morphology is distinct from that of reviewed acritarchs. Amoebozoans and rhizarians have tests with always present opening, through which the cytoplasm extrusions or cilia are exuded for feeding and moving, and are radially symmetrical. Preserved as fossils, they are not folded or wrinkled because their organic tests are rigid, even if not encrusted by minerals. Acritarchs occasionally show the opening, and their vesicles are often asymmetrical and mostly are preserved compressed and wrinkled because their organic cell wall is more plastic and thinner, never being originally mineralized. The existence of other protists with organic walls, i.e. ciliates and dinoflagellates in the Proterozoic, inferred exclusively by biomarkers (references in Porter 2006) is questionable. The consensus is that dinoflagellates are a highly derived group (Raven et al. 2005; Cavalier-Smith 2007; Delwiche 2007) that evolved in the late Permian–Triassic (Fensome et al. 1996). The biopolymers synthesized in the cell walls of algae and in land plants (‘plant cells’), such as sporopollenin/algaenan, are diagnostic for photosynthetic taxa and were inherited from early unicellular ancestors (Raven et al. 2005; De Leeuw et al. 2006; Falkowski and Raven 2007; Moczydłowska 2010). These preservable cell walls are resistant to acetolysis, hydrolysis and acids, prolonged burial conditions, and show diagnostic ultrastructures such as trilaminar sheath (TLS; Derenne et al. 1992a, b, 1996; Gelin et al. 1999; Allard and Templier 2000; Hagen et al. 2002).

The trilaminar sheath structure occurs within a certain cell wall layer, and it should not be confused with the three-layered ultrastructure of the entire cell wall seen in some algae (Hagen et al. 2002), or metazoan resting stages (Liu et al. 2009). The trilaminar sheath structure is composed of fine membranous sublayers of nanometre-scale (nm) thickness, and totally 10–20 nm, which are differentiated by their electron density in TEM (Allard and Templier 2000). This structure occurs within a single layer of the cell wall, usually the outmost or middle of a multilayered cell wall in microalgae (Derenne et al. 1996; Hagen et al. 2002; Damiani et al. 2006), and it has been recognized in certain Cambrian and Proterozoic leiosphaerid acritarchs (Arouri et al. 1999; Arouri et al. 2000; Talyzina and Moczydłowska 2000; Javaux et al. 2004; Moczydłowska and Willman 2009; Moczydłowska et al. 2010, figs 1–2). The latter taxa are most likely various species of Leiosphaeridia, which are difficult to identify phenotypically because of the lack of diagnostic features and occur in geochronologically distant levels between 1450 and 520 Ma (Text-fig. 4). Discrete layers of three-layered or multilayered cell wall of microalgae are distinguished by their texture and thickness of a scale of hundred nanometres each, thus of a different magnitude than that of the trilaminar sheath structure. In metazoan cysts, the cyst wall may also be multilayered with diverse textures, or even of tripartiate structure of embryonic cuticle (which is one of the layers), which individual layers range in thickness from hundred nm to a few micrometres (μm; Liu et al. 2009). The total thickness of a metazoan cyst wall thus exceeds the thickness of trilaminar sheath (TLS).

‘Plant cell’ walls differ by chemical compounds that give high preservation potential from fungal and animal cell walls (Falkowski and Raven 2007; Webster and Weber 2007). Fungal and animal cells are fossilized only by syngenetic permineralization (Bengtson 2004; Yuan et al. 2005; Raff et al. 2008), whereas ‘plant cells’ are fossilized as body fossils more ubiquitously and without mineralization. The terrestrial fungal spores are occasionally recovered by chemical palynological processing, demonstrating the cell wall resistance like acritarchs, but they are Cretaceous taxa (Vajda and McLoughlin 2004). Terrestrial organisms began to synthesize resistant biopolymers much later as adaptation to survive desiccation. This is seen in land plant spores in the Cambrian (Strother and Beck 2000) or Ordovician (Wellman et al. 2003). Land plants inherited this biochemical pathway from algae (Raven et al. 2005; Cavalier-Smith 2007; Falkowski and Raven 2007), and terrestrial fungi might have acquired this ability independently.

Microalgae radiated quickly in the Cambrian and Ordovician (Moczydłowska 1991, 1998, 2011; Servais et al. 2008; Molyneux 2009); however, several morphotypes with features related to the reproductive cycle occur in the Proterozoic, although they are not always recognized as such. These Proterozoic microbiota document the evolutionary divergence of unicellular eukaryotic clades, and their first appearance datum (FAD) constrains the minimum age of their origins. Radiometric ages of the strata in which the microfossils are found are compared to divergence times given by molecular clocks (Cavalier-Smith 2007; Hedges and Kumar 2009). Molecular clock timing seems to be underestimated for certain photosynthetic clades.

This report follows the genome-based, phylogenetic and phenotypic classification of green algae (Raven et al. 2005; O’Kelly 2007; Turmel et al. 2008). Green algae belong to two Viridiplantae lineages: the phyla Chlorophyta (prasinophytes and core chlorophytes) and Streptophyta (Charophytes and all land plants). The Streptophytes diverged from the Prasinophytes/Chlorophytes, and the paraphyletic lineages of the Prasinophytes diverged at the base of a shared lineage with the Chlorophytes (Text-fig. 4).

Many microfossils from the group of acritarchs have been interpreted as phytoplankton based on their morphology and palaeoecology, and certain genera referred to modern algal classes (Tappan 1980; Downie 1982; Strother 1996; Wicander 2007; Le Hérisséet al. 2009; Molyneux 2009; The PhytoPal Taxonomic Database website). New records and additional ultrastructural studies allowed some Cambrian, Ediacaran and Cryogenian microfossils to be recognized as algal (Talyzina and Moczydłowska 2000; Moczydłowska and Willman 2009; Moczydłowska et al. 2010; Willman 2009; Moczydłowska 2010, 2011). Other representative taxa, including new specimen preserved at the moment of ‘giving birth’ to the endocyst, that range through the Proterozoic to Ordovician are revised below (Text-figs 1–3).

Aspects of morphology and biochemistry

  1. Top of page
  2. Abstract
  3. Aspects of morphology and biochemistry
  4. Representative microfossils
  5. Phylogeny of Chlorophytes
  6. Conclusions
  7. References

The morphology of microfossils with resistant cell walls, their ornamentation and the functionally identifiable structures are the source of information used to assess their systematics. Reliance on morphology alone can lead to the problem of convergent morphology, which may be resolved by ultrastructure and biochemistry of the cell wall as exemplified by Leiosphaeridia spp. (Talyzina and Moczydłowska 2000; Moczydłowska et al. 2010). However, the combination of numerous independent characters is needed to reconstruct phylogeny with more accuracy. Microfossil cell walls, which are acid resistant and thus extractable by chemical processing from the host rocks, are composed of biopolymers with the properties of the sporopollenin/algaenan classes of biomolecules synthesized by green algae, the green dinoflagellates and the reproductive cells of higher plants (Gelin et al. 1999; O’Kelly 2007). These biota share primary biochemical pathways for the synthesis of biopolymers in construction of cell walls (Falkowski and Raven 2007).

The origin of sexually reproducing eukaryotes required the ‘invention’ of structures to protect the zygote and the meiosis, and the development of more sophisticated excystment. Sexual reproduction meant the appearance of polymorphic generations in a complex life cycle that included alternating planktic and periodically benthic life modes (‘benthic plankton’ is the resting stage). Morphologically dissimilar cells of the vegetative vs. reproductive/resting encysted stages in a single biological species are well known in modern chlorophytes, dinoflagellates and chrysophytes. Those among them reproducing sexually produce the greatest variety of morphotypes, which are related to haploid and diploid developmental stages in their life cycle (Margulis et al. 1989; Raven et al. 2005; Lee 2008).

Morphological structures and ornamentation of the cell walls of vegetative but predominantly resting/reproductive stages show that the innovations appeared in the basal groups of green algae (i.e. prasinophytes and chlorophytes). Certain cell wall morphological structures arose out of functional necessity (e.g. excystment structures) and by processes of morphogenetic change from somatic to reproductive cells (i.e. zygotic or resting cysts with ornament and processes). During cyst morphogenesis in modern algae, cell walls become thicker, more complex in ultrastructure and multilayered by secreting additional layers of various biochemical compounds, which strengthen the walls and make the cyst resistant (Hagen et al. 2002; Damiani et al. 2006). This is achieved by such ultrastructure features as TLS made of algaenan (Derenne et al. 1996; Gelin et al. 1999).

Sporopollenin/algaenan is recognized in various modern algae, although not in all taxa. It occurs in Prasinophytes, Chlorophytes and Charophytes. Algaenan and trilaminar sheath (TLS) occur in resting stages (cysts, autospores and aplanospores) and in mother cells (vegetative) of freshwater and marine microalgae (De Leeuw et al. 2006). Algal cell wall ultrastructure and chemistry differ from fungal and metazoan cells. Algal biopolymers are more resistant than those in fungal and metazoan cell walls, and therefore, few of the latter are preserved (by permineralization). Interestingly, since the appearance of metazoans in the late Neoproterozoic and their radiations through the Phanerozoic, which are seen in the numerically and taxonomically abundant occurrences, there are no records of nonmineralized egg cysts. This ‘negative evidence’ and lack of metazoan cysts more commonly in the Phanerozoic conform to the property of nonresistant and thus nonpreservable biopolymers in their cell walls or membranes. The exception is the occurrence of the Ordovician–Devonian chitinozoans, which are organic-walled microfossils made of resistant polymers initially erroneously called chitin but have been proved to be of nonchitinous composition and have been interpreted to be metazoan egg cases (Paris and Nõvlak 1999). However, such interpretation is contentious, and alternatively, chitinozoans could be considered as protists (Miller 1996).

The resistant cell walls in the acritarchs Trachyhystrichosphaera, Shuiyousphaeridium, Dictyosphaera and Tappania, which are extracted by chemical processing and proved such property, and their certain morphological features do not support Butterfield’s (2005) interpretation that they are fungi. Diagnostic morphological features of these taxa, which do not conform to fungi, are the vesicle processes and their shape and terminations, double-walled vesicle, multilayered wall and pylome (Vidal et al. 1993; Yin et al. 2005). The cell wall of fungi is typically, although not always, present and is based on glucans and chitin and rarely includes cellulose (Webster and Weber 2007). None of these major compounds or the proteins and lipids of the cell walls are preservable (Raven et al. 2005). Only fungal spores are potentially preservable because may contain resistant macromolecules similar in properties to algaenans and dinosporins (De Leeuw and Largeau 2006), but have a different morphology than that of acritarchs. Fungi have few morphological characters that are taxonomically useful, and fungal fossils are virtually absent in pre-Phanerozoic, with the exception of 551–635 Ma fungal hyphae in a lichen-like association preserved in phosphorites, i.e. permineralized (Yuan et al. 2005). Fungal spores have no morphologically complex processes like the above-mentioned acritarch genera.

Similarly, Appendisphaera could not be a metazoan egg cyst, as suggested by Cohen et al. (2009), because of the same composition of the cell wall and other morphological characteristics, notably the zygotic cyst-like vesicle and excystment structure. Previously in acritarchs, only Tianshushania represents evidently animal resting eggs and embryos (Xiao and Knoll 2000; Yin et al. 2004).

Representative microfossils

  1. Top of page
  2. Abstract
  3. Aspects of morphology and biochemistry
  4. Representative microfossils
  5. Phylogeny of Chlorophytes
  6. Conclusions
  7. References

The microfossils reviewed below have been selected because of their unique phenotypical features allowing systematic interpretation, i.e. overall morphology resembling zygotic or resting cysts, excystment structures, endocyst inside the cyst and preserved at the moment of release. They all have been extracted by chemical processing proving their cell wall resistance to acids, and they have survived hundred million years of diagenesis and burial, not just a short laboratory treatment. Such a combination of phenetic features and biogeochemical properties occurs only in microalgae. The illustrated species here are reviewed in the geochronological order, as indicated by their FAD. Other taxa, although may be older, i.e. the oldest spheroidal and some with complex morphology, are discussed in conjuncture with these species because of the morphological and ultrastructural features diagnostic for interpreting them as microalgae.

Trachyhystrichosphaera vidalii appears at c. 950 Ma and the genus at c. 1250 Ma (Samuelsson et al. 1999). T. vidalii has a spheroidal vesicle with numerous cylindrical hollow processes and is enveloped by a membrane (Vidal et al. 1993; Text-fig. 1D). T. vidalii resembles the reproductive cysts of modern microalgae of the order Zygnematales of the Charophyceae. Such cysts develop in several modern genera (e.g. Spirotaenia, Closterium, Cosmarium, Micrasterias and Staurastrum; Bold and Wynne 1985; Raven et al. 2005; Lee 2008). It is not suggested that T. vidalii is a direct ancestor of the class Charophyceae. However, it is apparent that the morphological features (i.e. internal cyst with processes surrounded by outer membrane) related to the protective function in zygotic cysts have evolved early in the evolution of green algae and are shared among lineages that split into the classes Chlorophyceae and Charophyceae (Text-fig. 4). Cyst with a membranous envelope is inferred to be an early adaptation, along with the method of forming the cyst inside the gametangial or vegetative cell, and was inherited by descendant clades. Although the double-walled cyst of T. vidalii is similar to the phycoma-like stage of prasinophyceaens (Moczydłowska and Willman 2009), the processes and the complex structure are more advanced, as in the Chlorophyceae. The processes are homomorphic, of equal length and evenly distributed (almost symmetrically; Text-fig. 1D), and such characters are not seen in Prasinophyceae. The double-walled cyst should not be confused with the two-layered cell wall ultrastructure, which is observed in prasinophytes and chlorophytes. The number of layers in the cell wall of algae depends on developmental stage and maturity and differs from single- to multilayered wall (Hagen et al. 2002; Moczydłowska et al. 2010).

A similar construction of a vesicle with the spheroidal body, which bears processes but solid and enveloped by a membrane, is characteristic of Vandalosphaeridium walcottii and Cymatiosphaeroides kullingii (Vidal and Ford 1985; not illustrated herein but see in Moczydłowska 2008). Their first appearance datum is at c. 750 Ma, and Vandalosphaeridium sp. occurs at c. 1250 Ma (Samuelsson et al. 1999). These species are interpreted to represent the encysted stages of microalgae, possibly of early lineage leading to the class Chlorophyceae. The morphological complexity and resemblance to zygotic cysts support such affinities more conclusively than of prasinophytes previously assumed (Moczydłowska 2008).

Four Mesoproterozoic taxa, Shuiyousphaeridium, Dictyosphaera, Tappania and Tasmanites, are among the oldest unicellular eukaryotes with a distinctive morphology and cell wall perforation (not illustrated) that suggest their systematic affinity. Their first appearance datum is shown in Text-figure 4. S. macroreticulatum is eukaryotic, as shown by its morphological and ultrastructural complexity, i.e. multilayered cell wall with reticulated surface (Javaux et al. 2004), overall vesicle habit and cylindrical processes that flare outward (Xiao et al. 1997;Yin 1997; Yin et al. 2005), and it resembles microalgal cysts. Previously, a combination of these features has been attributed to as the eukaryotic protist, however, finding little support for any specific lineage (Knoll et al. 2006). The same reticulated vesicle wall as in S. macroreticulatum is observed in D. delicata. The most significant feature of D. delicata (Yin et al. 2005, figs 2: 10 and 2: 5) is the ontogenetically formed pylome with a lid, although it was not identified as such. The species may be conspecific with S. macroreticulatum (Xiao et al. 1997), and it may represent a developmental stage in two ‘species’ complex life cycle. A pylome with a lid is diagnostic of cysts (Traverse 2007) and suggests a chlorophyceaen assignment of Dictyosphaera and Shuiyousphaeridium, consequently (Text-fig. 4). Dinoflagellate cysts may possess the operculate pylome (Traverse 2007), but dinoflagellates did not evolve until the Mesozoic, with the exception of a few perhaps in the Permian (Fensome et al. 1996; Raven et al. 2005; Delwiche 2007). For this reason, it is difficult to accept the interpretation by Meng et al. (2005) that Shuiyousphaeridium is a dinoflagellate, but mostly because the polygonal sculpture of the wall surface in this genus and in Dictyosphaera delicata is not an analogue to the tabulation of dinoflagellates. It is a superficial relief of the wall and not a set of the individual structural elements of the wall.

T. plana is the oldest ornamented microfossil (c. 1450 Ma). Its excystment structure and cell wall ultrastructure (Javaux et al. 2004) are similar to those in microalgae, and Yin et al. (2005) recognized the resemblance to conjugating desmid algae (Charophyceae). Knoll et al. (2006) considered this species to be an actively growing life stage of heterotrophic eukaryote, yet without any specific attribution. The inferred actively growing cell of T. plana does not exclude an option of being an algal zygotic cyst, which is also metabolically active and growing although at a lower rate. The algal cells change morphology during the transformation from a vegetative to reproductive stages, and this may be consistent with a diverse morphology of Tappania. The presence of the excystment structure only in some specimens (i.e. life stages) is supporting an algal affinity rather than heterotrophic organism, which should have an opening in the test permanently. Phagocytic heterotroph would not need to have it at all.

T. rifejicus has a spheroidal vesicle with perforated cell wall and is herein recognized as a prasinophyceaen phycoma. The genus has morphological, ultrastructural and biochemical characteristics of this class, and T. rifejicus may represent the order Pyramimonadales (Moczydłowska and Willman 2009). The Prasinophytes diversified at c. 1250 Ma, and Valeria, Simia, Pterospermella and Pterospermopsimorpha pileiformis (Moczydłowska 2008) appeared with T. rifejicus (Text-fig. 4).

The Cryogenian Leiosphaeridia sp. A (Vidal and Ford 1985; Text-fig. 1A, B) and Trachysphaeridium laufeldi (Text-fig. 2C) possess distinctive excystment structures. Excystment structure with a predetermined morphology, instead of just a random rupture, is evidence that microfossil is an alga, because it does not exist in any life stage of animals, heterotrophic protists or fungi, but only among green microalgae and dinoflagellates (Traverse 2007). Complex excystment structures like a pylome or the opening with perforations, lids, the collars surrounding the opening (Text-figs 1A, B, 2B, C, 3B, C, E) seen in the Proterozoic (Vidal and Ford 1985; Nagovitsin 2009) and more commonly the Phanerozoic acritarch species (probably because they are more diverse) enable identification of chlorophyte–chlorophycean affinities at c. 1250 Ma, when such fossils are recorded (Text-fig. 4).

Leiosphaeridia sp. A has a spheroidal psilate vesicle, whereas T. laufeldi is ornamented with fine spines. Both have circular pylomes with a collar or rimmed edge and are operculate, while the cyst is immature and closed (Text-fig. 1A, B; Text-fig. 2C, respectively). Leiosphaerids may represent both vegetative cells and aplanospores and zygotic cysts with a pylome. Advanced excystment structure is not found in prasinophytes but is seen in chlorophyceaens. Dinoflagellate origins, as mentioned above, were much later, and dinoflagellates form a highly derived group. Thus, Leiosphaeridia sp. A is probably of chlorophyceaen affinity, as is T. laufeldi.

The oldest spheroidal acritarchs are from the c. 1800 Ma Changzhougou Formation and c. 1730–1700 Ma Chuanlinggou Formation. They have resistant and multilayered cell walls with median splits and have been recognized as acritarchs within the stem group Eukarya (Lamb et al. 2009; Peng et al. 2009). They do not differ from other microfossils of the Leiosphaeridia type with characteristic multilayered cell wall and are assumed to have an algal affinity.

The Ediacaran Appendisphaera grandis (Moczydłowska 2005; Text-figs 1C, 2A, B) has a distinct morphology with homomorphic hollow processes that are densely arranged on a spheroidal body and a circular pylome. It resembles immature zygotic cysts (Text-figs 1C, 2A). After the release of the zoospores or endocyst/offspring cell, it has an open, circular pylome (Text-fig. 2B). Thus, the genus, of which A. grandis is the type species, is interpreted to be a chlorophyceaen. The specimens attributed to the taxon have resistant cell walls what is proved by acid chemical processing. Therefore, Appendisphaera could not be a metazoan egg cyst, as suggested by (Cohen et al. 2009), because of the ‘plant cell’ composition of the wall and the above characteristics, notably the presence of the pylome as the certain Ediacaran taxa (Grey 2005).

The assumption that large ornamented Ediacaran microfossils with resistant cell walls, such as Appendisphaera, Gyalosphaeridium and Tanarium, and extracted by acid dissolution, are animal resting cysts (Cohen et al. 2009) conflicts with animal cell wall biochemistry and preservation potential (Raff et al. 2006, 2008). Membranes and fertilization envelops formed by nonresistant biopolymers, which decompose shortly after hatching, surround unmineralized metazoan eggs. Therefore, they are found fossilized only in sediments preserved under such specific burial conditions as syndepositional mineralization by phosphate or silica, or mediated by bacterial replacement and mineralization (Bengtson 2004; Raff et al. 2008). The low preservation potential of metazoan eggs was overcome only by the evolution of the reptiles and then birds, which produce mineralized egg shells and much later. There are a few exceptions among invertebrate metazoans that are known to produce resistant encysted stages, like the brine shrimp Artemia, which additionally secretes a rigid noncellular shell around the diapause cyst (Liu et al. 2009). Cyst shells, which are made of organic material containing chitin, lipoproteins and haematin but otherwise of unclear molecular composition, are resistant to severe environmental stress. This includes dryness, osmotic pressure, temperature change and UV exposure. They may survive such conditions for days, months or perhaps a few years, and in laboratory conditions a test of resistance to organic solvents (methanol) and freezing for 3 months (−20°C; Liu et al. 2009). The biopolymers in cyst shells are nonpreservable at long term, and the most resistant among them, chitin, is known in the fossil record no longer than a few tens of million years (Traverse 2007). Notably, the chitinozoans, which are recorded at c. 480–470 Ma, are not made of chitin (Paris and Nõvlak 1999).

Metazoan egg envelopes resembling morphology of acritarchs (van Waveren and Marcus 1993) are excellent examples of convergent morphology, but they are not even semi-fossils, being of Holocene age and perhaps only a few thousand years old. They do not demonstrate any resistance properties of the egg cyst walls that would survive hundred million years of diagenesis and thus do not serve as a comparison to Proterozoic microfossils as possibly of metazoan origins (Cohen et al. 2009), but superficial morphological similarity. Examples of alleged resistant walls in diapause cysts of invertebrate metazoans (Cohen et al. 2009) have been studied only in modern and sub-modern occurrences and conditions, not the truly fossilized specimens. Comparison of the cell wall ultrastructure of the Ediacaran acritarch Gyalosphaeridium sp. to a modern brine shrimp Branchinella diapause cyst as an ‘analogue’ (Cohen et al. 2009) is unfortunate because the TEM images of the cell wall section of the former is falsified by knife-cut marks (Cohen et al. 2009, fig. 5B–C) in the same way as an another example of the acritarch Leiosphaeridia sp. (Cohen et al. 2009, fig. 4D). Conversely, the cell wall of T. conoideum contains an aliphatic hydrocarbon, like algaenan, and it supports the algal affinity (Marshall et al. 2005).

More evidence of phenotypic traits indicative of microalgal affinity is shown by the Cambrian Skiagia ornata (Text-fig. 2D–F), Timofeevia lancarae, Cristallinium cambriense and Polygonium varium (Text-fig. 3A–F), and the Ordovician Stelliferidium timofeevii (Text-fig. 2G–I). The set of features considered here, i.e. the overall morphology, excystment structure, presence of the endocyst, resistant cell walls and in some cases their recognized chemistry and ultrastructure, is diagnostic of such affinities in the studied taxa. The size difference, an order of magnitude between the Ediacaran and Cambrian taxa, and supposed morphological dissimilarities have been emphasized as indicating a lack of direct analogues between the Ediacaran and Cambrian acritarchs and to support the alternative interpretation that the Ediacaran taxa are metazoan egg cysts (Cohen et al. 2009). This is not so obvious, because there are overlapping size ranges between taxa of these ages (Vidal 1994), and their body-plan and particular morphotypes are similar or the same. There are several common form genera recognized by morphology, such as Tasmanites, Pterospermopsimorpha and Pterospermella, not to mention leiospherids, which occur in the Proterozoic and Cambrian strata (Moczydłowska 1991, 2008). Considering size, however, the putative fossil cleavage-stage embryos from the Ediacaran records are large by comparison with many modern embryos, and the validity of some putative fossils interpreted as larval forms has been questioned (Raff et al. 2006).

The larger dimensions of Ediacaran taxa may be explained as eco-phenotypical phenomenon known among single-celled microorganisms, both photosynthetic and heterotrophic, in warm climatic zones and periods of greenhouse conditions. Examples of enlarged sizes are observed in coccolithophores (Henderiks and Pagani 2008), dinoflagellates (van Mourik 2006) and foraminiferans (Zachos et al. 1996) during global warming episode in the Eocene and reversed at the transition into the icehouse conditions in the Oligocene. Large cell size and high species diversity are consistent with ecological conditions imposed by increased mean temperature and pCO2 during the greenhouse climatic intervals (Henderiks and Pagani 2007). Climate, affecting stratification in the ocean, is a universal abiotic factor responsible for evolutionary changes in the size of marine phytoplankton (diatoms and dinoflagellates) over the Cenozoic (Finkel et al. 2005, 2007). Nutrient pulses in instable environments select for large cells in diatoms (Litchman et al. 2009). The same effect may be observed in the Ediacaran when icehouse (so-called Snowball Earth) conditions changed into global warming, increase in mineral input attributed to weathering, mixing of the layers and oxygenation of the ocean (Canfield et al. 2007; Fairchild and Kennedy 2007). Ediacaran large acritarchs are good candidates that show features of rapidly radiating phytoplankton in warm seas, occupying empty ecological niches and having a little competition for nutrients. An additional point is that some evolutionary lineages of phytoplankton, to which some acritarchs belong, must have survived the Ediacaran–Cambrian transition and the extinction of the Ediacaran acritarch association, giving rise to the Cambrian radiation. They might not yet have been recorded or not have been preserved. The small sizes of the earliest Cambrian acritarchs (Moczydłowska 1991) are also typical of newly evolving photosynthetic species after extinction events (van Mourik 2006). There is no good evidence for decoupling genetically and evolutionarily the Ediacaran and Cambrian acritarch microfossils as being alien (Cohen et al. 2009). They are predominantly phytoplanktic and algal in affinities (Grey 2005; Moczydłowska 2005, 2010, 2011; Moczydłowska and Willman 2009).

Skiagia ornata has long processes with funnel-shaped tips and a median split (Moczydłowska 1998; Text-fig. 2E). The specimens show three stages of cyst development: immature closed (Text-fig. 2D), mature empted with a median split (Text-fig. 2E) and maturing cyst with an endocyst (Text-fig. 2F). The most conclusive evidence that S. ornata is a cyst is the latter morphotype (Zang 2001; Zang et al. 2004, 2007) with an endocyst that bears an offspring cell or a sack of swarmers (Moczydłowska 2010). The genus is likely of the chlorophyceaen order Chlorococcales (Moczydłowska 2010). Endocysts are preserved in other taxa (i.e. Skiagia ciliosa, S. orbicularis, P. varium (Text-fig. 3F) and S. timofeevii (Text-fig. 2G)). The new specimen and the best example of showing the release of the endocyst through the pylome is S. timofeevii (Text-fig. 2G), which is spheroidal, covered by processes and it has an apical pylome (Text-fig. 2G–H), clearly defined by a rimmed edge (Text-fig. 2I). The microfossils Stelliferidium and Polygonium are presumed to be zygotic stages of chlorophyceaens.

The process-bearing Timofeevia lancarae (Text-fig. 3A–C) forms a continuous series of developmental stages that include an immature cyst without an opening (Text-fig. 3A), the mature cyst with a circular operculate pylome with a lid (Text-fig. 3B) and the abandoned cyst with opened pylome (Text-fig. 3C). Timofeevia-type cysts may be compared to zygotic cysts in the Chlorophyceae (Volvocales, Chlorococcales) and Charophyceae (Zygnematales) by similar phenotypical (shape of processes and opening) and developmental features. The strongest similarity is to the modern genus Chlamydomonas (Bold and Wynne 1985; Raven et al. 2005), which is at the base of phylogenetic lineage within the Volvocales. Cristallinium cambriense has an ovoidal vesicle divided by low costae into polygonal fields (Moczydłowska 1998). The immature cyst stage and the abandoned cyst with two opened pylomes are known (Text-fig. 3D–E). This feature is rare, but double pylomes occur in the Cambrian Leiosphaeridia bipylomifera (Palacios and Moczydłowska 1998). Cristallinium is probably a chlorophyceaen zygotic cyst. These affinities of Timofeevia and Cristallinium are treated with a reservation by one of the authors (T. Palacios), who considers them morphologically similar to dinoflagellates.

Phylogeny of Chlorophytes

  1. Top of page
  2. Abstract
  3. Aspects of morphology and biochemistry
  4. Representative microfossils
  5. Phylogeny of Chlorophytes
  6. Conclusions
  7. References

The chronological sequence of microfossils with diagnostic traits of (1) phycoma-like cysts, (2) ornamented zygotic cysts, (3) pylomes, (4) double-walled vesicles and endocysts and (5) spheroidal vegetative cells and/or aplanospores with TLS, which are interpreted as green microalgae, is arranged on a phylogenetic scheme of the Viridiplantae (Text-fig. 4). The first appearance datum of these taxa provides the minimum age of the origin of the classes to which they are assigned. According to the inferred affinities of these microfossils, the sequence of evolutionary events is as follows:

The crown group of the Viridiplantae appeared prior to c. 1800 Ma, and the major branching nodes in this lineage at c. 1800 Ma for the Chlorophytes, c. 1650 Ma for the Prasinophyceae and c. 1450 Ma for the Chlorophyceae–Trebouxiophyceae–Ulvophyceae lineage. The divergence of the Ulvophyceae might have occurred before c. 950 Ma (Text-fig. 4).

The origin of the Chlorophytes is constrained by the earliest known Leiosphaeridia-type microfossils. The ‘leiosphaerid’ morphology, which occurs in the prasinophyceaen or chlorophyceaen microalgae (Moczydłowska and Willman 2009; Moczydłowska 2011), has deep roots in a common ancestral group and is not only the result of a later convergent morphology. This morphotype was inherited and shared in vegetative cells and reproductive/resting cysts, as the simplest and most functionally ‘perfect’ cell morphology.

The prasinophyceaen lineage is represented by Tasmanites rifejicus (Text-fig. 4), and co-occurring genera with phycoma-like, double-walled cysts (e.g. Simia, Pterospermopsimorpha and Pterospermella), and the striated vesicle (Valeria). Valeria appears at c. 1650 Ma in the Mallapunyah Formation (Javaux et al. 2004) and marks the minimum age at which the Prasinophyceae lineage splits from the basal Chlorophytes. Later phycoma-like microfossils are recorded at c. 950 Ma (Octoedryxium), c. 580 Ma (Tasmanites, Simia, Octoedryxium and Pterospermopsimorpha; Grey 2005), and after c. 540 Ma and throughout the Cambrian (Tasmanites, Granomarginata, Pterospermella and Cymatiosphaera; Moczydłowska 1991, 1998, 2011).

The chlorophyceaen lineage is known from various species of Leiosphaeridia with trilaminar sheath (TLS) in the cell walls at c. 1450 Ma (Javaux et al. 2004). They are likely the early members of the orders Volvocales and/or Chlorococcales. Leiosphaerids with such traits occur at c. 650 and 520 Ma (Moczydłowska et al. 2010). The systematically problematic Tappania is eukaryotic and considered algal herein; it appeared at c. 1450 Ma. Microfossils with morphologies of enveloped zygotic cysts (Trachyhystrichosphaera sp., Vandalosphaeridium sp., T. vidalii, V. walcottii and C. kullingii) occur between c. 1250 and 750 Ma. Distinctly ornamented zygotic cysts (S. macroreticulatum, and D. delicata) appeared at c. 1250–1300 Ma and are subsequently found at c. 580 Ma (Appendisphaera), c. 520–500 Ma (Skiagia, Timofeevia, Polygonium and Cristallinium) and c. 480 Ma (Stelliferidium). Microfossils with defined pylomes are found at c. 1250–1030 Ma (Osculosphaera; Nagovitsin 2009) and then 750 Ma (Leiosphaeridia sp. A and T. laufeldi; Text-fig. 4).

The divergence of the Ulvophyceae prior to c. 950 Ma is suggested by the dasycladacean Archaeoclada and Variaclada in the Lakhanda Group, and the siphonocladacean Proterocladus from the c. 750–700 Ma Svanbergfjellet Formation (Hermann 1990; Butterfield et al. 1994; Text-fig. 4). A process-bearing Papillomembrana from the c. 650 Ma Hedmark Group has been also assigned to the ulvophyceaen algae (Tappan 1980).

The minimum ages of the origin of the Viridiplantae and the divergence of the major microalgal clades differ from the molecular clock estimates made by Cavalier-Smith (2007) and from the synthesis of Hedges and Kumar (2009). The molecular clock estimates of these events conflict with the microfossil records, and the interpretation of some of them as photosynthesizing taxa (Text-fig. 4), and seem to be too late. The evolutionary transition between bacteria and Eukaryota in the interval 1000–800 Ma (Cavalier-Smith 2007) is obviated by the fossil evidence. In the timetree of life (Hedges and Kumar 2009), the average time for the origin of eukaryotes at 1594 Ma is too late. Eukaryotic organisms existed prior to 1000 Ma as shown by body and molecular fossils (Javaux et al. 2004; Butterfield 2005; Javaux and Marshal 2006; Knoll et al. 2007). A consensus exists on the eukaryotic nature of the spheroidal microfossils at c. 1.8 Ga (Javaux and Marshal 2006; Lamb et al. 2009; Peng et al. 2009), and morphologically complex taxa with a cytoskeleton existed by 1.5 Ga (Javaux et al. 2004; Yin et al. 2005).

Conclusions

  1. Top of page
  2. Abstract
  3. Aspects of morphology and biochemistry
  4. Representative microfossils
  5. Phylogeny of Chlorophytes
  6. Conclusions
  7. References

The assignment of Proterozoic unicellular microfossils with resistant cell walls to specific eukaryotic groups is tentative. However, we argue that the new interpretations of their functional morphology, combined with cell wall ultrastructure and biochemistry, allow their assignment to microalgal classes. Microfossils with complex ornamentation and ontogenetically formed excystment structures or endocysts, which prove that they are cysts in a complex life cycle with sexual reproduction, are related to the basal lineage of the Chlorophytes and the class Chlorophyceae. A cell wall ultrastructure with a trilaminar sheath supports the affinity of some spheroidal taxa to the Chlorophytes (Text-fig. 4).

The phylogeny of the Chlorophytes shows a sequence of branching nodes from a crown group of the Viridiplantae that leads to the classes Prasinophyceae and Chlorophyceae, and then the Ulvophyceae (O’Kelly 2007; Moustafa et al. 2009). Based on the present interpretation of the microfossil record, the timing of these nodes is deduced to be prior to c. 1650 Ma for the Prasinophyceae, c. 1450 Ma for the Chlorophyceae and c. 950 Ma for the Ulvophyceae. The origin of the Chlorophytes, and in general the Viridiplantae, predates 1.8 Ga. These ages, based on microfossils, are earlier than the estimates based on molecular clocks.

Acknowledgements.  The work by M. Moczydłowska was supported by a research grant from the Swedish Research Council (VR).

Editor. Svend Stouge

References

  1. Top of page
  2. Abstract
  3. Aspects of morphology and biochemistry
  4. Representative microfossils
  5. Phylogeny of Chlorophytes
  6. Conclusions
  7. References
  • ALLARD, B. and TEMPLIER, J. 2000. Comparison of neutral lipid profile of various trilaminar outer cell wall (TLS)-containing microalgae with emphasis on algaenan occurrence. Phytochemistry, 54, 369380.
  • AROURI, K., GREENWOOD, P. F. and WALTER, M. R. 2000. Biological affinities of Neoproterozoic acritarchs from Australia: microscopic and chemical characterisation. Organic Geochemistry, 31, 7589.
  • AROURI, K., GREENWOOD, P. F. and WALTER, M. R. 1999. A possible chlorophycean affinity of some Neoproterozoic acritarchs. Organic Geochemistry, 30, 13231337.
  • BENGTSON, S. 2004. Tracing metazoan roots in the fossil record. 289300. In LEGAKIS, Sfenthourakis, S., Polymeni, R. and Thessalou-Legaki, M. (eds). The new panorama of animal evolution. PENSOFT Publishers Sofia, Moscow, 738 pp.
  • BOLD, H. C. and WYNNE, M. J. 1985. Introduction to the algae, Second Edition. Prentice- Hall, Inc., Englewood Cliffs, NJ, 720 pp.
  • BUTTERFIELD, N. J. 2005. Probable Proterozoic fungi. Paleobiology, 31, 165182.
  • BUTTERFIELD, N. J., KNOLL, A. H. and SWEET, K. 1994. Paleobiology of the Neoproterozoic Svanbergfjellet Formation, Spitsbergen. Fossils and Strata, 34, 84 pp.
  • CANFIELD, D. E., POULTON, S. W. and NARBONNE, G. M. 2007. Late-Neoproterozoic deep-ocean oxygenation and the rise of animal life. Science, 315, 9295.
  • CAVALIER-SMITH, T. 2007. Evolution and relationships of algae: major branches of the tree of life. 2155. In BODIE, J. and LEWIS, J. (eds). Unravelling the algae, the past, present, and future of algal systematics. Natural History Museum. The Systematics Association Special Volume Series 75, CRC Press, Taylor & Francis Group, London, 376 pp.
  • COHEN, P. A., KNOLL, A. H. and KODNER, R. 2009. Large spinose microfossils in Ediacaran rocks as resting stages of early animals. Proceedings of the National Academy of Sciences, 106, 65196524.
  • DAMIANI, M. C., LEONARDI, P. I., PIERONI, O. I. and CÁCERES, E. J. 2006. Ultrastructure of the cyst wall of Haematococcus pluvialis (Chlorophycea): wall development and behaviour during cyst germination. Phycologia, 45, 616623.
  • DE LEEUW, J. W. and LARGEAU, C. 2006. A review of macromolecular organic compounds that comprise living organisms and their role in kerogen, coal, and petroleum formation. 2372. In ENGEL, M. H. and MACKO, S. A. (eds). Organic geochemistry: principles and applications. topics in geobiology. Plenum Press, New York, 884 pp.
  • DE LEEUW, J. W., VERSTEEGH, G. J. M. and VAN BERGEN, P. F. 2006. Biomacromolecules of plants and algae and their fossil analogues. Plant Ecology, 189, 209233.
  • DELWICHE, C. F. 2007. The origin and Evolution of Dinoflagellates. 191205. In FALKOWSKI, P. G. and KNOLL, A. H. (eds). Evolution of primary producers in the sea. Academic Press, Elsevier, Amsterdam, 441 pp.
  • DERENNE, S., LARGEAU, C., BERKALO, C., ROUSSEAU, B., WILHELM, C. and HATCHER, P. 1992a. Non-hydrolysable macromolecular constituents from outer walls of Chlorella fusca and Nanochlorum eucaryotum. Phytochemistry, 31, 19231929.
  • DERENNE, S., LE BERRE, F., LARGEAU, C., HATCHER, P., CONNAN, J. and RAYNAUD, J. F. 1992b. Formation of ultralaminae in marine kerogens via selective preservation of thin resistant outer walls of microalgae. Organic Geochemistry, 19, 345350.
  • DERENNE, S., LARGEAU, C. and BERKALO, C. 1996. First example of an algaenan yielding an aromatic-rich pyrolysate: possible geochemical implications on marine kerogen formation. Organic Geochemistry, 24, 617627.
  • DOWNIE, C. 1982. Lower Cambrian acritarchs from Scotland, Norway, Greenland and Canada. Transactions of the Royal Society of Edinburgh, Earth Sciences, 72, 257282.
  • FAIRCHILD, I. J. and KENNEDY, M. J. 2007. Neoproterozoic glaciation in the Earth System. Journal of Geological Society, London, 164, 895921.
  • FALKOWSKI, P. G. and RAVEN, J. A. 2007. Aquatic photosynthesis. Princeton University Press, Princeton and Oxford, 484 pp.
  • FENSOME, R. A., MACRAE, R. A., MOLDOWAN, J. M., TAYLOR, F. J. R. and WILLIAMS, G. L. 1996. The early Mesozoic radiation of dinoflagellates. Paleobiology, 22, 329338.
  • FINKEL, Z. V., KATZ, M. E., WRIGHT, J. D., SCHOFIELD, O. M. E. and FALKOWSKI, P. G. 2005. Climatically driven macroevolutionary patterns in the size of marine diatoms over the Cenozoic. Proceedings of the National Academy of Sciences, USA, 102, 89278932.
  • FINKEL, Z. V., SEBBO, J., FEIST-BURKHARDT, S., IRWIN, A. J., KATZ, M. E., SCHOFIELD, O. M. E. and YOUNG, J. R. 2007. A universal driver of macroevolutionary change in the size of marine phytoplankton over the Cenozoic. Proceedings of the National Academy of Sciences, USA, 104, 2041620420.
  • GELIN, F., VOLKMAN, J. K., LARGEAU, C., DERENNE, S., SINNINGHE DAMSTÉ, J. S. and DE LEEUW, J. W. 1999. Distribution of aliphatic, nonhydrolyzable biopolymers in marine microalgae. Organic Geochemistry, 30, 147159.
  • GREY, K. 2005. Ediacaran palynology of Australia. Memoirs of the Association of Australasian Palaeontologists, 31, 1439.
  • HAGEN, C., SIEGMUND, S. and BRAUNE, W. 2002. Ultrastructural and chemical changes in the cell wall of Haematococcus pluvialis (Volvocales, Chlorophyta) during aplanospore formation. European Journal of Phycology, 37, 217226.
  • HEDGES, S. B. and KUMAR, S. (eds). 2009. The timetree of life. Oxford University Press, Oxford, 551 pp.
  • HENDERIKS, J. and PAGANI, M. 2007. Refining ancient carbon dioxide estimates: significance of coccolithophore cell size for alkenone-based pCO2 records. Paleoceanography, 22, PA3202.
  • HENDERIKS, J. and PAGANI, M. 2008. Coccolithophore cell size and the Paleogene decline in atmospheric CO2. Earth and Planetary Sciences Letters, 269, 575583.
  • HERMANN, T. N. 1990. Organic World billion years ago. Leningrad, Nauka, 50 pp. (In Russian).
  • JAVAUX, E. J. and MARSHAL, C. P. 2006. A New Approach in deciphering early protist palaeobiology and evolution: combined microscopy and microchemistry of single Proterozoic acritarchs. Review of Palaeobotany and Palynology, 139, 115.
  • JAVAUX, E. J., KNOLL, A. H. and WALTER, M. R. 2004. TEM evidence for eukaryotic diversity in mid-Proterozoic oceans. Geobiology, 2, 121132.
  • KNAUTH, L. P. and KENNEDY, M. J. 2009. The late Precambrian greening of the Earth. Nature, 460, 728732.
  • KNOLL, A. H., JAVAUX, E. J., HEWITT, D. and COHEN, P. 2006. Eukaryotic organisms in Proterozoic oceans. Philosophical Transactions of the Royal Society B, 361, 10231038.
  • KNOLL, A. H., SUMMONS, R. E., WALDBAUER, J. R. and ZUMBERGE, J. E. 2007. The geological succession of primary producers in the ocean. 133163. In FALKOWSKI, P. G. and KNOLL, A. H. (eds). Evolution of primary producers in the sea. Academic Press, Elsevier, Amsterdam, 441 pp.
  • LAMB, D. M., AWRAMIK, S. M., CHAPMAN, D. J. and ZHU, S. 2009. Evidence for eukaryotic diversification in the ∼1800 million-year-old Changzhougou Formation, North China. Precambrian Research, 173, 93104.
  • LE HÉRISSÉ, A., DORNING, K. J., MULLINS, G. L. and WICANDER, R. 2009. Global patterns of organic-walled phytoplankton biodiversity during the Late Silurian to earliest Devonian. Palynology, 33, 2575.
  • LEE, R. E. 2008. Phycology. Cambridge University Press, Cambridge, 547 pp.
  • LITCHMAN, E., KLAUSMEIER, C. A. and YOSHIYAMA, K. 2009. Contrasting size evolution in marine and freshwater diatoms. Proceedings of the National Academy of Sciences, USA, 106, 26652670.
  • LIU, Y.-L., ZHAO, Y., DAI, Z.-M., CHEN, H.-M. and YANG, W.-J. 2009. Formation of diapause cyst shell in brine shrimp Artemia parthenogenetica, and its resistance role in environmental stresses. Journal of Biological Chemistry, 284, 1693116938.
  • MARGULIS, L., CORLISS, J. O., MELKONIAN, M. and CHAPMAN, D. J. (eds). 1989. Handbook of protoctista. Jones and Bartlett Publishers, Boston, 914 pp.
  • MARSHALL, C. P., JAVAUX, E. J., KNOLL, A. H. and WALTER, M. R. 2005. Combined micro-Fourier transform infrared (FTIR) spectroscopy and micro-Raman spectroscopy of Proterozoic acritarchs. A new approach to Palaeobiology. Precambrian Research, 138, 208224.
  • MENG, F. W., ZHOU, C. M., YIN, L. M., CHEN, Z. L. and YUAN, X. L. 2005. The oldest known dinoflagellates: morphological and molecular evidence from Mesoproterozoic rocks at Yongji, Shanxi Province. Chinese Scientific Bulletin, 50, 12301234.
  • MILLER, M. 1996. Chitinozoa. 81107. In JANSONIUS, J. and McGREGOR, D. C. (eds). Palynology: principles and applications. American Association of Stratigraphic Palynologists Foundation 1, Publishers Press, Salt Lake City, 462 pp.
  • MOCZYDŁOWSKA, M. 1991. Acritarch biostratigraphy of the Lower Cambrian and the Precambrian–Cambrian boundary in southeastern Poland. Fossils and Strata, 29, 127 pp.
  • MOCZYDŁOWSKA, M. 1998. Cambrian acritarchs from Upper Silesia, Poland – biochronology and tectonic implications. Fossils and Strata, 46, 121 pp.
  • MOCZYDŁOWSKA, M. 2005. Taxonomic review of some Ediacaran acritarchs from the Siberian Platform. Precambrian Research, 136, 283307.
  • MOCZYDŁOWSKA, M. 2009. The Ediacaran microbiota and the survival of Snowball Earth Conditions. Precambrian Research, 167, 115.
  • MOCZYDŁOWSKA, M. 2010. Life cycle of early Cambrian microalgae from the Skiagia- plexus acritarchs. Journal of Paleontology, 84, 216230.
  • MOCZYDŁOWSKA, M. 2011. The early Cambrian phytoplankton radiation: acritarch evidence from the Lükati Formation, Estonia. Palynology, 35, 101143.
  • MOCZYDŁOWSKA, M. and WILLMAN, S. 2009. Ultrastructure of cell walls in ancient microfossils as a proxy to their biological affinities. Precambrian Research, 173, 2738.
  • MOCZYDŁOWSKA, M., SCHOPF, J. W. and WILLMAN, S. 2010. Micro-scale and nanoscale ultrastructure of cell walls in Cryogenian microfossils revealing their biological affinity. Lethaia, 43, 129136.
  • MOLYNEUX, S. G. 2009. Acritarch (marine microphytoplankton) diversity in an Early Ordovician deep-water setting (the Skiddow Group, northern England): implications for the relationship between sea-level change and phytoplankton diversity. Palaeogeography, Palaeoclimatology, Palaeoecology, 275, 5976.
  • MOUSTAFA, A., BESHTERI, B., MAIER, U. G. and BOWLER, C. 2009. Genomic footprints of a cryptic plastid endosymbiosis in diatoms. Science, 324, 17241726.
  • NAGOVITSIN, K. 2009. Tappania-bearing association of the Siberian platform: biodiversity, stratigraphic position and geochronological constraints. Precambrian Research, 173, 137145.
  • O’KELLY, C. J. 2007. The origin and early evolution of green plants. 287309. In FALKOWSKI, P. G. and KNOLL, A. H. (eds). Evolution of primary producers in the sea. Academic Press, Elsevier, Amsterdam, 441 pp.
  • PALACIOS, T. and MOCZYDŁOWSKA, M. 1998. Acritarch biostratigraphy of the Lower- Middle Cambrian boundary in the Iberian Chains, Province of Soria, northeastern Spain. Revista Española de Paleontologia, n° extr. Homenaje al Prof. Gonzalo Vidal, 6582.
  • PARIS, F. and NÕVLAK, J. 1999. Biological interpretation and palaeodiversity of a cryptic fossil group: the ‘chitinozoan animal’. Geobios, 32, 315324.
  • PENG, Y., BAO, H. and YUAMN, X. 2009. New morphological observations for Paleoproterozoic acritarchs from the Chuanlinggou Formation, North China. Precambrian Research, 168, 223232.
  • PORTER, S. M. 2006. The Proterozoic Fossil Record of Heterotrophic Eukaryotes. 121. In XIAO, S. and KAUFMAN, A. J. (eds). Neoproterozoic geobiology and paleobiology, Springer, Dordrecht, 300 pp.
  • RAFF, E. C., VILLINSKI, J. T., TURNER, F. R., DONOGHUE, P. C. J. and RAFF, R. A. 2006. Experimental taphonomy shows the feasibility of fossil embryos. Proceedings of the National Academy of Sciences, USA, 103, 58465851.
  • RAFF, E. C., SCHOLLAERT, K. L., NELSON, D. E., DONOGHUE, P. C. J., THOMAS, C.-W., TURNER, F. R., STEIN, B. D., DONG, X., BENGTSON, S., HULDTGREN, T., STAMPANONI, M., CHONGYU, Y. and RAFF, R. A. 2008. Embryo fossilization is a biological process mediated by microbial biofilms. Proceedings of the National Academy of Sciences, USA, 105, 1935919364.
  • RAVEN, P. H., EVERT, R. F. and EICHHORN, S. E. 2005. Biology of plants. W. H. Freeman and Company Publishers, New York, 686 pp.
  • SAMUELSSON, J., DAWES, P. R. and VIDAL, G. 1999. Organic-walled microfossils from the Proterozoic Thule Supergroup, Northwest Greenland. Precambrian Research, 96, 123.
  • SERVAIS, T., LEHNERT, O., LI, J., MULLINS, G. L., MUNNECKE, A., NÜTZEL, A. and VECOLI, M. 2008. The Ordovician Biodiversifiction: revolution in the oceanic trophic chain. Lethaia, 41, 99109.
  • STROTHER, P. K. 1996. Chapter 5. Acritarchs. 81107. In JANSONIUS, J. and McGREGOR, D. C. (eds). Palynology: principles and applications, American Association of Stratigraphic Palynologists Foundation 1, Publishers Press, Salt Lake City, 462 pp.
  • STROTHER, P. K. and BECK, J. H. 2000. Spore-like microfossils from Middle Cambrian strata: expanding the meaning of the term cryptospore. 413424. In HARLEY, M. M., MORTON, C. M. and BLACKMORE, S. (eds). Pollen and spores: morphology and biology. Royal Botanical Gardens, Kew, England, 567 pp.
  • TALYZINA, N. M. and MOCZYDŁOWSKA, M. 2000. Morphological and ultrastructural studies of some acritarchs from the Lower Cambrian Lükati Formation, Estonia. Review of Palaeobotany and Palynology, 112, 121.
  • TAPPAN, H. 1980. The paleobiology of plant protists. WH Freeman, San Francisco, CA, 1028 pp.
  • TRAVERSE, A. 2007. Paleopalynology, Second Edition. Springer, Dortrecht, 813 pp.
  • TURMEL, M., BROUARD, J.-S., GAGNON, C., OTIS, C. and LEMIEUX, C. 2008. Deep division in the Chlorophyceae (Chlorophyta) revealed by chloroplast phylogenetic analyses. Journal of Phycology, 44, 739750.
  • VAJDA, V. and McLOUGHLIN, S. 2004. Fungal proliferation at the Cretaceous-Tertiary boundary. Science, 303, 1489.
  • Van MOURIK, C. A. 2006. The Greenhouse –Icehouse Transition: a dinoflagellates perspective. Meddelanden från Stockholmsuniversitets institution för geologi och geochemi, Vol. 327, 132. Doctoral Thesis. Marine Geoscience at Stockholm University, Sweden 2006.
  • VAN WAVEREN, I. M. and MARCUS, N. H. 1993. Morphology of copepod egg envelopes from Turkey Point (Gulf of Mexico). Special Papers in Palaeontology, 48, 111124.
  • VIDAL, G. 1994. Early ecosystems: limitations imposed by the fossil record. 298311. In BENGTSON, S. (ed.). Early life on earth. Nobel Symposium No. 84, Columbia U.P., New York, 630 pp.
  • VIDAL, G. and FORD, T. 1985. Microbiotas from the late Proterozoic Chuar Group (northern Arizona) and Uinta Mountain Group (Utah) and their chronostratigraphic implications. Precambrian Research, 28, 349489.
  • VIDAL, G., MOCZYDŁOWSKA, M. and RUDAVSKAYA, V. 1993. Biostratigraphical implications of a Chuaria-Tawuia assemblage and associated acritarchs from the Neoproterozoic of Yakutia. Palaeontology, 36, 387402.
  • WEBSTER, J. and WEBER, R. W. S. 2007. Introduction to fungi. Cambridge University Press, Cambridge, 841 pp.
  • WELLMAN, C. H., OSTERLOFF, P. L. and MOHLUDDIN, U. 2003. Fragments of the earliest land plants. Nature, 425, 282285.
  • WICANDER, R. 2007. Acritarchs and prasinophyte phycomata. Short Course. CIMP Lisbon’ 07. Acritarch Subcommisssion, Lisbon, Portugal, 24–28 September 2007, 15 pp.
  • WILLMAN, S. 2009. Morphology and wall ultrastructure of leiosphaeric and acanthomorphic acritarchs from the Ediacaran of Australia. Geobiology, 7, 820.
  • XIAO, S. and KNOLL, A. H. 2000. Phosphatized animal embryos from the Neoproterozoic Doushantuo Formation at Weng’an, Guzhou, South China. Journal of Paleontology, 74, 767788.
  • XIAO, S., KNOLL, A. H., KAUFMAN, A. J., YIN, L. and ZHANG, Y. 1997. Neoproterozoic fossils in Mesoproterozoic rocks? Chemostratigraphic resolution of a biostratigraphic conundrum from the North China Platform. Precambrian Research, 84, 197220.
  • YIN, C. Y., BENGTSON, S. and YUE, Z. 2004. Silicified and phosphatized Tianshushania, spheroidal microfossils of possible animal origin from the Neoptorterozoic of South China. Acta Palaeontological Polonica, 49, 112.
  • YIN, L. 1997. Acanthomorphic acritarchs from Meso-Neoproterozoic Shales of the Ruyang Group, Shanxi, China. Review of Palaeobotany and Palynology, 98, 1525.
  • YIN, L., YUAN, X., MENG, F. and HU, J. 2005. Protists of the Upper Mesoproterozoic Ruyang Group in Shanxi Province, China. Precambrian Research, 141, 4966.
  • YUAN, X., XIAO, S. and TAYLOR, T. N. 2005. Lichen-like symbiosis 600 million years ago. Science, 308, 10171020.
  • ZACHOS, J. C., QUINN, T. M. and SALAMY, K. A. 1996. High resolution (104 years) deep- sea foraminiferal stable isotope records of the Eocene–Oligocene climate transition. Paleoceanography, 11, 251266.
  • ZANG, W. 2001. Acritarchs. 7485. In ALEXANDER, E. M., JAGO, J. B., ROZANOV, A. Yu. and ZHURAVLEV, A. YU. (eds). The Cambrian biostratigraphy of the Stansbury Basin, South Australia. IAPC Nauka, Moscow, 344 pp.
  • ZANG, W., JAGO, J. B., ALEXANDER, E. M. and PARASCHIVOIU, E. 2004. A review of basin evolution, sequence analysis and petroleum potential of the frontier Arrowie Basin, South Australia. PESA Eastern Australian Basins Symposium II, Adelaide, 19–22 September, 2004, 243256.
  • ZANG, W., MOCZYDŁOWSKA, M. and JAGO, J. B. 2007. Lower Cambrian acritarch assemblage zones in South Australia and global correlation. Memoirs of the Association of Australasian Palaeontologists, 33, 141177.