The Neoproterozoic Era has recently been divided into three Periods: Tonian ("stretching", after continued expansion of the platform covers": 1000 - 850 Ma), Cryogenian ("ice origins", after the Snowball Earth glaciations: 850 - 630 Ma), and Ediacaran (After Ediacara Springs, Australia: 630 - 542 Ma). The boundaries of the Tonian are arbitrarily defined, while the uppermost boundary of the Cryogenian is the the end of the Marinoan glaciation (and hence may be shifted to about 635.5 Ma pending further revisions), and the uppermost boundary of the Ediacaran is the first appearance of the trace fossil Trichophycus pedum (and thus the oldest biostratigraphically-determined boundary).

Climates of the Proterozoic

Good evidence of rising levels of oxygen in atmosphere (and hence oceans):

• BIFs may require extremely low levels of oxygen, but clearly cannot form at higher levels: their presence in Archean and early Paleoproterozoic and almost total absence after 2.0 Ga suggests that levels had risen too high by 2.0 Ga
• Uranite and pyrite are unstable at surface temperatures and pressures in presence of atmospheric oxygen; disappear by 2.3 Ga
• Red beds require oxygen to form: do not appear until 2.0-1.8 Ga
• Stromatolites (and thus potential oxygenators of atmosphere) common throughout Proterozoic

During Paleoproterozoic, good evidence of widespread glaciation (tillites, striations, carbon shifts).
During Neoproterozoic (in particular, from c. 750-580 Ma), evidence for superglaciations:

• Carbon isotope shifts far greater than seen during Phanerozoic glaciations
• Tillites, striations, etc. in paleoequatorial regions

Strong evidence for a Neoproterozoic Snowball Earth

• Break-up of Rodinia leaves many smaller continents near equator
• Climate change due to new weather patterns: lots of carbon dioxide sucked out of atmosphere due to erosion
• Temperature drops due to reduced greenhouse, pack ice builds up on poles and continental glaciers in mountains
• Positive feedback loop: ice increases albedo, drops temperature, ice spreads, albedo increases, etc.
• Pole-to-equator-to-pole glaciation:
  • Hydrologic cycle stops (and thus erosion stops)
  • Ice thickens to 100s of m to 1 km, so photic zone forbidden to life; oceans become anoxic (reappearance of BIFs)
• Life clings to refuges in ice cracks & along mid-ocean ridges
• Glaciers cannot build up after end of hydrologic cycle (no new snow!), and gradual creep off of continents
• Glacial conditions persist until slow (10 Myr or so?) build up of carbon dioxide due to volcanic activity to form a supergreenhouse (many MANY times modern levels)
• EXTREMELY fast melting, superhot world, massive rapid deposition of carbonates, massive blooms of organisms
• Cycle seems to have happened at least three (possibly four or more times in Neoproterozoic: the Sturtian (~720 Ma), Marinoan (~636 Ma), and Gaskiers (~580 Ma) are the best studied and established, but there may be others.

The reason Neoproterozoic superglaciations stop and haven't been seen again may be due to the rise of complex animals, which were able to liberate the carbon in the sediment through bottom-feeding (previously was simply trapped in sediment). However, another factor may be the ever-increasing brightness of the Sun.

Proterozoic Life: Stromatolites still common, becoming even more common c. 2.2 Ga, becoming more complex c. 1.2 Ga.

Origin of eukaryotes by endosymbiosis:

• Some large prokaryote (almost certainly an archaean) has close association (symbiosis) with a smaller bacterium (specifically a proteobacterium): the latter provides chemical energy as a byproduct of its metabolism
• The larger prokaryote absorbs the smaller bacterium into its cell: the smaller proteobacterium is the ancestor of mitochondria
  • Presence of mitochondria allows for more energetic heterotrophic life style
• In one branch of new eukaryotes (the ancestor of plants), a second case of endosymbiosis occurs when cyanobacteria become absorbed into the cell: origin of chloroplasts
• Eukaryotic fossils date back to about 1.68-1.78 Ga (chemical signatures from 2.7 Ga or so, but these may be the prokaryote ancestors of eukaryotes)

Eukaryotes remain unicellular for most of the Proterozoic. Life remains entirely aquatic, but food chains get more complex with diversifying levels of heterotroph consumers and detritivores and phototroph producers.

Fossils from Bitter Springs Formation of Australia (1 Ga) looks as if they show cells dividing.

Oldest known acritarchs (fossils of uncertain origin: some likely cysts of some kind of fossil photosynthesizers, possibly dinoflagellates, others are eggs of animals) at 1.4 Ga; become complex around 600 Ma; the main index fossils for the Proterozoic

Metabionts: multicellular organisms include:

• Rhodophyta (red algae, some make calcareous skeletons): may form a larger group Primoplantae with the viridiplants (green plants) and the unicellular glaucophytes
• Phaeophyta (brown algae or kelp; a multicellular group within Chromista, and thus relatives of diatoms and coccolithophorids)
• Viridiplantae, or green algae and plants: basal forms are unicellular, but paraphyletic Charophyta (one of the green algae) and Plantae or Embryophyta (true plants) are multicellular
• Fungi: together with animals and various unicellular groups form larger clade Opisthokonta
• Animalia: closest relatives are unicellular choanoflagellates; animal phylogenetic interrelationships are a matter of a LOT of study!

Some of these groups have records back to the Proterozoic:

• Possible rhodophytes from 1.25 Ga, but only clearly present from Cambrian
• Possible phaeophytes from Ediacaran Period but not particularly convincing: better possible Ordovician forms, and oldest definite kelps not until Cenozoic
• Possible charophytes from Neoproterozoic, but not definite until Cambrian (true plants do not show up until Ordovician)
• Fungi not clearly present until Ordovician: no strong argument for multicellular fungi in Proterozoic (unless the vendobionts are lichens)

Animal record is somewhat better, but still much debate:

• Molecular data suggests many true animal lineages had diverged at around 1 Ga
• Some controversial trace fossils from c. 800 Ma: only animals (with muscles) can move sediment around
• Possible animal-generated trace fossils by 570 Ma (but might be generated by giant single-celled eukaryotes!!)
• Phosphatized clusters of cells from Doushantou Fm., China from 600 Ma were thought to be animal embryos and eggs, but new evidence from later 2011 show these are most likely protist-grade eukaryotes rather than animals
• Tiny (1 mm or smaller) calcified shells:
  • cone-shaped Cloudina
  • spheroidal Namacalathus, a possible sponge (or sponge-like) suspension feeder
• The Ediacaran fauna (from Ediacara Hills, Australia, and elsewhere from c. 570 Ma onward)

The Ediacaran fauna is preserved only as impressions in sediment. Indicate a variety of organisms. Originally were pigeon-holed into modern groups, but new evidence suggests that a number of types of animals are present:

• Early cnidarians (members of the coral-sea anemone-jellyfish group), large polyps with their butts in the sand
• "Vendobionts"
  • A unique (and extinct) radiation of multicellular forms with a quilt-like body construction: see here and here and here.
  • May have fed by chemosynthesis (there is no evidence for mouths or anus in these creatures)
  • Some up to 1 m or more across, but thinner than a slice of bread
  • Recent work suggests that there are several branches of these, some outside of true Metazoa, some within Metazoa
• Kimberella: apparently an early mollusk or mollusk-relative; known from impressions of its foot and probable grazing marks

Interestingly, almost all the evidence for multicellular organisms comes AFTER the Gaskiers Glaciation. Some speculate that selective pressures from these hard times led to development of complex life; others that the appearance of creatures able to mobilize the carbon in the sea sediments kept atmospheric carbon dioxide from getting low enough to trigger the super ice ages.

Recent studies suggest that the size (and complexity) increases seen with the rise of endosymbiosis and of multicellularity both coincide with major increases in the amount of oxygen available.

To Next Lecture.
To Previous Lecture.
To Syllabus.

Last modified: 19 January 2012