Ediacaran Period

Abstract

Keywords: stratigraphy, Ediacaran Period, Ediacaran biota, fossil record, evolution, extinction

Introduction

The name ‘Ediacaran,’ in its geochronologic sense, provides the uppermost subunit of the Precambrian Eon, approximately 635 to 541 Ma (Cohen et al. 2015), with a stratotype in South Australia. Confusingly, the same term is also used in a biogenic sense, and in two different ways: Many authors apply the term ‘Ediacaran’ to any Ediacaran-age macrofossil, whereas others restrict the term to the unique and distinctive assemblage of enigmatic organisms best known from the Ediacara Hills of South Australia.

The base of the Ediacaran marks the end of the Marinoan glaciation, the last of the truly massive global glaciations of the middle Neoproterozoic, characterised by worldwide perturbations in carbon isotopes and which were immediately overlain by unique “cap carbonates” precipitated during the recovery from the glaciations. Geochemical evidence indicates increasing oxygenation of the deep ocean environment. The Ediacaran Period also marks a pivotal position in the history of life, between the microscopic assemblages that had dominated the classic “Precambrian” and the large, complex, commonly shelly animals that dominate the Cambrian and younger Phanerozoic periods” (after Gradstein et al. 2012, p. 431).

Stratigraphy

Type Section/Sections

Lower (Cryogenian-Ediacaran) Boundary

The late Precambrian was punctuated by a number of profound ice ages – the so-called “snowball” events – which produced widespread continental glaciations, even though much of the continental landmass of the time is thought to have been clustered at near-equatorial latitudes. More controversially, it has been proposed that the seas were also frozen over. The last two of these ice ages were the older and truly severe Varanger/Marinoan Glaciation, and the younger, somewhat milder, Gaskiers Glaciation which was immediately followed by the appearance of the largely soft-bodied Ediacara biota (Gradstein et al. 2012, p. 413).

After the usual protracted debates, it was agreed the base of the Ediacaran Period should be placed at the top of the Varanger/Marinoan glacial deposits, and at the base of the immediately overlying “cap-dolostone” carbonate rocks which were deposited during the climatic recovery from the ice-age.

The GSSP for the base of the Edicaran System is located in Enorama Creek, South Australia, at the base of the 6 m thick Nuccaleena Dolomite.

Strikingly similar “cap carbonates” or “cap dolomites” occur on the top of Marinoan glacial deposits (or an unconformity surface corresponding to this glaciation) worldwide, and serve as a superb global lithostratigraphic and chemostratigraphic marker for the base of the Ediacaran (after Gradstein et al. 2012, p. 415).

For further reading, see Knoll et al. 2004, 2006.

Upper (Ediacaran-Cambrian) Boundary

Since 1947, when H.E. Wheeler initiated debate with the suggestion that the Precambrian-Cambrian boundary should be based upon the first appearance of trilobites, much has ensued. Progress has largely been facilitated by the International Geological Congress (IGC) and the establishment in 1960 of a Subcommission on Cambrian Stratigraphy. The classical idea of placing the boundary at an unconformity has been displaced by the search for monofacial, continuous deposition sequences across the boundary, with the view to selecting a stratotype.

The search itself produced a wealth of data from around the world – including the Palaeotethyan Belt, Siberian Platform, and England – eventually focusing upon south-east Newfoundland. In 1991 the International Subcommission on Cambrian Stratigraphy (through its Working Group on the Precambrian-Cambrian Boundary) made the official decision to draw the base on the Cambrian at the first appearence date (FAD) of Trichophycus pedum in the reference section at Fortune Head.

Chronology

The age range for the Ediacaran Period is given by Cohen et al. 2015 as from approximately 635 Ma to 541.0 ± 1.0 Ma.

Paleogeography

Major Tectonic Events

Land and Sea

Climate

Paleontology

Introduction

Trace fossils ...

Body fossils typically of cnidarian grade dating from as early as 600 or 610 Ma – e.g. the Twitya fossils are simple cup-shaped animals, possibly similar to the sea anemones of today.

Doushantuo Phosphate embryos

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 Dickinsonia and Kimberella 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).

The “classic” Ediacarans ...

Mineralised skeletons of uncertain affinity – the ‘small shelly fauna’ – appear just before the beginning of the Cambrian, ~550 Ma, increasing in numbers and diversity towards the Tommotian. The most common skeletal materials are calcium carbonate (aragonite or calcite) and varieties of calcium phosphate. Many of the latter may originally have been carbonates, phosphatized during preservation.

The oldest of these to occur abundantly are Cloudina and the allied genera comprising the family Cloudinidae: small, conical fossils made of calcium carbonate, first (?) appearing in the Ediacaran Stirling Quartzite of California (Langille 1974) and persisting into the Cambrian. Anabarites and Cambrotubulus are other Ediacaran ‘small shelly’ taxa, known from Siberia and Mongolia.

Major Taxa

Cyanobacteria

Acritarchs

Porifera

Twitya ?cnidarians

Olivoiides?

The “classic” Ediacarans are a diverse assemblage of enigmatic organisms. We will briefly discuss one genus, Dickinsonia, here – see my Ediacaran Biota page for more.

Mollusca – Kimberella – material to right is used above; find something more suitable for this section. ”The fossil Kimberella quadrata was originally described from late Precambrian rocks of southern Australia. Reconstructed as a jellyfish, it was later assigned to the cubozoans (’box jellies’), and has been cited as a clear instance of an extant animal lineage present before the Cambrian. Until recently, Kimberella was known only from Australia, with the exception of some questionable north Indian specimens. We now have over thirty-five specimens of this fossil from the Winter Coast of the White Sea in northern Russia. Our study of the new material does not support a cnidarian affinity. We reconstruct Kimberella as a bilaterally symmetrical, benthic animal with a non-mineralized, univalved shell, resembling a mollusc in many respects. This is important evidence for the existence of large triploblastic metazoans in the Precambrian and indicates that the origin of the higher groups of protostomes lies well back in the Precambrian” (Fedonkin & Waggoner 1997, Abstract).

Arthropods may be indicated by some Ediacaran fossils such as Parvancorina minchami.

Echinodermata – Arkarua adami (Pound Subgroup, South Australia; Gehling 1987)

Cloudina and the allied genera comprising the family Cloudinidae Hahn & Pflug 1985 are small, conical fossils made of calcium carbonate. Cloudinids are the oldest shelly animal fossils to appear abundantly in the fossil record, first appearing in the Stirling Quartzite of California (Langille 1974) and persisting into the Cambrian. It is not known what kind of organism produced Cloudina.

Unfortunately, it is equally true that the relatively few body fossils known from the late Precambrian do not shed much light on the sequence of evolutionary advances that led to the famously diverse Cambrian taxa. There are a few sign-posts, however:

There can be little doubt, on the basis of trace evidence alone, that bilaterian metazoans existed from early in the Ediacaran. Although some traces are simple, rather featureless, winding trails, “others display transverse rugae and contain pellets that can be interpreted as of fecal origin. The bilaterian nature of these traces is not in dispute. Furthermore, such traces must have been made by worms, some of which had lengths measured in centimetres, with through guts, which were capable of displacing sediment during some form of peristaltic locomotion, implying a system of body wall muscles antagonized by a hydrostatic skeleton. Such worms are more complex than flatworms, which cannot create such trails and do not leave fecal strings” (Valentine 1995, p. 90). Sets of paired hypichnial ridges strongly hint at an arthropod s.l. presence.

Unfortunately, it is equally true that the relatively few body fossils known from the late Precambrian do not shed much light on the sequence of evolutionary advances that led to the famously diverse Cambrian taxa. There are a few sign-posts, however:

• Sponges are widely recognised (e.g. Nielsen 2001, pp. 30, 506-507) to be the most primitive of living metazoans, occupying a basal position in metazoan phylogeny, as a sister group to all other Metazoa. Thus their first occurrence in the fossil record is a metric of particular interest. However, only rare occurrences of Precambrian sponges have been reported. The earliest record is of presumed sponge remains from the Doushantuo phosphates, dated around 570 Ma (Li et al. 1998), and the earliest described species is Paleophragmodictya reticulata from the ?555 Ma Ediacara locality. However, sponges could have occurred earlier and not been recognised; spicules are not necessarily diagnostic, even in living sponges (Dr. Allen Collins, pers. comm.)
• Fossils of the Twitya Formation are generally presumed to be cnidarians, or at least as metazoans of cnidarian grade. “Interpretation as colonial aggregates of prokaryotes (e.g. Nostoc-like balls) is possible but is difficult to reconcile with the morphology and relatively high relief of the remains, their occurrence at the bottom of turbidite beds, and the lack of a carbonaceous film outlining them, particularly in view of the of the fact that carbonaceous compressions are present in the formation” (Hofmann et al. 1990, p. 1202). Of principal significance is this occurrence of cnidarian-grade metazoans in pre-Varanger sediments, since the Varanger glaciation is sometimes cited as an evolutionary ‘bottleneck’ which arrested metazoan evolution.
• In preserving evidence of bilaterians, the Ediacaran record provides constraints on the protostome-deuterostome split. If Kimberella is indeed a mollusc, as suggested by Fedonkin & Waggoner 1997, or if the Ediacara/Zimnie Gory traces are correctly interpreted as radula scratches, we have evidence for derived protostomes at 555 Ma. Similarly, if Arkarua adami (from the Pound Subgroup, South Australia; Gehling 1987) is correctly interpreted as an echinoderm, we have evidence for a derived deuterostome of similar age. In either case, it follows that the P-D split must have occurred well before 555 Ma, which is in accordance with most ‘molecular clock’ studies.

There can be little doubt, on the basis of trace evidence alone, that bilaterian metazoans existed from early in the Ediacaran. Although some traces are simple, rather featureless, winding trails, “others display transverse rugae and contain pellets that can be interpreted as of fecal origin. The bilaterian nature of these traces is not in dispute. Furthermore, such traces must have been made by worms, some of which had lengths measured in centimetres, with through guts, which were capable of displacing sediment during some form of peristaltic locomotion, implying a system of body wall muscles antagonized by a hydrostatic skeleton. Such worms are more complex than flatworms, which cannot create such trails and do not leave fecal strings” (Valentine 1995, p. 90). Sets of paired hypichnial ridges may hint at an arthropod s.l. presence, although the enigmatic Yilingia spiciformis is apparently not an arthropod (Chen et al. 2019).

Lagerstätten

New Zealand Occurrences

Precambrian rocks are almost unknown in New Zealand. Possibly the most likely contender is parts of the Balloon Formation, which is exposed in the Cobb Valley area: see Cooper & Grindley 1982, p. 50.

References

Chen, Z.; Zhou, C.; Yuan, X.; Xiao, S. 2019: Death march of a segmented and trilobate bilaterian elucidates early animal evolution. Nature Letters 573: 412-415.

Cohen, K.M.; Finney, S.C.; Gibbard, P.L.; Fan, J.X. 2015: The ICS international chronostratigraphic chart v 2015/01. Episodes 36: 199-204.

Cooper, R.A.; Grindley, G.W. 1982: Late Proterozoic to Devonian sequences of southeastern Australia, Antarctica and New Zealand and their correlation. Geological Society of Australia Special Publication 9: 1-103.

Fedonkin, M.A.; Waggoner, B. M. 1997: The Late Precambrian Fossil Kimberella is a Mollusc-Like Bilaterian Organism. Nature 388: 868-871.

Gehling, J.G. 1987: Earliest known echinoderm – a new Ediacaran fossil from the Pound Subgroup of South Australia. Alcheringa 11: 337-345.

Gradstein, F.M.; Ogg, J.G.; Schmitz, M.D.; Ogg, G.M. 2012: The Geologic Time Scale 2012. Elsevier 1-2.

Hofmann, H.J.; Narbonne, G.M.; Aitken, J.D. 1990: Ediacaran remains from intertillite beds in northwestern Canada. Geology 18: 1199-1202. Geology.

Knoll, A.H.; Walter, M.R.; Narbonne, G.M.; Christie-Blick, N. 2004: A new period for the geologic time scale. Science 305: 621-622. Science.

— 2006: The Ediacaran Period: A new addition to the geologic time scale. Lethaia 39: 13-30.

Langille, G.B. 1974: Problematic Calcareous Fossils from the Stirling Quartzite, Funeral Mountains, Inyo County, California. Geological Society of America Abstracts with Programs 6: 204-205.

Li, C.; Chen, J.; Hua, T. 1998: Precambrian Sponges with Cellular Structures. Science 279: 879-882.

Nielsen, C. 2001: Animal evolution: Interrelationships of the living phyla (second edition). Oxford University Press: 1-378.

Rogers, J.J.W. 1996: A history of the continents in the past three billion years. Journal of Geology 104: 91-107.

Valentine, J.W. 1995: Late Precambrian bilaterians: Grades and clades. In Fitch, W.M.; Ayala, F.J. 1995: Tempo and mode in evolution: Genetics and paleontology 50 years after Simpson. National Academy of Sciences: 87-107.

Wei, G.; Ling, H.; Shields, G.A.; Chen, T.; Lechte, M.; Chen, X.; Qiu, C.; Lei, H.; Zhu, M. 2019: Long-term evolution of terrestrial inputs from the Ediacaran to early Cambrian: Clues from Nd isotopes in shallow-marine carbonates, South China. Palaeogeography, Palaeoclimatology, Palaeoecology 535: 109367.