|Ediacaran - 3|
This section deals with the unique Ediacaran biota. The material, in small measure, repeats some of the earlier text on localities. We could invent some important didactic for doing this. However, the truth of the matter is that the repetition simply results from our haphazard editing of different materials which were subsequently combined.
The name 'Ediacaran' has a geochronologic meaning, providing an upper subunit of the Vendian, approximately 565 to 543 Ma (Bowring & Erwin 1998), 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 Vendian (latest Precambrian, 620 to ~543 Ma) times – 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.
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 occurrences in Mexico, England, Ireland, Scandinavia, Ukraine, and the Ural Mountains (read more).
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). Other occurrences are now widely understood to be in situ marine assemblages, also.
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).
"The sand is of unusual texture; rather like foundry sand, which is a factor in this uncommon mode of preservation" (Clarkson 1993, p. 59).
"The clay lenses were subsequently highly compacted and altered and are now mostly only thin, lenticular partings between the quartzite flags. Most of these partings can be opened only by natural weathering. They reveal fossils mostly on the lower surfaces of the quartzite flags" (Glaessner & Wade 1966, pp. 599-600).
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 occurrence 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 lato 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 (Furongian) 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.
Simple disc-like impressions, interpreted as cnidarian-grade body fossils, from the intertillite beds of the 610 to 600 Ma 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 probable metazoan origin, 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) 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 (Seilacher 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.
Fig 1: Cyclomedusa is probably the most common and widespread Vendian fossil. It also has one of the largest size ranges, ranging from a few millimetres to about a meter in diameter. Formerly thought to represent a planktonic (floating) jellyfish of some sort, Cyclomedusa is now considered by some to have been a benthic (bottom-dwelling) polyp, somewhat like a sea anemone, and by others to be the anchoring holdfasts of colonial, soft octocorals. The latter hypothesis may also explain why they are so common as fossils – because they were already buried – and posits that most of the 'species' are artefacts of differing preservation.
(A) 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" (McMenamin 1996, fig. 2A).
(B) 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.]
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 Furongian assemblage of just two taxa, one of which is Nimbia occlusa, from Booley Bay in Co. Wexford, Ireland.
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), indicating that at least some Ediacarans co-existed with 'modern' taxa for perhaps 20 or 30 Ma – and certainly throughout the Cambrian Explosion.
More than 30 different genera have been named. Ediacarans are a diverse group and earlier attempts to pigeon-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.
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).
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 simple and more or less horizontal; conspicuously 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).
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; but see Dzik & 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.
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 or may truly be indicative of predation.
Fig. 2: (A) Spriggina floundersi Glaessner – From the Vendian Pound Quartzite of the type locality, Ediacara, South Australia. Overall length about 10 cm. Specimen from the Yale collection (YPM 63257).
(B) 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.
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 approximately 610 to 600 Ma, circular impressions from the Twitya Formation of the Mackenzie Mountains provide evidence for the earliest metazoans, simple cup-shaped organisms, possibly cnidarians. Somewhat later, perhaps 590 to 565 Ma, but 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).
Fig. 3: (A) 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.]
(B) 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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page uploaded 16 May 2002
text © by Chris Clowes 2004
last revision ATW070108, edited RFVS111126