Cryogenian Period

An age of planet-wide glaciations

The Cryogenian Period is the middle Neoproterozoic Era was preceded by the Tonian Period and followed by the Ediacaran Period.

Large-scale glaciation events

Snowball Earth hypothesis (Kirschvink 1992)

Glaciation extended to about 30° North and South latitudes
Snow and ice reflect most incoming sunlight back into space
Causes further glaciation, with ice covering the entire Earth, even equator
Water cycle severely restricted by ice over oceans which inhibited the normal flow of sediments into the sea
- Although, research indicates that subglacial meltwater provided oxygenation to eukaryotic organisms (Lechte et al. 2019)
Volcanism underneath the sea changed the ocean chemistry: gases could not escape
The carbon cycle was affected
- Rain washes carbon dioxide out of the atmosphere into the ground
- Carbonic acid reacts with rocks to form carbonates/bicarbonates
- Carbonates are washed into the sea and eventually deposited on the seafloor
Carbon dioxide levels increase due to volcanism which triggers a greenhouse effect; ice sheets melt rapidly and temperatures rise, perhaps reaching as high as 50 °C temporarily
An influx of minerals into oceans causes a spike in algae growth, creating food webs with more efficient nutrient and energy transfers, driving ecosystems towards larger and increasingly complex organisms (Brock et al. 2017)

Supercontinent Rodinia breaks up (~700 mya)

Rift zones opened up between Laurentia (which occupied the central position in Rodinia) and continents to both the north and south
In the north lay Ur, which was separated from Laurentia as Laurentia started to drift eastwards.
An ocean along the west coast of Laurentia becomes the Panthallasic Ocean (the proto-Pacific)
Rifts also appeared along the southern edge of Laurentia, separating it from Baltica and Atlantica (Amazonia and West Africa)
These rifts opened up the ancient Iapetus Ocean between Laurentia and Atlantica.
The increase in Earth's tectonics may have been the distal cause for the above proximate causes (Stern and Miller 2017)
Landmasses clustered around the south pole
- Super-continent Rodinia may have disrupted the flow of ocean currents, which transfer heat between the equator and the poles
Banded iron-ore formations reappear

Above: Reconstruction of Earth during "Snowball" event

Above: Testate amoebae

Acritarchs decreased dramatically
Red algae and dinoflagellates diversify
Green algae diversified
- In the marine green algae (Chlorophytes), origin of the phycoplast, a structure that forms during cytokinesis,
- 750 mya: filamentous fossil (Proterocladus) attributed to siphonous green algae, Cladophorales (Butterﬁeld et al. 1994; Butterﬁeld 2000)
- Early terrestrial or glacier-bound green algae, called the anydrophytes, which had adaptations to drought conditions
  - There would have had a morphology similar to Coleochaete
- Earliest true plants, called embryophytes, probably arose from these anydrophytes during the mid- to late Cryogenian (Žárský et al., 2022)
Derived fungi lose their flagella (e.g. Basidiomycetes, Ascomycetes)
- Symbiosis between green algae and fungi, creating lichens, was probably formed by this time
Sponges appear in the fossil record around 760 mya (Brain et al. 2012)
Testate amoebae found; first evidence of heterotrophic eukaryotes