Ediacaran Assemblage

Abstract

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).

Keywords: Ediacaran, Vendian, Ediacaran fossils

Introduction

The further we look back in time, the less familiar the fossils are. In the infancy of biostratigraphy, a modified form of this principle was once used to coarsely date rocks. As far back as the Ordovician, most of the organisms we find as fossils as easily assigned to modern lineages. (Many modern groups, notably terrestrial plants and animals, are missing from Ordovician rocks – they had not evolved yet – but those that are present almost always have an obvious home among the living.) Further back in time, in the Cambrian, we find many more problematic forms. Even among the creatures that are obviously arthropods, many of them bear little resemblance to any modern kinds. Further back in time still, in the Ediacaran Period, the fossils become virtually unrecognisable. Although some few can be assigned to modern phyla, and sometimes even lower (i.e., more specific) taxonomic ranks, the affinities of far more of them are controversial. Many are so difficult that they are not assigned to modern groups at all, but instead are placed in “form taxa” which group things together purely on the basis of what they look like.

For some years a number of authors (e.g. Seilacher 1984, Seilacher 1989, McMenamin 1986) have argued that the Ediacarans were were not just difficult to classify, but were genuinely unrelated to any living group of organisms; that they represented a new kingdom (Vendobionta Seilacher 1992) which disappeared around the Ediacaran-Cambrian boundary, perhaps wiped out by a mass extinction event. However, this view has always encountered opposition and now appears to have lost much of its support.

The name ‘Ediacaran’ has a geochronologic meaning, as the period immediately before the Cambrian, approximately 635 to 538.8 Ma (Gradstein et al. 2020), 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’ in a broad sense to any Ediacaran age macrofossil, whereas others restrict the term narrowly to the unique and distinctive assemblage of forms with unknown affinities. (MacGabhann 2024 tries to develop this small confusion into a reason to eschew the term ‘Ediacaran biota’ and variants, altogether. Although most or all of the points he makes in support of this proposal are inarguable, it is an over-reaction to a non-problem.)

The first of these enigmatic fossils was described from the Fermeuse Formation on the Avalon Peninsula – specifically, Aspidella terranovica was named – by E. Billings in 1872. A second assemblage was described from Namibia sixty years later (Gürich 1933). Nevertheless, the assemblage acquired its name from a third and even later discovery, made by Reginald Sprigg in March 1946, at an abandoned copper/lead/zinc mine in the Ediacara Hills, Flinders Range, north of Adelaide in South Australia (Sprigg 1949). Since then, occurrences have been located on most continents.

The accumulation of increasingly accurate dates for the Ediacaran forms has allowed three broad assemblages to be recognised: the Avalon assemblage (571 to 560 Ma), White Sea assemblage (560 to 550 Ma), and Nama assemblage (550 to 538 Ma). Each of these exhibits some innovation which is interpreted as a significant evolutionary development (after Gradstein et al. 2020, vol. 1, p. 528-529).

The assemblage comprises marine life forms first appearing in latest Precambrian times – about 579 Ma, placing them among the oldest multicellular fossils known – and persisting into the basal Cambrian. The Ediacaran hey-day predates by a distinct interval of perhaps 20 Ma or more, the so-called ‘Cambrian Explosion’ when ‘modern’ multicellular life began to diversify rapidly.

Chronology

Some treatments (perhaps inadvertently including this one) tend to consider “the Ediacarans” as a single biota. In fact, however, various localities and taxa that have been labelled “Ediacaran” span both the globe and more than 30 million years of time; they are not a single unit. “Patterns of taxonomic, morphological, and ecospace diversity change dramatically during the Ediacaran..., which has led to the suggestion of several evolutionary radiations” (Eden et al. 2022).

The oldest characteristic Ediacaran fossils are those of the Drook Formation from south-eastern Newfoundland, believed to date from around 575 Ma, but the oldest of the ‘classical’ localities is the 565 Ma occurence at Mistaken Point, Newfoundland. Youngest of the classical localities is in Namibia, where Ediacarans co-occur with small shelly faunas and range up to the Ediacaran-Cambrian boundary (541 Ma; see Grotzinger et al. 1995, Martin et al. 2000, Gradstein et al. 2012). In round numbers, the “Ediacaran biology” occupied the last 38 million years or so of the Proterozoic Eon.

However, there are contenders to push these boundaries in both directions. One might even argue that the temporal range of the Ediacarans sensu ampla essentially equates to the ~100 Ma range of Nimbia occlusa. The Twitya fossils (pre-Varanger) are the oldest fauna which have been termed ‘Ediacaran,’ and Booley Bay (Late Cambrian) the youngest: both include forms assigned to Nimbia occlusa. With the single exception of the Twitya Formation occurrence, all appear to post-date the last Varangian glaciation.

First (pre-Varanger) Appearances

Simple disc-like impressions, interpreted as cnidarian-grade body fossils, from the intertillite beds of the >635 Ma (Xiao & Laflamme 2008, p. 32) Twitya Formation in the Mackenzie Mountains, north-western Canada (Hofmann et al. 1990), include the “Ediacaran taxon” Nimbia occlusa and are sometimes described as “Ediacarans.” They are simple, circular impressions and, while it is true that many taxa from the classic Ediacaran localities are little more, the Twitya assemblage as a whole does not exhibit the morphological diversity nor the complexity of the classical Ediacaran assemblages. Unless one regards the Ediacaran assemblage as simply a hold-all for any Ediacaran-aged macrofossil, the Twitya fossils do not belong here.

In a report of claimed Ediacaran taxa and associated trace fossils from the Clemente Formation of north-western Sonora, Mexico, Mark McMenamin (1996, p. 4990) rejects the authenticity of the Twitya assemblage and explicitly lays claim to having found “the oldest known remains of the Ediacaran biota” himself (see Fig. 1). McMenamin appears committed to this interpretation (e.g. 1998, p. 204-207) though neither his assertions for the age of the material (”600 million years or more”) nor the biological affinities of his putative fossils have been embraced with any enthusiasm by others. The proposed age, in particular, is based upon an extremely tenuous chain of reasoning, all under-pinned by the supposed stratigraphic range of a single, poorly-documented, ichnotaxon, Vermiforma antiqua (McMenamin 1996, p. 4993), which may, in fact, turn out to be a tectonic artefact (Meschede et al. 2000). Thus the Sonora material cannot be seriously considered without additional corroboration and is not further discussed here.

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 “classical” Ediacaran fossil assemblages post-date the Varanger-Marinoan ice ages.

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Fig. 1: McMenamin’s form “cf. Cyclomedusa plana Glaessner and Wade” (= Aspidella terranovica Billings 1872) from Sonora, Mexico: “A discoid fossil preserved in hyporelief. Note annular ridge occurrence at the margin (arrowhead) of the central cone. Greatest dimension of rock specimen is 6.0 cm. Sample 1 of 3/17/95; fossil occurrence is approximately 75 meters below the Clemente Formation oolite, in unit 1 of the Clemente Formation”. [Reproduction of fig. 2A from McMenamin 1996.]

Fig. 2: Cyclomedusa sp. from the Winter Coast of the White Sea. This specimen is about 5 cm across. [Image courtesy of University of California Museum of Paleontology.]

Avalon assemblage (579 to 559 Ma)

The Avalon assemblage (579 to 559 Ma) is the oldest of the “classical” Ediacaran assemblages. It consists largely of those frond-like fossils exhibiting self-similar branching, where centimetre scale modules build decimetre- to metre-scale constructions (rangeomorphs such as Charnia) and “exhibits relatively limited ecological and morphological diversity” (Eden et al. 2022).

It is known only from deep-water deposits in Newfoundland, England, and the Sheepbed Formation of NW Canada. >/ref -f Grazhdankin et al. 2008/) showed that some long-ranging, cosmopolitan taxa of the Avalon assemblage, such as Charnia and Hiemalora) persist in deepwater deposits to the end of the Ediacaran, but these younger deep-water assemblages typically also contain Ediacaran fossils typical of the younger Ediacaran assemblages. Palaeopascichnus and other body fossil impressions consisting of serial arrangements of hollow chambers also make their first appearance at this time. In general, trace fossils and other evidence of mobility are either absent or exceedingly rare.

The oldest representatives occur approximately 150 m below an ash dated at 579 Ma in the Drook Formation of eastern Newfoundland (Narbonne & Gehling 2003) which is only 3 million years after the end of the Gaskiers glaciation. Ediacaran fossils have not been found in similarly aged shallow-water deposits, implying that these early experiments in complex megascopic life originated in deep sea settings (Narbonne 2005). The youngest example appears to be the Charnwood Forest occurrence. (After Gradstein et al. 2012, p. 417-419.)

White Sea assemblage (558 to 550 Ma)

The White Sea assemblage (558 to 550 Ma) shows “a large increase in morphological diversity [over the preceding Avalon assemblage], including putative blilaterians [Shen et al. 2008], in tandem with greater ecological diversity that includes the appearance of grazing, herbivory, and widespread motility” (Eden et al. 2022). In some contrast, Gradstein et al. (2012), p. 419, describes the association as “a depauperate [species-poor], assemblage of rangeomorph taxa, many of them holdovers from the preceding Avalon assemblage.”

New developmental plans include erniettomorphs, such as Pteridinium and Phyllozoon, which show a modular construction of soda-straw-shaped elements, and segmented forms such as Dickinsonia, some of which show polarity and possible cephalization (e.g., Spriggina, Kimberella).

The oldest abundant and reasonably unequivocal bilaterian animal burrows appear worldwide 555 million years ago, more or less coincident with the White Sea assemblage (Martin et al. 2000).

The assemblage is probably a shallow-water association, best known from the “original” Ediacara site in the Flinders Ranges of Australia and the Zimnie Gory/White Sea locality of the Urals. It is also known from Ukraine in eastern Europe and the Blueflower Formation of NW Canada is probably a deep water equivalent.

Nama assemblage (549 to 542 Ma)

The Nama assemblage (549 to 542 Ma) is a species-poor association of Ediacara-type fossil impressions, mainly rangeomorphs and erniettomorphs, mostly holdovers from the preceding assemblages, but distinguished by the appearance of the oldest biomineralising (calcified) megafossils, principally Cloudina and Namacalathus. Trace fossils include the first simple treptichnids (Jensen et al. 2000).

It also “records a decrease in taxonomic diversity.... This reduction in taxonomic diversity, sometimes referred to as the ‘diversity drop’, has been suggested to correspond to a post-White Sea extinction around 550 Ma, which eliminated the majority of Ediacaran soft-bodied organisms.... This diversity drop has been suggested to be caused by either an environmental driven catastrophic environmental extinction or biotic replacement driven extinction” (Eden et al. 2022). However, these authors prefer an ecological explanation: “metacommunity structure increased in complexity, with increased specialisation and resulting in competitive exclusion, not a catastrophic environmental disaster, leading to diversity loss in the terminal Ediacaran” (Eden et al. 2022, Abstract). They further explain that their results “show that these Ediacaran organisms underwent the niche contraction and specialisation that is traditionally associated with Cambrian diversification.... Therefore, we find that the eco-evolutionary dynamics of metazoan diversification known from the Cambrian started earlier in the Ediacaran with the Avalon assemblage and increased in complexity towards the Phanerozoic as new anatomical innovations appeared….”

The Nama assemblage is best known from shallow-water occurrences in Namibia, Oman, and the Dengying Formation of China, and also the deeper-water Khatyspyt Formation of eastern Siberia (Gradstein et al. 2012, p. 419).

Last Appearances

At the younger end of their range, “Ediacara type” fossils have been increasingly reported from Cambrian sediments. Youngest and most exciting of these are the Upper Cambrian Ediacarans from a turbidite sequence exposed at Booley Bay, near Duncannon in Co. Wexford, Ireland, which includes just two taxa: ‘Ediacaria booleyi’ (possibly yet another variant of Aspidella terranovica) and the cosmopolitan Nimbia occlusa (Crimes et al. 1995). The Booley Bay occurrence is dated by acritarchs of sufficient “diversity and quantity to constrain biostratigraphically the relative age of this succession ... to the upper part of the Upper Cambrian” (Moczydlowska & Crimes 1995, p. 125), indicating that at least some Ediacarans co-existed with ‘modern’ taxa for perhaps 20 or 30 Ma – and certainly throughout the Cambrian Explosion.

Geological Setting

Occurrences are scattered at low paleolatitudes on every continent except (so far) Antarctica. Additionally, the South American Mato Grosso occurrence from southwestern Brazil (Hahn et al. 1982) is questionable. The best known are the ‘classic’ localities in southern Namibia, the Flinders Range locality in Australia, Mistaken Point in south east Newfoundland, and on the White Sea coast of northern Russia, but there are also reported occurences in Mexico, England, Ireland, Scandinavia, Ukraine, and the Ural Mountains.

Habitat and Habit

Among the first comprehensive treatments of the Ediacarans were those of Martin Glaessner, in the 1960s. However, although we may now agree with many of his conclusions, his ideas were predicated on an incorrect interpretation of the paleoenvironment: Glaessner (1961; also Glaessner & Wade 1966; Jenkins 1981) believed the Rawnsley depositional sequence to have been semi-emergent (”sandy shoals ... [with] areas of temporary quiescent conditions between the shifting current tracks, where fine particles could settle until they were covered again by sand waves” – Glaessner & Wade 1966, p. 599-600) and the body fossil assemblage transported. Glaessner envisaged the assemblage as a mass stranding, thereby predisposing himself to accept the radial forms as ‘medusoids.’ More recently, however, Gehling (1991, 2001) has demonstrated that the South Australian fossils occur above a valley fill facies, on sandstone partings within upward-shoaling, delta-front environments between storm- and fair-weather wave base (Gehling 2001, p. 30).

The Australian fossils occur in preservational windows in the Ediacara Member of the Rawnsley Quartzite, a formation of the Pound Subgroup, bounded above by the Early Cambrian Uratanna sequence. The Rawnsley depositional sequence is developed over an erosional surface having some 250 m of relief, where southeasterly directed paleovalleys are filled with sequences of massive sandstone and laminated siltstones, passing up into up into well-bedded sandstone. The fossils occur above the valley fill facies, on sandstone partings within upward-shoaling, delta-front environments between storm- and fair-weather wave base (Gehling 2001, p. 30).

Other occurrences are now widely understood to be in situ marine assemblages, also. In particular, the Mistaken Point and nearby localities of south eastern Newfoundland are believed to preserve a deep water slope environment.

Morphologies

More than 30 different genera have been named. Ediacarans are a diverse group and earlier attempts to pidgeon-hole them into a limited number of phylogenetic types appear now to have been misguided. However, for a quick overview it may be useful to consider four broad morphological categories (after Briggs et al. 1994, p. 44), bearing in mind that these do not indicate evolutionary relationship.

1. Most abundant are circular impressions, some of which plausibly recall jellyfish and similar medusoid cnidarians, although many are so simple that convergence rather than affinity cannot be ruled out (Fig. 1, 2). Others of this form are believed to be the holdfasts of the frond-like Ediacarans (see below).
2. Next are the trace fossils of various tracks and burrows made, at least in part, by bilaterian animals. Although there is evidence of burrowing, the traces are mostly simple and more or less horizontal (but see Jensen & Runnegar 2005); typically absent is any evidence for wide-scale churning up of the sea bed by animals living in the sediment (infauna) (Conway Morris 1998a, p. 30).
3. Third most abundant are a number of problematic benthic forms. Whereas some of these seem familiar enough to suggest affinities with extant groups such as annelids (e.g. Dickinsonia and Spriggina, see Fig. 3, 4; however, also see Dzik & Ivantsov 1999 for a contrary view), echinoderms (e.g. Arkarua), or arthropods (e.g. Diplichnites and Parvancorina, Fig. 5), others such as Praecambridium, Vendia and Tribrachidium (Fig. 6) are more problematic.
4. Least abundant, though perhaps most characteristic of the assemblage as a whole, are the attached, frond-like organisms (Fig. 7), which some propose have affinities with the sea pens and other soft corals (Fig. 8).

Leaving aside the trace fossils, a typical Ediacaran of any form had a soft body – there is no evidence of any skeletal hard parts except, possibly, for the head-shield of Spriggina (Fig. 3) – yet they most commonly occur in silt- and sandstones which typically form in quite turbulent conditions: not the sort of sediments where one would ordinarily expect to find good soft tissue preservation.

Many of the forms display a morphology which has been described as “quilted.” Some researchers consider this to be a real characteristic, which indicates a phylogenetic relationship between otherwise dissimilar forms: that all the “Ediacara fossils” are members of the same high-level taxon; that they form a single clade with a single bauplan (see below).

Interestingly, it appears that Ediacaran communities were largely free of large predators; no species appears to have possessed a jaw apparatus suitable for seizing and tearing prey, and few fossils show clear evidence of predatory damage. A possible exception, however, are some Chinese Cloudina fossils with tiny boreholes, which may simply be a diagenetic effect or may truly be indicative of predation.

Phylum Ctenophora Eschscholtz 1829 Class unknown Order unknown Family Stromatoveridae Shu et al.2006  Stromatoveris Phylum Cnidaria Hatschek 1888 Class Cyclozoa Fedonkin 1983 Order unknown Family Cyclomedusidae Gureev 1987 Aspidella (incl. Cyclomedusa, Ediacaria, Spriggia, Tateana, Tirasiana, etc.) Class Hydrozoa Owen 1843 Order unknown Families unknown Eoporpita, Ovatoscutum, Wigwamiella Class Anthozoa Ehrenberg 1834 Order Pennatulacea Verrill 1865 (Subclass Octocorallia) ?Family Pteridiniidae Richter 1955 [though maybe this family belongs with the Petaliform problematica or with the Trilobozoa?] Pteridinium (but not Charniodiscus, in my view) Orders unknown Families unknown Beltanelliformis, Hiemalora, Inaria Classes unknown Orders unknown Families unknown Namapoikia Nimbia Phylum Mollusca Linnaeus 1758 Class unknown Order unknown Family unknown Kimberella Phylum Arthropoda von Siebold 1848 Class unknown Orders unknown Families unknown ?Archaeaspinus, Onega, Parvancorina, Praecambridium, Redkinia Family Sprigginidae Glaessner 1958 Spriggina Phylum Echinodermata Klein 1734 ex De Brugière 1789 Class ?Edrioasteroidea Order unknown Family unknown Arkarua Insertae sedis 1. Triradially Symmetric Taxa (= Phylum Trilobozoa Fedonkin 1985) Classes unknown Orders unknown Family unknown Triforillonia Family Albumaresidae Fedonkin 1985 Ablumares, Anafesta, Skinnera Family Anabaritidae Glaessner 1979 Anabarites Family Tribrachididae Runnegar 1992 Tribrachidium 2. ‘Metameric’ Taxa Classes unknown Orders unknown Family Dickinsonidae Harrington & Moore 1955 Dickinsonia Family Vendomiidae Keller 1976 ?Vendia, Vendomia Family Yorgiidae Ivantsov 2001 Yorgia 3. Petaliform Taxa (= Phylum Petalonamae Pflug 1972; Kingdom Vendobionta Seilacher 1992) Class Erniettamorpha Pflug 1972 Order(s) unknown Family Erniettidae Pflug 1972 Ernietta, ?Swarpuntia Family Pteridiniidae Richter 1955 [though maybe this family belongs with the Anthozoa or even the Trilobozoa?] ?Phyllozoan, Pteridinium Class Rangeomorpha Pflug 1972 Order(s) unknown Family Rangeidae Rangea Family Charniidae Charnia, ?Charniodiscus, Paracharnia 4. ‘Vermiform’ Taxa (= Phylum "Vermes" of Runnegar 1992) ?Archaeichnum, ?Cloudina, Cochlichnus, ?Didymaulichnus, Gordia, Harlaniella, Helminthoidichnites, Planolites, Sellaulichnus 5. Serial Growth Forms Neonereites, Palaeopascichnus, Yelovichnus 6. Others Ausia, Bomakiella, Bonata, Lorenzinites, ?Wigwamiella Table 1: Taxonomic outline of some Ediacaran forms. Blue = form-taxa; red = trace fossils (ichnotaxa).

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Fig. 3: Spriggina floundersi Glaessner – From the Ediacaran Pound Quartzite of the type locality, Ediacara, South Australia. Overall length about 10 cm. Specimen from the Yale collection (YPM 63257). [Image courtesy of the Yale Peabody Museum of Natural History, Yale University.]

Fig. 4: Dickinsonia costata Sprigg – Vendian, from the Brachina Gorge, Flinders Ranges, South Australia. Specimen from the Yale collection (YPM 35467). Dickinsonia has been known to reach dimensions of up to a metre. [Image courtesy of the Yale Peabody Museum of Natural History, Yale University.]

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Fig. 5: Parvancorina minchami – A candidate arthropod, possibly a trilobite (see Fortey et al. 1996). In this scenario, the central axial ridge and the strongly arched anterior ‘lobes’ may be analogous to the midgut and gastric diverticulae. The scale bar is in centimetres.
[Image and interpretation courtesy of Chris Nedin, Department of Industry, Science and Resources, Canberra.]

Fig. 6: Tribrachidium heraldicum – Few fossils of Ediacaran animals are so compellingly bizarre as this unusual disc-shaped form with three-part (triradial) symmetry. Affinities have been proposed with either the Cnidaria (corals and anemones) or Echinodermata (urchins and starfish); nor can the possibility that it is a holdfast be entirely eliminated.
[Image and interpretation courtesy of University of California Museum of Paleontology.]

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Fig. 7: Charniodiscus arboreus – One of the frond-like Ediacaran fossils considered by some to be a ‘conventional’ cnidarian, possibly a pennatulacean. Collected from the Ediacaran Member, Rawnsley Quartzite, Bunyeroo Gorge, Flinders Ranges, South Australia. Specimen from the South Australian Museum Collection (SAM P19690). Overall length 40cm.
[Image courtesy of the South Australian Museum.]

Fig. 8: Thaumaptilon walcotti Conway Morris 1993 – From the Middle Cambrian Burgess Shale. Proposed by Conway Morris as a possible pennatulacean (sea pen). However, this view is by no means universally accepted; for example, Nielsen 2001 notes (p. 59) that the branches of both Charniodiscus and Thaumaptilon “were united with a membrane which makes the interpretation dubious on functional grounds, and the structures tentatively interpreted as polyps are very small and show no tentacles.” Specimen from the US National Museum collection (USNM 468028). Overall length about 20 cm. If this animal is a descendant of Charniodiscus and its Ediacaran allies, as proposed by Conway Morris, the holdfast has been modified from the original bulky disc, possibly to facilitate withdrawal into a burrow.
[Reproduction of fig. 2E from Conway Morris 2000.]

Antecedents

Genetic evidence has been used to suggest significant metazoan diversity far pre-dating the Ediacaran fossils (e.g. Wray et al. 1996: “Calibrated rates of molecular sequence divergence were used to test this hypothesis. Seven independent data sets suggest that invertebrates diverged from chordates about a billion years ago, about twice as long ago as the Cambrian. Protostomes apparently diverged from chordates well before echinoderms, which suggests a prolonged radiation of animal phyla.”)

Other estimates (e.g. see Conway Morris 1998a, Ayala et al. 1998, Knoll & Carroll 1999) are lower, but still require the existence of some animal diversity as early as 750 Ma ago, implying that for the first 150 Ma or more they left no fossil record. (Inexplicably, Ayala et al. claim that their results are “consistent with paleontological estimates.”) The general rarity of soft-part preservation may explain this in part, but one would still expect to find some trace fossils – tracks and burrows – of any animals large enough to disturb sea-floor sediments. “Thus, if they really were present, we can be fairly sure that any pre-Cambrian animals would have been tiny, only a few millimetres long.... What later triggered their initial emergence as the Ediacaran faunas, and subsequently the even more spectacular Cambrian explosion, remains a significant topic for debate” (Conway Morris 1998a, p. 144).

At more than 635 Ma (Xiao & Laflamme 2008), circular impressions from the Twitya Formation of the Mackenzie Mountains provide evidence for the earliest macroscopic metazoans, simple cup-shaped organisms, possibly cnidarians, predating the Marinoan Glaciation. Somewhat later, around 600 Ma, and still predating any known Ediacaran assemblage, the Doushantuo phosphate deposit in China is slowly yielding a surprisingly diverse biota, including probable algae, sponges, cnidarians and bilaterians.

Affinities

Until now we have considered the ‘Ediacaran fauna’ in abstract terms, without any attempt to delimit the concept. Real difficulties stand in our way – it is genuinely difficult to map the characters of most Ediacaran fossils onto the body plans of living invertebrates; certainly there are similarities, but they are “worryingly imprecise” (Conway Morris 1998a, p. 28). Nevertheless, failure to make the attempt is no less reprehensible for having a long pedigree.

Initially, Ediacarans were interpreted in terms of extant phyla, such as cnidarians, annelids, etc. Much of this early work was completed by Martin Glaessner. Fellow Australian, Jim Gehling (1991, p. 182), reports that Glaessner left many Ediacaran forms unassigned, and in some cases undescribed, because he “refused to include new taxa in extant phyla without considered taxonomic evidence, stating that from a ‘historical perspective, failures are not phyla.’” The German paleontologist, Hans Pflug was clearly unconvinced, however, and in 1972 erected the new phylum Petalonamae to accommodate some of the frondose Ediacaran taxa (Pflug 1972).

”Vendozoa”

Through the mid-1980s to mid-1990s, Adolf Seilacher and some others further questioned assignments of Ediacaran taxa to living phyla, and even the metazoan affinities of many Ediacarans. He contended that, in spite of their apparent diversity, nearly all of the genera share a striking basic uniformity: they are thin and flattened, round or leaf-like, possess a ridged or ‘quilted’ upper surface, and lack clear indications of a mouth or gut. Seilacher believed that the Ediacaran body plan comprises tough organic walls surrounding fluid-filled internal cavities.

Seilacher 1992 recognises three groups: cryptic bilaterians which left only trace fossils, the Psammocorallia – coelenterates which utilised sand for an internal skeleton – and the ‘quilted’ Vendobionta, a concept roughly comparable to Pflug’s Petalonamae. The last of these groupings has attracted some vigorous criticism: “In proposing the separation of Ediacaran organisms from the Metazoa, Seilacher (1989) has attempted to unify them with a common constructional model. … By emphasising the untested generalisation that all [of the ‘quilted’] Ediacaran organisms were flat and constructed of tubular elements, and uncomplicated by internal organs, Seilacher (1989, fig. 2) was able to argue that the observed variation between taxa was solely based on modes of growth, involving the addition of tubular elements. … Only with a very broad brush could all Ediacaran organisms be represented as fractal growth variations based on the same units of construction” (Gehling 1991, pp. 192-193, 202).

Gehling’s last point is supported by observation of considerable variation between taxa, in the style of preservation, indicating that there were “different classes of organic construction involved in the Ediacara fauna” (Gehling & Rigby 1996, p. 185).

The idea that “the quilted Ediacarans” may comprise their own kingdom, separate from the metazoans, only ever gained sporadic support, and had largely fallen from favour even before analyses of lipid biomarkers demonstrated (almost beyond doubt) that one of the most iconic Ediacarans, Dickinsonia, was an animal (Bobrovskiy et al. 2018, Summons & Erwin 2018).

Even the less extreme proposal that most or all Ediacarans were taxonomically closely related has mostly fallen from favour – “It’s clearly not true for, say, the Burgess Shale. Why should it apply to the Ediacarans?” (Waggoner 1999) – and few adherants remain.

However, Seilacher’s argument must be seen in the context of its time; it was predicated on knowledge of far fewer Ediacarans (in general, the larger taxa) than are known today, and underpinned by the beliefs – prevalent at the time – that:

1. the Ediacarans died out completely well before the Vendian-Cambrian boundary (the Kotlin crisis);
2. a considerable age separated the disappearance of the Ediacarans and the appearance of the first calcareous ‘skeletons’ (the so-called ‘small shelly fauna,’ see below);
3. the assemblages represented mass strandings rather than in situ associations; and perhaps stemming from this,
4. that coeval trace fossils could not be attributed to the Ediacaran taxa.

However, none of these beliefs can be sustained today except, arguably, the last. Even that contention is less tenable than it was in the 1980s, when the Ediacarans were thought to be mostly large taxa. We now know that small taxa make up a large part of the assemblage; the small bilaterian forms are potentially the missing trace-makers.

Bruce Runnegar and Mikhail Fedonkin (in Schopf & Klein 1992, p. 373) were next to tackle the overall taxonomy of the assemblage. Their approach – by far the most convincing to date – combines the conservative reference of taxa to modern phyla, where such assignments are not obviously forced, with a pragmatic recognition of the large number of undeniably enigmatic forms. Probably the most significant message evident in Runnegar & Fedonkin’s classification – obvious today, but enlightening in 1992 – is that the Ediacaran fauna is not a single, monolithic, taxonomic group. Rather, different Ediacaran taxa represent a variety of metazoan (and possibly other) lineages. Even today, some authors (e.g. McMenamin 1998) appear uncommitted to this view. Table 1 is an updated version of Runnegar & Fedonkin’s work.

Seilacher’s original constructional analysis is not debated a great deal today, and though it still claims adherents (e.g. see McMenamin 1998), the majority of authors speak of ‘Ediacaran metazoans’ and ‘the Ediacaran fauna.’

Traces

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 further hint at an arthropod s.l. presence.

A trace fossil presumed to represent the radula scratches of a mollusc is found at Zimnie Gory and in the Ediacara Hills (Martin et al. 2000, p. 844).

’Metameric’ (Segmented) Forms

Whereas there may be a general acceptance that the majority of Ediacarans are stem metazoans of some sort, most are still notoriously problematic. One large and important group of these, arguably the most crucial to our understanding of the overall pattern of metazoan evolution, are those exhibiting real or apparent metamerism. Most are small, though some of the dickinsonids can be enormous: up to about a metre.

Some authors, notably M.A. Fedonkin and A. Yu. Ivantsov, argue that many of these organisms are pseudosegmented, with segments alternating on either side of the mid line, thereby casting doubt on their bilaterian affinities. However, their published photographs (e.g. of Archaeaspinus and Yorgia) are often based upon relatively few specimens and the asymmetry is not always clear (Jim Gehling, pers. comm.)

In some specimens of Dickinsonia, the segments do not appear to correspond across the mid-line on the dorsal surface. However, as noted in Gehling 1991 (though the original observation is attributed to Bruce Runnegar), in all specimens where the ventral side is preferentially preserved, segments clearly continue across the mid-line, so offset on the dorsal side must be a product of flattening. It is only in rare specimens of Dickinsonia elongata that the alternate insertion and overlap of segments along the mid line is difficult to explain.

“Fedonkin (1983, 1984, 1985a, 1986) has not only attempted to assess the phyletic relationships of Ediacaran taxa, but has tackled the problem of comparative morphology. However, his approach has been strongly dependent on a two dimensional body plan analysis of symmetry. This concept of “promorphology” almost entirely disregards the original three dimensional architecture, palaeobiology, and ontogeny of the organisms. Bergstrom (1990, figure 2) illustrated four taxa with apparent alternation of regular elements on each side of the axis; but in each case the sketches represent unrestored images of flattened animals. Order within symmetry groups may be useful in classification of minerals, but in organisms, the superficial symmetry of body plans may be a secondary product of adaptation to different life styles” (Gehling 1991, p. 203.)

Summary

Some similar organisms – which post-date the major Ediacaran biotas and are generally better preserved – have been identified with conventional zoological taxa. Conway Morris 1998a cites as examples Thaumaptilon (Fig. 8), which he believes is a conventional pennatulacean (p. 28-29), Mackenzia (p. 83-84), and Emmonsaspis (p. 134, note 7). Advocates of this viewpoint explain the unusual ‘Ediacaran preservation’ by suggesting a paucity of burrowing and scavenging organisms to disturb the remains, once buried.

However, it remains true that the differences between the Ediacaran and overlying Cambrian faunas are far more striking than any similarities.

There can be little doubt, on the basis of trace evidence alone, that bilaterian metazoans existed in the Vendian. 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 may be a few sign-posts, however:

1. Sponges are widely recognised as the most primitive of living metazoans, occupying a basal position in metazoan phylogeny, as a sister group to all other Metazoa (e.g. Nielsen 2001, p. 30, 506-507, but see Dunn et al. 2008, Schierwater et al. 2009, and Ryan et al. 2013 for alternative possibilities). 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 (Gehling & Rigby 1996). However, sponges could have occurred earlier and not been recognised; spicules are not necessarily diagnostic, even in living sponges (Allen Collins, pers. comm.)
2. 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.
3. As already mentioned, some of the frond-like forms may be conventional pennatulaceans (Conway Morris 1998a).

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 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.

Decline and Disappearance

Decline

Although some taxa are now known to have persisted, and others may have evolved into different forms, most of the Ediacarans simply vanish from the fossil record near the beginning of the Cambrian. The characteristic assemblage persists in full bloom – at least in Namibia – right up until the Vendian-Cambrian boundary after which the assemblage, as a whole, abruptly disappears. It is uncertain whether a mass extinction event struck at this time, or if we are simply observing the closure of some form of “taphonomic window” – both have been suggested.

One school of thought holds that Ediacarans may have been largely wiped out – possibly by the supposed Kotlin nutrient crisis, see Brasier 1992 – immediately prior to the Vendian-Cambrian boundary.

“In the past few years, evidence has accumulated for a remarkable perturbation in the carbon cycle close to the Proterozoic-Cambrian boundary. Globally distributed sedimentary successions document a strong (7 to 9 per mil) but short-lived negative excursion in the carbon-isotopic composition of surface seawater at the stratigraphic breakpoint between Ediacaran-rich fossil assemblages and those that document the beginning of true Cambrian diversification. The causes of this event remain uncertain, but the only comparable events in the more recent Earth history coincide with widespread extinction – for example, the Permo-Triassic crisis, when some 90% of marine species disappeared, is marked by an excursion similar to but smaller than the Proterozoic-Cambrian boundary event. An earliest Cambrian increase in bioturbation shuttered the taphonomic window on Ediacaran biology. Thus, while Chengjiang and Sirius Passet fossils indicate that Ediacaran-grade organisms were not ecologically important by the late Early Cambrian, biostratigraphy admits the possibility that Ediacarans were eaten or outcompeted by Cambrian animals. It is biogeochemistry that lends substance to the hypothesis that Ediacaran and Cambrian faunas are separated by mass extinction” (Knoll & Carroll 1999, p. 2135).

In Oman, the ‘early’ SSFs, Cloudina and Namacalathus, are reported to go extinct very shortly after the Vendian-Cambrian boundary, at 542.0 ± 0.5 Ma (Kerr 2002).

Other researchers observe that a mass extinction event is not necessary to explain the disappearance of the Ediacarans from the fossil record; conditions may simply have ceased to be favourable to the unique ‘Ediacaran preservation’ with the arrival of more numerous and more diverse scavenging and bioturbating organisms.

The preservational characteristics of typical Ediacaran assemblages are undeniably unusual (‘characteristic’ might be a better word), and evidence for more widely spread and deeper bioturbation certainly does increase sharply at the base of the Cambrian. Indeed, as we have seen, the lower boundary of the Cambrian is now defined by the occurrence of the burrow trace fossil, Trichophycus pedum. However, to offer this as a complete explanation for the abrupt disappearance of a distinctive, cosmopolitan fauna simply feels a little too convenient for my taste; I believe something did happen to the Ediacarans near the end of the Vendian or in the earliest Cambrian. If not, then another explanation must be found for the pronounced carbon isotope excursions.

Occasionally the idea of predation is raised. However, it should be noted that the only evidence of predation of Vendian organisms is confined to a few possibly bored Cloudina tubes.

Cambrian Occurences

For many years, Ediacarans were believed to have been confined to the Ediacaran/Vendian Period. Indeed, prior to accurate dating of the Nama occurrences in the mid-1990s, they were widely conceived to have disappeared perhaps 10 Ma before the end of the period. A variant of this view is speculation (e.g. Seilacher 1984; Knoll & Carroll 1999) that a mass extinction terminated the Ediacaran and eliminated the Ediacaran biota. But although the Ediacarans were certainly no longer ecologically important by Chengjiang times, since about 1990 there has been a steadily accumulating body of Cambrian age discoveries, including the following.

Lower Cambrian

A South Australian discovery, including frond-like forms very similar to those found in the White Sea coast, and the disc-like Kullingia, occurs in the basal Cambrian Uratanna Formation of the Flinders Ranges (Jensen et al. 1998).

From Lower Cambrian strata on the Digermul Peninsula, Norway, Crimes & McIlroy 1999 describe the widely occurring Ediacaran species, Nimbia occlusa and Aspidella terranovica (as Tirasiana sp.), from approximately 80 m above the base of the Ediacaran–Cambrian boundary (Nemakit-Daldynian), and a further specimen of Aspidella terranovica (this time as ‘Cyclomedusa’ sp.) from about 600 m above the boundary, in rocks of trilobite-bearing age (Atdabanian; as indicated by Cruziana).

Ediacarans have been known from the Great Basin, California, at least since 1991. A number of taxa, including ?Tirasiana disciformis, cf. Swartpuntia, Cloudina-like tubes, and Ernietta plateauensis, have been described from several localities (Horodyski 1991; Hagadorn 1998; Hagadorn et al. 2000). In this region, Swartpuntia persists through several hundred metres of section, extending up as far as the Nevadella trilobite zone (Atdabanian).

Middle Cambrian

Simon Conway Morris (1989, 1993, 1998a) claims to recognise Ediacaran forms hiding among ‘conventional’ Cambrian faunas. He cites as examples Thaumaptilon (Conway Morris 1998a, p. 28-29), Mackenzia (ibid., p. 83-84), and Emmonsaspis (ibid., p. 134, note 7). Thaumaptilon, from the Burgess Shale (Middle Cambrian), which Conway Morris believes to be a conventional pennatulacean, is proposed as a relative of forms such as Charniodiscus. The comparison is unsatisfying, however. The holdfast of Thaumaptilon in no way resembles the disc-shaped structure so characteristic of many Ediacaran fronds. Moreover, the pennatulacean idea itself requires further testing yet; as Nielsen 2001, p. 59, notes the branches of both Charniodiscus and Thaumaptilon “were united with a membrane which makes the interpretation dubious on functional grounds, and the structures tentatively interpreted as polyps are very small and show no tentacles.” Also note that the “Burgess Shale fronds lack evidence of the structural complexity found in the primary branches of Charniodiscus, and may be structurally closer to other Ediacaran fronds, such as Pteridinium” (Gehling 1991, p. 204).

Upper Cambrian

Youngest and most intriguing are the Upper Cambrian Ediacarans from a turbidite sequence exposed at Booley Bay, near Duncannon in Co. Wexford, Ireland, which includes two taxa: ‘Ediacaria’ booleyi and the ubiquitous Nimbia occlusa (Crimes et al. 1995). The ‘Ediacaria’ taxon is preserved three-dimensionally through nearly 100 m of sediment. Preservational details suggest the organism possessed a rigid wall. The Booley Bay occurrence is dated by acritarchs of sufficient “diversity and quantity to constrain biostratigraphically the relative age of this succession … to the upper part of the Upper Cambrian” (Moczydlowska & Crimes 1995, p. 125), indicating that at least some Ediacarans co-existed with ‘modern’ taxa for perhaps 20 or 30 Ma – and certainly survived the Cambrian Explosion.

Transition to Cambrian Faunas

Whether by mass extinction or some other mechanism, soft-bodied fossil lagerstätten such as the Chengjiang fauna indicate that Ediacaran-grade organisms were no longer ecologically significant by the Botomian (late Early Cambrian). Although some taxa persisted throughout the Cambrian, as we have seen, most of the Ediacarans simply vanish from the fossil record near the beginning of the Cambrian.

“We cannot tell how abruptly the Ediacaran Faunas became extinct, but only a very small number are represented by possible survivors...” (Briggs et al. 1994, p. 46).

“Although most Ediacaran fossils have no post-Proterozoic record, they were not immediately succeeded in lowermost Cambrian rocks by diverse crown group bilaterians. Earliest Cambrian assemblages contain few taxa, and the diversity of trace and body fossils grew only over a protracted interval. Hyoliths and halkierids (extinct forms thought to be related to mollusks), true conchiferan mollusks and, perhaps, chaetognaths enter the record during the first 10 to 12 million years of the Cambrian, but crown-group fossils of most other bilaterian phyla appear later: the earliest body fossils of brachiopods, arthropods, chordates, and echinoderms all post-date the beginning of the period by 10 to 25 million years. Trace fossils suggest earlier appearances for some groups, notably arthropods, but the observation remains that the Early Cambrian contains considerable time for the assembly and diversification of crown group morphologies” (Knoll & Carroll 1999).

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