Arthropods may be indicated by some Ediacaran fossils such as .

The “classic” Ediacarans ...

Table 1: Taxonomic outline of some Ediacaran forms.

Claims of “oldest Ediacaran” require the fossils to pre-date the Varanger-Marinoan ice age, approximately 635 Ma, which may have been a time of widespread extinction. “Late Proterozoic carbon isotopic profiles display strong negative as well as positive excursions. Negative excursions are specifically associated with the major ice ages that mark immediately pre-Ediacaran time. Much research is currently focused on this unusual coupling of climate and biogeochemistry, and both paleoceanographic models and clustered phytoplankton extinctions suggest that these ice ages had a severe impact on the biota – potentially applying brakes to early animal evolution” (Knoll & Carroll 1999). Although presumed body fossils, such as the Twitya assemblage occur earlier, all of the diverse Ediacaran fossil assemblages post-date the Varanger-Marinoan ice ages.

McMenamin, M.A.S. 1986: The garden of Ediacara. Palaios 1: 178-182.

Study of among fossils of Ediacaran Fauna in a White Sea site in Russia has found what appeared to be a group of 4 individuals, all the same size and shape, 3 of which were impressions of one side but the 4th was an imprint of the other side, prompted the suggestion that 3 of the impressions were made by the animal as it flopped around on the sea floor, the 4th being an impression of the actual organism as it died. Study of the Ediacaran Hills site has turned up the same pattern. The fact that these largish animals, up to about 50 cm long, could apparently move enough to flop around on the sea bed indicates that they had already developed musculature that was obviously capable of being operated in an organised manner requiring some level of nervous system. This leads to the conclusion that they were apparently more complex than was originally believed.

— 1998: The garden of Ediacara. Columbia: 1-295.

By contrast, “Ediacaran glacial deposits are rare and typically isolated, despite their generally more polar positions”. Best known is the Gaskiers Formation from Newfoundland which is dated at 584 to 582 Ma, which is broadly similar to the older Cryogenian glacial deposits in many ways, including the presence of localised cap-carbonates. However, there is gathering evidence that there may have been more than one glaciation within the Ediacaran Period. (After Gradstein et al. 2012, p. 422.)

The Cambrian "explosion" and the Burgess Shale

The Ediacaran Period (635 to 541 Ma) marks a pivotal position in the history of life, between the microscopic, largely prokaryotic assemblages that had dominated the classic “Precambrian” and the large, complex, and commonly shelly animals that dominated the Cambrian and younger Phanerozoic periods. Diverse large spiny acritarchs and simple animal embryos occur immediately above the base of the Ediacaran and range through at least the lower half of the Ediacaran (after Gradstein et al. 2012, p. 413).

Ediacaran (Vendian) Fauna - Species list - …

material from the Ediacaran Hills site in South Australia in the Natural History Museum, Oxford, has found that all the characteristics of the can be explained by the hypothesis that its body was based on a simple hydraulic system, such as that found in coelenterates where movement is achieved by changes in turgor pressure in the body

Largest Ediacaran discs from the Jodhpur Sandstone, …

It has been suggested that the Varanger-Marinoan ice ages which immediately preceded the Ediacaran, from approximately 655 to ~635 Ma (Gradstein et al. 2012, p. 401), were “snowball” events in which glaciation extended to very low latitudes; possibly right to the equator. It may have been a time of widespread extinction, a contention based mainly on carbon isotopic profiles, which display strong negative excursions.

The Cambrian "explosion" and the Burgess Shale

The mid-Ediacaran Gaskiers glaciation (584 to 582 Ma) was almost immediately followed by the appearance of the Avalon assemblage of the largely soft-bodied Ediacara biota (579 Ma). The earliest abundant bilaterian burrows and impressions (555 Ma) and calcified animals (550 Ma) appear towards the end of the Ediacaran Period. Ediacara-type fossils are centimeter- to meter-scale impressions of soft-bodied organisms that typically were preserved at the bases of event beds of sand or volcanic ash. The affinities of the Ediacara biota are contentious - some groups such as the rangeomorphs and erniettomorphs may not be ancestral to any Phanerozoic or living life forms, whereas other forms such as and preserving evidence of locomotion and feeding arguably represent stem-group animals. A few possible Ediacaran precursors and Ediacaran survivors are known, but in general Ediacara-type fossil impressions are strictly restricted to the upper half of the Ediacaran System (after Gradstein et al. 2012, p. 413, 415).