|Neoproterozoic||Ediacaran - 4|
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 1998, 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), 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).
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. 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). 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).
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:
the Ediacarans died out completely well before the Ediacaran-Cambrian boundary (the Kotlin crisis);
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);
the assemblages represented mass strandings rather than in situ associations; and perhaps stemming from this,
that coeval trace fossils could not be attributed to the Ediacaran taxa.
However, none of these beliefs except, arguably, the last can be sustained today, and 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.'
Taxonomic outline of some Ediacaran forms:
Old text (no longer needed?):
Some, such as the German paleontologist Adolph (Dolf) Seilacher, have expressed doubts that the 'quilted' Ediacarans were animals at all; suggesting instead that they represent a quite separate evolutionary lineage. 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.
However, most workers reject this idea. "It's clearly not true for, say, the Burgess Shale. Why should it apply to the Ediacarans?" (Waggoner 1999).
Initially, Ediacarans were interpreted in terms of extant phyla, such as cnidarians, annelids, etc. Through the mid-1980s to mid-1990s, however, Adolf Seilacher has questioned these assignments and even the metazoan affinities of many Ediacarans. 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. It is the last of these groups which has attracted the most 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).
However, Seilacher's argument must be seen in the context of its time; it was predicated on far fewer Ediacarans (in general, the larger taxa) than are known today, and underpinned by the beliefs (justifiable at the time) that:
the Ediacarans died out completely at or before the Ediacaran-Cambrian boundary (the Kotlin interval);
a considerable age separated the disappearance of the Ediacarans and the appearance of the small shelly faunas;
the assemblages represented mass strandings rather than in situ associations; and perhaps stemming from this,
that coeval trace fossils could not be attributed to the Ediacaran taxa.
None of these beliefs except, arguably, the last, can be sustained today, and 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, the majority of authors speak of 'Ediacaran metazoans' and 'the Ediacaran fauna.'
Bruce Runnegar and Mikhail Fedonkin (in Schopf & Klein 1992, p. 373) were next to tackle the 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.
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).
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 'Archaeaspis' 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.)
Some similar organisms – which post-date the major Ediacaran biotas and are generally better preserved – have been identified with conventional zoological taxa. Conway Morris 1998 cites as examples Thaumaptilon (fig. 4B), which he believes is a conventional pennatulacean (pp. 28-29), Mackenzia (pp. 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.
In view of the fauna's supposed primitive metazoan affinities, the virtual absence of sponges (phylum Porifera) is intriguing. However, rare occurrences of sponges have been reported: Gehling & Rigby 1996 describes a probable sponge, Paleophragmodictya from the Ediacaran of South Australia (read more) and Li et al. 1998 reports sponge remains from the extremely ancient Doushantuo phosphate deposit in China, which predates any known Ediacaran assemblage (read more).
There can be little doubt, on the basis of trace evidence alone, that bilaterian metazoans existed in the Ediacaran. Unfortunately, it is equally true that the relatively few body fossils known from the late Precambrian do not shed much light on the sequence of evolutionary advances that led to the famously diverse Cambrian taxa. There are a few sign-posts, however:
Sponges are widely recognised (e.g. Nielsen 2001, pp. 30, 506-507) to be the most primitive of living metazoans, occupying a basal position in metazoan phylogeny, as a sister group to all other Metazoa. Thus their first occurrence in the fossil record is a metric of particular interest. However, only rare occurrences of Precambrian sponges have been reported. The earliest record is of presumed sponge remains from the Doushantuo phosphates, dated around 570 Ma (Li et al. 1998), and the earliest described species is Paleophragmodictya reticulata from the ?555 Ma Ediacara locality. However, sponges could have occurred earlier and not been recognised; spicules are not necessarily diagnostic, even in living sponges (Dr. Allen Collins, pers. comm.)
Fossils of the Twitya Formation are generally presumed to be cnidarians, or at least as metazoans of cnidarian grade. "Interpretation as colonial aggregates of prokaryotes (e.g. Nostoc-like balls) is possible but is difficult to reconcile with the morphology and relatively high relief of the remains, their occurrence at the bottom of turbidite beds, and the lack of a carbonaceous film outlining them, particularly in view of the of the fact that carbonaceous compressions are present in the formation" (Hofmann et al. 1990, p. 1202). Of principal significance is this occurrence of cnidarian-grade metazoans in pre-Varanger sediments, since the Varanger glaciation is sometimes cited as an evolutionary 'bottleneck' which arrested metazoan evolution.
In preserving evidence of bilaterians, the Ediacaran record provides constraints on the protostome-deuterostome split. If Kimberella is indeed a mollusc, as suggested by Fedonkin & Waggoner 1997, or 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.
Fig. 4: (A) 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.]
(B) Thaumaptilon walcotti Conway Morris 1993 – From the Middle Cambrian Stephens Formation 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.]
Most 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.
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.
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 Ediacaran 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.
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).
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.
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.)
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‰ δ13C 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 radula scratchings of a mollusc (possibly Kimberella).
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.
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.
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 Ediacaran-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 Ediacaran-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 Ediacaran-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 Ediacaran 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 Ediacaran organisms is confined to a few possibly bored Cloudina tubes.
For many years, Ediacarans were believed to have been confined to the Ediacaran. 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.
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 Ediacaran–Cambrian boundary (Fortunian), 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).
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).
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.
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).
Fig 5: The horizontal burrow trace fossil, Trichophycus (formerly Phycodes) pedum defines the lower boundary of the Cambrian in the reference section at Fortune Head, southeastern Newfoundland. It has been suggested that newly evolved, burrowing organisms like this may have closed the taphonomic door on the peculiar 'Ediacaran preservation'. [Image courtesy of Dr. Gerd Geyer, Institut für Paläontologie, Bayerische Julius-Maximilians-Universität, Würzburg, Germany.]
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unless otherwise indicated, text © Chris Clowes 2002
last revised ATW060108, edited RFVS111203