Precambrian Jellyfish Fossil

Nucleus (eukaryote)

Nucleus (eukaryote)

Amoeboid prokaryote

DNA aggregation

Archaeobacterlum

DNA aggregation

Nucleate pre-eukaryote

Archaeobacterlum

Phagotrophic eubacterium

Nucleate pre-eukaryote

Phagotrophic eubacterium

Inflexible cell wall

Flexible cell wall

Gram-negative eubacterium

Fig. 7.1 (a) The Serial Endosymbiotic Theory suggests that eukaryote organelles arose from successive endosymbioses between different kinds of prokaryote and an amitochondriate host bacterium such as Thermoplasma. (b) The Neomuran Hypothesis indicates a common ancestry in a gram-negative bacterium followed by secondary acquisition of mitochondria and chloroplasts by serial endosymbiosis in different lineages.

diameter (Schopf & Klein 1992; Knoll 1994) while sterane biomarkers typical of eukaryotes have been obtained from the Barney Creek Formation of northern Australia (~1.64 Ga; Summons & Walter 1990). Between 1.3 and 1 Ga, the diversity of acritarchs began to increase rapidly, to include not only simple sphaeromorphs but also megasphaeromophs larger than 200 |im and spiny forms known as acantho-

morphs (e.g. Schopf 1992; Schopf & Klein 1992; Knoll 1994; Chapter 9). The enigmatic tetrad form Eotetrahedrion (Fig. 7.2c) also appears in this interval as did the red algae (Fig. 7.2d; Butterfield et al. 1990), and according to recent rRNA sequence data this was followed by a major eukaryotic radiation which is thought to have involved ciliates, brown algae, green algae, plants, fungi and animals.

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87Sr/86Sr 0.704 0.708

Tectonic events

813C%o (PDB) Glaciations

87Sr/86Sr 0.704 0.708

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—Earliest red algae, brown algae Eukaroytes diversify

—Earliest red algae, brown algae Eukaroytes diversify

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-5 o 5 10 Carbon isotopes

Biological evolution

-5 o 5 10 Carbon isotopes

Biological evolution

co co

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Fig. 7.2 Summary of evolutionary and geochemical changes through the Early Proterozoic to Cambrian. (Adapted from Brasier 2000 and sources therein.)

Cellular differentiation, root-like structures and the presence of nucleus-like spots are arguable indications of eukaryotic organization in Neoproterozoic chert microfloras (Fig. 7.3). There is controversial evidence for vegetative reproduction and sexual reproduction (meiotic spore tetrads, Fig. 7.3b,c) in the Bitter Springs Chert (about 800 Ma). The existence of branched cells like those of siphonalean green algae

(Fig. 7.3e) suggests sexual reproduction had evolved by this time.

The sexual revolution

The appearance of sexual reproduction, the exchange of genes to form new genetic recombinations, has many evolutionary advantages over asexual reproduction. For

Fig. 7.3 Early fossil 'eukaryotes'. (a) The carbonaceous film 'Helmmthoidkhmtes'(=Grypama) meeki from the Greyson Shale, Montana. Scale bar = 2 mm. (b) Sequence of Precambrian fossils claimed to indicate mitosis in Glenobotrydion. Scale bar = 10 |m. (c) Tetrad of Precambrian Eotetrahedrion. Scale bar = 10 |lm. (d) Precambrian Eosphaera. Scale bar = 10 |lm. (e) Precambrian siphonalean-like filament. Scale bar = 10 |m. ((a) and (b) based on Schopf 1972; (c) and (d) based on Cloud 1976.)

Fig. 7.3 Early fossil 'eukaryotes'. (a) The carbonaceous film 'Helmmthoidkhmtes'(=Grypama) meeki from the Greyson Shale, Montana. Scale bar = 2 mm. (b) Sequence of Precambrian fossils claimed to indicate mitosis in Glenobotrydion. Scale bar = 10 |m. (c) Tetrad of Precambrian Eotetrahedrion. Scale bar = 10 |lm. (d) Precambrian Eosphaera. Scale bar = 10 |lm. (e) Precambrian siphonalean-like filament. Scale bar = 10 |m. ((a) and (b) based on Schopf 1972; (c) and (d) based on Cloud 1976.)

example, 10 genetic mutations in an asexual population can result in only 11 genotypes, the original type plus those of the 10 mutants. The same number of mutations in a primitive diploid sexual population could be combined to produce up to 59,049 distinct genotypes (Schopf et al. 1973). Hence, in theory, the evolution of eukary-otic sexuality must have resulted in a prodigiously increased genetic variety of organisms, expressed by an increased rate of biological evolution in the fossil record. A plausible explanation for the explosion in the diversity of microfossils after about 1.3 Ga, therefore, is the evolution of meiosis (a reduction division of the cells) and syngamy (the fusion of gametic cells). Prior to this, primitive eukaryotes are likely to have reproduced asexually, by means of mitosis. If sexual reproduction in modern eukaryotes is a shared character that has been inherited from a single common ancestor, then it may be that the major groups of eukaryotes did not diverge much before about 1.3 Ga.

The Cambrian explosion

The biological revolution: microfossils from the Neoproterozoic-Cambrian transition

From about 600 Ma onward, marked changes began to take place in the fossil record, indicating a major revolution in the biosphere. Changes in the marine phytoplankton are heralded by dramatic changes in the diversity and composition of acritarch assemblages. Large acanthomorph acritarchs had appeared by about 1 Ga but experienced extinctions over the Varangian glacial interval (c. 600-560 Ma) and again during the Early Ediacarian, before the appearance of the Ediacara fauna. A new more diverse assemblage of small acanthomorphs, including Cymatiosphaera, appeared just above the Precambrian-Cambrian boundary, after which acritarch groups diversified dramatically (Vidal & Moczydlowska 1997). These changes in the marine phytoplankton were coincident with the appearance of the first phosphatized animal embryos at c. 580 Ma (Fig. 7.4a), with the Ediacara biota (impressions of large, soft-bodied multicellular animals or possibly of giant protists; see Seilacher et al. 2003; Brasier & Antcliffe 2004) after c. 575 Ma, with the first unequivocal animal trace fossils close to 555 Ma, and the appearance of diverse assemblages of small shelly fossils close to the Precambrian-Cambrian boundary at 543 Ma.

Most of the earliest skeletal microfossils are only a few millimetres in diameter and have to be studied using micropalaeontological techniques. Cloudina is a small, irregularly curved tube with a double layered CaCO3 wall of stacked half rings. It occurs with Ediacara fauna in Namibia in rocks dated between 550 and 543 Myr and was possibly made by a sedentary, suspension-

Platysolenites

feeding worm living in shallow algal mounds on carbonate platforms. Siliceous biomineralization was also beginning offshore at this time, where the remains of both hexactinellid sponges (Fig. 7.41) and demo-sponge microfossils have been found in phosphatic and siliceous facies (e.g. Brasier et al. 1997).

The base of the Cambrian is marked by the appearance of a complexly branching trace fossil, Treptichnus (or Phycodes) pedum (Brasier et al. 1994). This is accompanied in places by small shelly fossils typical of the Nemakit-Daldynian Stage at the base of the Cambrian System. Platysolenites is an agglutinated tube, now believed to be the earliest foraminiferid test (Mcllroy et al. 2001). Anabarites is a small tapering CaCO3 tube with a three-lobed cross section, commonly preserved as phosphatic iternal moulds (Fig. 7.4 o, p). This was probably the skeleton of a sedentary, suspension-feeding cnidarian, related to corals and jellyfish. Micromolluscs such as Latouchella also made their appearance in this stage. Latouchella has a planispirally coiled, flared and bilaterally compressed shell with strong transverse ribs (Fig. 7.4 g). The seven-rayed calcareous spicules of Chancelloria (Fig. 7.4 j) differed from those of sponges in being hollow and articulated and were formed by an animal of unknown biology. Later examples, such as Allonnia, have a reduced number of rays (Fig. 7.4 k). Protohertzina is a small, phosphatic protoconodont (Fig. 7.4 b, e, f; see Chapter 21) and is thought to have been part of the feeding apparatus of a predatory, pelagic invertebrate resembling a modern chaetognath worm. Other tooth-shaped objects are also found but are of unknown affinity (e.g. Maldeotaia, Fig. 7.4 c, d).

The Tommotian Stage marks a further step in the Cambrian radiation, with the appearance of archaeo-cyathan sponges, inarticulate brachiopods and a range of possibly related small shelly fossils known as tommotiids. These appear to have had a multielement skeleton of sclerites, which may show right- or left-handed symmetry and symmetry transition series (see Qian & Bengtson 1989) as, for example, in the saddle-

shaped phosphatic sclerite of Camenella (Fig. 7.4 m). Helically coiled microgastropods such as Aldanella (Fig. 7.4 a) are particularly characteristic of the Tom-motian stage, while snails with more rapidly expanding whorls, such as Pelagiella, appeared in the following Atdabanian Stage (Fig. 7.4 h).

The Atdabanian is notable for the appearance of arthropod skeletal remains, including not only tri-lobites but also the first bradoriids (see Chapter 20). The elaborately sculptured phosphatic nets of Micro-dictyon (Fig. 7.4 n) appear widely at this time. They appear to have been part of the dorsal skeleton of the caterpillar-like onycophoran arthropod formerly called Hallucigenia. A decline took place in the diversity of small shelly fossils during the succeeding Botomian Stage, which also brought about the first well-documented extinction of major reef-building ecosystems. This extinction coincided with a major episode of transgression, which brought anoxic waters onto the shelves (Brasier 1995; Wood & Zhuravlev 1995). The evolutionary trends and stratigraphic utility of these earliest skeletal microfossils are comprehensively reviewed in Brasier (1989).

Rifting of major supercontinents after ~580 Ma was accompanied by an episodic and prolonged rise in sea level through to the end of the Cambrian (Brasier & Lindsay 1998). These changes brought oxygen-depleted and nutrient-enriched oceanic waters over drowning platforms (Brasier 1995; Wood & Zhuravlev 1995). Under these conditions phosphatization was widespread and led to the remarkable preservation of animal embryos from the Duoshantuo Formation of China (Fig. 7.5 a; Ediacarian), phosphatized molluscs from the earliest Cambrian and micro-arthropods (the Orsten microbiota) from the Upper Cambrian of Sweden (Fig. 7.5 b-d).

Whilst the first metazoans (multicelled animals) appear abruptly in the fossil record at the end of the Precambrian, some fundamental aspects of this event remain unclear. Are the metazoans a monophyletic

Fig. 7.4 (opposite) Representative early skeletal microfossils. All from Lower Cambrian except where stated: (a) from Oxford,

UK; (b, g, o from Elburz Mts, Iran; (c-f) from Lesser Himalaya, India; (h, k) from Sichuan, China; (i) from Estonia; (l) from

Gobi-Altay, Mongolia; (m) from Nuneaton, UK; (n) from Newfoundland, Canada; (j, p) from Siberia. (a) Aldanella attleborensis.

(b, e, f) Protohertzina unguliformis. (c, d) Maldeotaia bandalica. (g) Latouchella korobkovi. (h) Pelagiella emeishanensis. (i) Platysolenites antiquissimus. (j) Chancellorie lenaica. (k) Allonnia erromenosa. (l) Hexactinellid spicule from latest Proterozoic of Mongolia.

(m) Camenella baltica. (n) Microdictyon cf. effusum, width of view 0.3 mm. (o, p) Anabarites trisculatus. Scale bar = c. 100 |m unless otherwise stated.

Cambrian Precambrian Chert Fossils

Fig. 7.5 Exceptional Precambrian and Cambrian fossils preserved in calcium phosphate. Scale bars = 100 |m. (a) Fossil embryo from the Doushantuo Formation (570 ± 20 Ma), South China. (b) Hesslandona sp. from the Upper Cambrian, Orsten, Vestergötland, Sweden. (c) Microarthropod Martinssonia elongata (Müller & Walosseck) from the Upper Cambrian of Sweden. (d) Arthropod larva, dorsal view, from the Upper Cambrian of Sweden. ((a) From Xiao & Knoll 2000, figure 7(2) (with permission of the Paleontological Society); (b) from Müller 1985, plate 1, figure 8 (with permission of the Royal Society, London); (c) from Walosseck & Müller 1990, figure 6 (with permission of the Lethaia Foundation); (d) from Müller & Walosseck 1986, figure 1h (with permission of the Royal Society, Edinburgh).)

Fig. 7.5 Exceptional Precambrian and Cambrian fossils preserved in calcium phosphate. Scale bars = 100 |m. (a) Fossil embryo from the Doushantuo Formation (570 ± 20 Ma), South China. (b) Hesslandona sp. from the Upper Cambrian, Orsten, Vestergötland, Sweden. (c) Microarthropod Martinssonia elongata (Müller & Walosseck) from the Upper Cambrian of Sweden. (d) Arthropod larva, dorsal view, from the Upper Cambrian of Sweden. ((a) From Xiao & Knoll 2000, figure 7(2) (with permission of the Paleontological Society); (b) from Müller 1985, plate 1, figure 8 (with permission of the Royal Society, London); (c) from Walosseck & Müller 1990, figure 6 (with permission of the Lethaia Foundation); (d) from Müller & Walosseck 1986, figure 1h (with permission of the Royal Society, Edinburgh).)

group, that is derived from a single unicelled organism (ciliated or aciliate unicells?) or perhaps from multicellular eukaryotes? Were Late Precambrian soft-bodied organisms, so widespread in the Ediacara biota, different from those of the Early Palaeozoic and if so was there a mass extinction in the Late Precambrian? How and when did the major phyla of living animals evolve? 18S ribosomal RNA sequence data suggest the Metazoa are monophyletic whilst other methods have supported a monophyletic ancestry for at least the Eumetazoa (all animals except sponges) and that the coelenterates (Cnidaria and Ctenophora) are the sister group to all other living higher Metazoa (the Bilateria).

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  • nichole
    Where have precambrain era jellyfish fossils been found?
    2 years ago

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