The Ediacaran Assemblage

Abstract

This page describes the geological and chronological settings of the Ediacaran forms, the range of morphologies of the fossils themselves, and concludes with some remarks about their relationships with other organisms.

Keywords: Ediacaran fauna, Vendian fossils

Introduction

The name ‘Ediacaran’ has a geochronologic meaning, providing an upper subunit of the Vendian, approximately 620 to 543 Ma (Knoll et al. 2004), 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 Vendian (or Ediacaran) age macrofossil, whereas others restrict the term narrowly to the unique and distinctive assemblage of enigmatic organisms best known from the Ediacara Hills of South Australia, and characterised by problematic oval, frondose, and spindle-shaped forms of unknown affinity.

Ediacaran fossils were 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. Since then, occurrences have been located on most continents.

The assemblage comprises marine life forms first appearing in latest Precambrian times – about 575 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.

For some years a number of authors (e.g. Seilacher 1984, McMenamin 1986) have argued that the Ediacarans were unrelated to any living group of organisms; that they represented a new kingdom (Vendobionta Seilacher 1992) which disappeared around the Vendian-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.

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 (read more).

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, pp. 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).

Lithology

"The sand is of unusual texture; rather like foundry sand, which is a factor in this uncommon mode of preservation" (Clarkson 1993, p. 59).

Chronology

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 Vendian-Cambrian boundary (543 Ma; see Grotzinger et al. 1995, Martin et al. 2000). Thus, paleontologists have had to come to terms with a "relatively short time frame of Ediacaran biology. Diverse Ediacaran assemblages from Australia, northern Russia, and Namibia were all deposited within the last 15 to 20 million years of the Proterozoic Eon" (Knoll & Carroll 1999).

However, there are contenders to push these boundaries in both directions. The stratigraphic 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 Appearance

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 Vendian macrofossil of possible metazoan affinity, the Twitya fossils do not belong here.

In a report of 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. 1A). McMenamin appears committed to this interpretation (e.g. 1998, pp. 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 (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 600 Ma plus require these fossils to pre-date the Varanger-Marinoan ice age, approximately 600 to 590 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.

Acme

Oldest occurrences, such as those from the Twitya and Drook  Formations, are taxonomically impoverished. The assemblage becomes rich around 565 Ma (e.g. at the Mistaken Point locality) but does not achieve full diversity until about 555 Ma. From then it continues in full bloom until the Vendian-Cambrian boundary after which, although some taxa linger on, the characteristic 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.’ It has been suggested that more widely spread and deeper bioturbation, evidence for which increases sharply at the base of the Cambrian, is incompatible with the unique ‘Ediacaran preservation.’ Somehow, I find this view unconvincing.

A number of ‘Ediacarans’ are reported from the Cambrian. The youngest of them all is an impoverished Late Cambrian assemblage of just two taxa, one of which is Nimbia occlusa, from Booley Bay in Co. Wexford, Ireland.

Last Appearance

At the younger end of their range, "Ediacara type" fossils have been increasingly reported from Cambrian sediments (see below). 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 two taxa: 'Ediacaria booleyi' (possibly yet another variant of Aspidella terranovica) and the cosmopolitan Nimbia occlusa (Crimes, Insole and Williams 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.

Morphology

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). Others of this form are believed to be the holdfasts of the frond-like Ediacarans (see below, also fig. 4A).
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 1998, 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. 2; however, also see Dzik and Ivantsov 1999 for a contrary view), echinoderms (e.g. Arkarua), or arthropods (e.g. Diplichnites and Parvancorina, fig. 3A), others such as Praecambridium, Vendia and Tribrachidium (fig. 3B) are more problematic.
4. Least abundant, though perhaps most characteristic of the assemblage as a whole, are the attached, frond-like organisms (fig. 4A), which some propose have affinities with the sea pens and other soft corals.

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. 2A) – 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 of may truly be indicative of predation.

Antecedents

Genetic evidence has been used to suggest significant metazoan diversity far pre-dating the Ediacaran fossils (e.g. Wray, Levinton & Shapiro 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 1998, 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 1998, 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 (read more).

Affinities

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 believes 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 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.

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.’

Recent Views

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.

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. Bergström (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.)

Metazoan Origins

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 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; read more), and the earliest described species is Paleophragmodictya reticulata from the ?555 Ma Ediacara locality (Gehling & Rigby 1996; read more). 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 Vendian 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.

Nutritional Hypotheses

Many Ediacarans appear to have been very thin. Some researchers, particularly those from the Seilacher camp, propose that their tissues may have housed symbiotic algae. Mark McMenamin (1986, 1998) coined the phrase ‘garden of Ediacara’ to encapsulate the concept and he has largely championed this theory. However, other workers such as Bruce Runnegar (1992, p. 83) point out that the hypothesis is both difficult to test and unlikely anyway, because some of the fossils appear to have been deposited below storm wave base – about 100 m – where the intensity of sunlight is very much diminished, perhaps below useful limits for such organisms.

Major Vendian Occurences

An impoverished but characteristic Ediacaran assemblage occurs in the upper beds of the Drook Formation, south-eastern Newfoundland, 1,500 m stratigraphically below the well-known Mistaken Point fossils. These are the oldest of the large, architecturally complex fossils found so far (Anderson 1978; Hofmann et al. 1979; King 1980; Narbonne & Gehling 2003). The published age constraints on these fossils are from 595 Ma (Varangian glacial diamictites of the Gaskiers Formation) to 565 Ma (well-dated Ediacaran fossils at Mistaken Point occurring 1.5 km stratigraphically higher). Unpublished data noted in Walker 2003, p. 220, indicates an age of 575 Ma.

Current-aligned fronds attributable to the cosmopolitan Ediacaran, Charnia masoni, and those of a large (up to nearly 2 m in length) new species, Charnia wardi, occur on the shaley tops of turbidite beds under volcanic ashes. Their position above the glacial marine rocks of the Gaskiers Formation (595 Ma) provides our earliest window on life following the Varanger ice age.

~570 to 560 Ma: ‘Old’ Trace Fossils

Crimes in Cowie & Brasier 1989, p. 167, lists the earliest occurring trace fossils as Planolites, Didymaulichnus, Arenicolites, Neonereites and Gordia.

The lower Elkera Formation in the Georgina Basin, central Australia, contains the ichnofossil Planolites ballandus, pre-dating the Australian Ediacaran fauna (Walter et al. 1989, p. 218) though the genus is elsewhere known to range up into the Phanerozoic (Crimes in Cowie & Brasier 1989, p. 169). Planolites ballandus is a sub-horizontal, simple, straight to gently curved, unbranched, smooth, cylindrical burrow-fill, sometimes with fine longitudinal striations, approximately 1 mm wide (Walter et al. 1989, p. 239).

Didymaulichnus is another horizontal burrow-fill, comprising a double-lobed, slightly sinuous to tightly curved, convex hyporelief, ~10 mm wide by ~3 mm deep, the lobes separated centrally by a distinct, shallow furrow, and ranging up into the Atdabanian. Arenicolites is highly unusual – possibly unique among Vendian traces – insofar as it is not confined to the horizontal plane. It comprises a U-shaped tube, approximately 10 mm deep. It, too, ranges up into the Phanerozoic.

565 Ma: Mistaken Point – Oldest of the ‘Classic’ Ediacaran Assemblages

The oldest of the diverse Ediacaran assemblages yet described is that from Mistaken Point, eastern Newfoundland, where fossils are spectacularly preserved on large bedding surfaces along the sea-cliffs of the Avalon Peninsula. Zircons from interbedded ash have been dated at 565 ± 3 Ma (Benus 1988).

The Mistaken Point assemblage contains a few cosmopolitan taxa such as Charnia and Aspidella, but most are either endemic or shared only with the Charnwood Forest locality in central England (King 1980).

?565 Ma: Charnwood Forest

The volcaniclastic turbidites of the Charnian Supergroup contain an Ediacaran fauna comprising frondose forms, either as simple fronds or multi-fronded balls, discs which are usually ovoid and contain variable numbers of concentric rings, worm burrows, and other forms which do not fit into these groups. Worm burrow traces occur in the lower Brand Group of Charnwood Forest. The discovery of Charnia masoni in 1957 has become an essential part of Ediacaran folk lore; Aspidella (as Cyclomedusa) and Pseudovendia have also been reported from this site (Brasier in Cowie & Brasier 1989, p. 85).

Age constraints on the Charnwood Forest assemblage are poor. Taxonomic similarities with Mistaken Point suggest an age around 565 Ma; a K-Ar determination from a nearby porphyroid emplacement yielded a date of 583 ± 25 Ma.

555 Ma: White Sea

The two most abundant and diverse Ediacaran trace and body fossil assemblages are those from the White Sea coast of Russia and from the Flinders Ranges in South Australia, which together account for 60% of the well-described Ediacaran taxa.

"Many exposures in the White Sea region contain known Ediacaran biotas; however, the best fossil occurrences are found along the shoreline cliffs at Zimnie Gory. These unmetamorphosed and nondeformed (except for present-day cliff-face slumping) siliciclastic rocks belong to the uppermost Ust’ Pinega Formation and form the northern flank of the Mezen Basin along the southeast flank of the Baltic Shield" (Martin et al. 2000, p. 842). Zircons from a volcanic ash in the lower part of the sequence preserved between Medvezhiy and Yeloviy Creeks (ibid., fig. 2) yielded a date of 555.3 ± 3 Ma, the minimum age for the "oldest definitive triploblastic bilaterian, Kimberella, and the oldest well-developed trace fossils; and it documents that spectacularly diverse and preserved Ediacaran fossils formed more than 12 million years before the base of the Cambrian" (ibid., p. 843).

The fossil Kimberella, originally described from southern Australia but subsequently found elsewhere, including from the White Sea in northern Russia, has been persuasively reconstructed as a benthic bilaterian animal with a non-mineralised, univalved shell, resembling a mollusc (Fedonkin & Waggoner 1997). This interpretation provides evidence for the existence of large triploblastic metazoans in the Precambrian, and requires the origin of the higher groups of protostomes to have occurred deep in the Precambrian, at least prior to ~560 Ma.

Presumed radula scratchings are found at Zimnie Gory, as is Hiemalora, another problematic form which cannot with certainty be categorised as either a body fossil or a feeding trace (Martin et al. 2000, p. 844). Irrespective, this evidence establishes the existence of actively crawling organisms, almost certainly bilaterians, and almost certainly above the grade of planarians because of the implied hydrostatic skeleton.

Most provocative of all the Ediacaran forms are those exhibiting real or apparent metamerism (e.g. Dickinsonia and Spriggina). Most are small, though some of the dickinsonids can be enormous: up to ~1 metre. Several authors – notably M.A. Fedonkin (e.g. 1986) and A. Yu. Ivantsov (e.g. 2001) – 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 approach has been strongly dependent on a two dimensional body plan analysis of symmetry, with little regard for the original three dimensional architecture of the organisms. Similarly, Bergström (1990, figure 2) illustrates 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. James Gehling leaves little doubt where he stands on the matter: "To reconstruct the small Ediacaran segmented taxa as other than vagile metazoans requires an appeal to the absurd" (Gehling 1991, p. 205; also see pp. 199 and 203.)

?555 Ma: South Australia

Material from the Ediacara Hills (Flinders Ranges) has still not been precisely dated; it is assumed to be approximately coeval with the White Sea fossils, in the region of 555 Ma (see below), but it could be as young as the +1 to +2‰ d ¹³C interval, dated at 549 to 543 Ma in southern Namibia (Martin et al. 2000, p. 844). It is the assemblage from this site that is most widely associated with the Ediacaran biota.

The Ediacara Hills locality is also the provenance of the earliest taxonomically-resolved poriferan, Paleophragmodictya reticulata, and the possible echinoderm, Arkarua adami.

Although best known for the ‘classical’ body fossils, the region also provides interesting traces. One ichnotaxon has been interpreted as the rudula scratchings of a mollusc (possibly Kimberella).

549 to 543 Ma: The Nama Group

The Nama Group is a thick (> 3 km) shallow marine and fluvial foreland basin succession, partitioned into northern and southern sub-basins by an intervening arch, across which most stratigraphic units thin, located in southern Namibia. The age range of the Ediacaran assemblages from the Nama Group is the interval 548.8 ± 1 to 543.3 ± 1 Ma (Grotzinger et al. 1995).

In addition to typical Ediacaran taxa, such as the cosmopolitan Pteridinium, the shelly fossil Cloudina first appears slightly below the earliest Ediacaran fossils, extends throughout the Ediacaran range, and into the Cambrian. A second shelly taxon, Namacalathus (the "goblet-shaped shelly fossils" of Grotzinger et al. 1995) coexists with Cloudina from at least 545 Ma through into the Cambrian.

549 to 543 Ma: Southwestern Mongolia

Ediacaran faunas and, notably, unquestionable hexactinellid (glass sponge) spicules, have been reported from limestones just above a phosphorite-chert-black shale marker bed in the upper Tsagaan Gol Formation of southwestern Mongolia (Brasier et al. 1997). The dates have been established chemostratigraphically, based on carbon and ⁸⁷Sr/⁸⁶Sr isotope correlations.

Disappearance of the Ediacaran Assemblage

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.

• 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.

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

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 and 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 Vendian–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, 1998) claims to recognise Ediacaran forms hiding among ‘conventional’ Cambrian faunas. He cites as examples Thaumaptilon (Conway Morris 1998, pp. 28-29), Mackenzia (ibid., pp. 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 (fig. 4A). 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.

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Transition to Cambrian Faunas

"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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