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The Ediacaran Period of the Neoproterozoic Era:

630 to 542 Mya

The Ediacaran, formerly known as Vendian, is the last of the , immediately preceeding the . It stretches from the end of the great Neoproterozoic glaciation ("") to shortly before the of animal life. The Ediacaran itself is characterized by a radiation of enigmatic creatures, known as the Ediacaran fauna, whose relationships to later animals remain largely unclear.

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The Ediacaran Period and System were first proposed by Sokolov 1952, from drill core sequences on the Platform. Sokolov & Fedonkin (1984). Although the Ediacaran was not embraced quickly, it is now recognised by the Subcommission on Precambrian Stratigraphy and has now come into almost universal usage - although frequently still called the Vendian.

As with the better-known Period, the absolute age constraints on the Ediacaran interval have ebbed and flowed over the past few years; the best current best guess is from 630 to 542 million years (Ma) ago.

The Precambrian was a period in earth history before the evolution of hard-bodied and complex organisms. Throughout the extent of both periods, dominant Precambrian and Ediacaran organisms were simple, entirely marine, and for the most part soft-bodied: hard-bodied organisms did not occur until nearly the beginning of the Cambrian Period when the so-called "" appeared.

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The Ediacaran Period and System were first proposed by Sokolov (1952), from drill core sequences on the Siberian Platform. It was officially adopted by the in 2004. The base of the Ediacaran (the Global Standard Stratotype Section and Point, or GSSP) is by reference to the base of the Marinoan cap carbonate (Nuccaleena Formation), immediately above the Elatina diamictite in the Enorama Creek section, Flinders Ranges, South Australia. See discussion at .

While we follow the ICS designation, we agree with the dissenting opinion of the Russian members. This may not have been the best pick for a number of reasons:

1) The age of the GSSP is very poorly constrained.

2) At least one widespread "Ice Age" post-dates the GSSP.

3) There are no signs of metazoan life anywhere near the GSSP, much less any Ediacaran assemblage.

4) the name is confusing, since it suggests an Ediacaran assemblage.

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

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

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The name "Ediacaran," in its geochronologic sense, used to mean an upper subunit of the Vendian, approximately 565 to 543 Ma (Bowring & Erwin 1998), with a stratotype in South Australia. This was supposed to take the age of the Ediacaran fauna, while the Varangian sub-period took in the age of the Varangian glaciation - presumably the first half of the Vendian.

Unfortunately, both nomenclature and dating have changed considerably:

The Ediacaran is still the age of the Ediacaran fauna, sort of. We can conceive of the Ediacaran as a period during which the Ediacaran fauna presumably evolved and in which conditions were generally right for it to live, even if we have yet to establish its presence with certainty. The earliest signs of life at Chengjiang almost reach the 600 My mark, and the earliest Twitya trace fossils push 610 My. The GSSP lies somewhere between 600 and 635 Ma, with the most likely date being 630 Ma. Thus the correspondence between the GSSP and metazoan life is not all that unreasonable. The Ediacaran-Cambrian boundary (i.e. the base of the Cambrian, is still dated at 542 Ma - about the same as it has been for the last decade.

The is a chronostratigraphic unit. That is, its base is defined in terms of years. It cannot move, and it has no GSSP. Thus its base is fixed at 850 Ma and its end is defined as the beginning of the Ediacaran, or about 630 Ma.

The "Varangian" is not a recognized unit. Fortunately, the age of the Varanger glaciation has also been re-dated - at 800 to 630 Ma. Thus, the Cryogenian covers essentially the same turf. Despite the sanction of the ICS, the term "Varangian" is used far more often than "Cryogenian." The broad (but short-lived) Ice Age which occurred about 600 or 580 My is a distinct event. The tendency in the recent literature has been to call it the Marinoan and to distinguish it from the world-wide Varanger (or Varangian) Ice Age which ended about 630 My.

The "Vendian" no longer exists as a stratigraphic unit. This is a good thing. Personally, we liked the name. It honored the important Russian contributions to Pre-Cambrian stratigraphy. It was more widely used than "Ediacaran," and less easily misspelled. However, it was always hopelessly ambiguous. We suspect the Russians will have their day when the Early and Middle Cambrian ages are finally agreed on.

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The Precambrian supercontinent usually known as (or, rarely, as Proto-Pangea or Ur-Pangea) formed ~1,000 Ma from the amalgamation of three or four pre-existing continents, in an event known as the Grenville Orogeny. Perhaps beginning ~700 Ma, but protracted over many millions of years, Rodinia began breaking up into three major blocks: West Gondwana, East Gondwana, and . Subsequently - perhaps ~540 Ma - West and East Gondwana merged in the mountain-building event known as the Pan-African Orogeny. (After Rogers 1996.)

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605 to 585 Ma: The Varanger-Marinoan Ice Ages

It has been suggested that the Varanger-Marinoan ice ages, which lasted from approximately 605 to 585 Ma (Martin et al. 2000), were "snowball" events in which glaciation extended to very low latitudes; possibly right to the equator. It may have been a time of widespread extinction, a contention based mainly on carbon isotopic profiles, which display large negative excursions. Anyone who thinks they understand the "snowball" phenomenon should consult the defense of our strongly held position of on this subject.

Post-Glacial Ediacaran

The post-glacial Ediacaran was warm to hot and relatively arid at low and most middle latitudes. Even at high latitudes in the south, there is evidence of a warm, humid climate. The latest Ediacaran and earliest Cambrian were marked by a return to colder conditions, with some glaciation at high latitudes. (mod. ATW060108)

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Trace fossils

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

Doushantuo Phosphate

Mineralised skeletons of uncertain affinity - the - appear just before the beginning of the Cambrian, ~550 Ma, increasing in numbers and diversity towards the . The most common skeletal materials are calcium carbonate (aragonite or calcite) and varieties of calcium phosphate. Many of the latter may originally have been carbonates, phosphatized during preservation. The oldest of these to occur abundantly are Cloudina and the allied genera comprising the family Cloudinidae: small, conical fossils made of calcium carbonate, first (?) appearing in the Ediacaran Stirling Quartzite of California (Langille, 1974) and persisting into the Cambrian. Anabarites and Cambrotubulus are other Ediacaran SSF taxa, known from Siberia and Mongolia.

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Simple disc-like impressions, interpreted as cnidarian-grade body fossils, have been reported from the inter-tillite beds of the 610 to 600 Ma Twitya Formation, in the Bluefish Creek area of the McKenzie Mountains, north-western Canada. Hofmann et al. (1990). The Twitya Formation is a ~800 m thick succession of siliciclastic turbidites within the Windermere Supergroup. Fossils occur in the interval 170 to 200 m below the top of the formation.

The age of the assemblage (approximately 610 Ma, Martin et al. 2000: fig. 1), indicates deposition during the early part of the Varanger-Marinoan ice age. This placement is supported by overlying deep-water diamictite containing dropstones, rare striated clasts, and relict ice-cemented boulders.

The Twitya impressions are preserved as "very simple, convex discoidal and annular reliefs on the lower surfaces of thin sandstone beds, and as counterparts on the tops of the underlying shale beds." Hofmann et al. (1990: 1199). This mode of preservation is quite characteristic of the Ediacaran fauna, which typically occurs in somewhat younger rocks. Inorganic sedimentary structures such as gas-evasion marks, load and dewatering structures, are common in the Twitya Formation. However, systematic differences between the presumed fossils and obviously inorganic structures, as well as a high degree of morphological consistency exhibited by the hundred or more specimens, argue strongly for a biogenic origin.

Hofmann et al. provisionally assigned the fossils to three taxa: Nimbia occlusa, Vendella?, and Irridinitus? The last two of these, however, are both probably junior synonyms of Aspidella terranovica (refer Gehling et al., 2000: 448) so probably only two taxa are represented. Their occurrence at the bottom of turbidite beds suggests a sessile habit. They are most widely interpreted as the basal impressions of cnidarians – either simple pedestal impressions or possibly the remains of a sand ‘ballast’ retained within the organisms during life – 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: 1202).

The principal significance of this occurrence of cnidarian-grade metazoans is their stratigraphic position below ?Varangian glaciomarine tillites (Aitken 1988, 1989) [2]. This is the only Ediacaran-like assemblage found below Varanger glacial deposits anywhere, and provides a useful test for those models positing metazoan evolution to have been arrested during one or more of the Neoproterozoic glaciations. See, e.g., Runnegar (2000); Peterson & Davidson (2000).

In addition to the putative cnidarian impressions, the Twitya Formation contains the carbonaceous film taxa, Morania and Beltina, and also some poorly preserved filamentous microfossils and .

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An impoverished but characteristic Ediacaran assemblage occurs in the upper beds of the Drook Formation, south-eastern Newfoundland, 1500 m stratigraphically below the well-known Mistaken Point fossils; these are the oldest of the large, architecturally complex fossils found so far (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: 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 shaly 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.

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Soft-tissue fossils preserving cellular structures, notably including the earliest record of (see below), occur in the Doushantuo Formation phosphates, exposed near Weng'an in Guizhou (south central China), providing evidence of a diverse biota.

Age constraints on the Doushantuo Formation are rather weak. Chemostratigraphic profiles suggest that Doushantuo fossils predate the last strongly positive carbon isotope excursion of the Proterozoic, dated as 549 ± 1 Ma in Namibia (Grotzinger et al. 1995). Similarly, Doushantuo microfossils provide biostratigraphic evidence that this formation predates the 555 ± 3 Ma sandstones of the Redkino Series, northern Russia, which contain diverse Ediacaran body and trace fossils. Bio- and chemostratigraphic correlations further suggest that Doushantuo fossils are older than diverse Ediacaran assemblages found in Australia, Ukraine, and northern Siberia. However, in the absence of direct radiometric constraints, it is uncertain whether Doushantuo fossils predate frondose Ediacaran remains from Newfoundland, dated at 565 ± 3 Ma, although even the age of 570 Ma for the Doushantuo fossils proposed by Martin et al. (2000) (much less the c. 600 Ma found by Knoll et al., 2000) places them some 5 Ma earlier. Knoll 2003, p. 141, suggests a range of 600 to 590 Ma. Whichever is correct, the deposit seems certain to be post-Varangian.

The reported biota now includes probable algae, cnidarians and – the last two largely known from fossil embryos. Unfortunately, diagenetic effects are sometimes difficult to distinguish from genuine biological structures, and much of the evidence from this source, though widely accepted, remains equivocal.

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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.

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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: 842). Zircons from a volcanic ash in the lower part of the sequence preserved between Medvezhiy and Yeloviy Creeks (id.) yielded a date of 555.3 ± 3 Ma, the minimum age for the "oldest definitive triploblastic bilaterian, , 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." Id. at 843.

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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 d13C interval, dated at 549 to 543 Ma in southern Namibia. Martin et al. (2000: 844). It is the assemblage from this site that is most widely associated with the base of the Ediacaran biota. Although best known for the 'classical' body fossils, the region also provides interesting traces. One , similar to that from Zimnie Gory, has been interpreted as the radula scratchings of a (possibly Kimberella).

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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. Moreover, a second, unnamed, shelly taxon ("goblet-shaped shelly fossils") coexists with Cloudina from at least 545 Ma through into the Cambrian. Id.

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The following passage is from Benchley and Harper, Palaeoecology, pp.121-123

"The main and most conspicuous elements of the Vendian biota belong to the Ediacara fauna. The fauna is entirely soft-bodied and was probably adapted to relatively low oxygen conditions in a variety of usually nearshore marine environments. The apparently unique morphology and mode of preservation of the Ediacara fauna led to much debate bout the identity and origins of the assemblage. Are the Ediacarans one of the first true metazoans, or the impressions of an entire ecosystem populated by quite a different type of organism? Seilacher (1989) has reinterpreted the fauna in terms of its constructional and functional morphology. Apart from a distinctive mode of preservation, the fauna shares the following features: quilted pneu (rigid, hollow, balloon-like) structures with sometimes additional struts and supports together with a significant flexibility. If the Ediacara animals are in fact divorced from the true metazoans and indeed may be grouped together as a separate grade of organization - termed by Seilacher and others, the Vendozoa or Vendobionta (Buss and Seilacher, 1994) - certain generalizations about their anatomy and behaviour, some speculative, may be made. Reproduction may have been by spores or gametes, and growth was achieved by both isometric and allometric modes. The skin or integument had to be flexible, although it could crease and fracture. Moreover the skin must have acted as an interface for diffusion processes, whilst providing a water-tight seal to the animal. This stimulating and original view of the fauna, however, remains controversial. A range of adaptive morphologies has been recognized in the fauna.
There is little doubt that the Ediacara biotas dominated the latest Precambrian marine ecosystem, occupying a range of ecological niches and pursuing varied life strategies probably within the photic zone. It is also possible that these flattened animals hosted photosymbiotic algae, maintaining an autotrophic existence in the tranquil 'Garden of Ediacara' (McMenamin, 1986). The ecosystem, however, was dominated by medusoid pelagic animals and attached, sessile benthos; infaunal animals were sparse; food chains were probably short and the trophic structure was apparently dominated by suspension- and deposit-feeders."

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

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

  • Sponges are widely recognised (e.g. Nielsen, 2001: 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 metazoans of cnidarian grade. "Interpretation as colonial aggregates of (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: 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 - 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 (but see, Mooi (2001) and our discussion of Arkarua at ). 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.

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

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

While it is not known what kind of organism produced Cloudina, and many other SSFs are equally problematic, some of the Cambrian representatives have been tied back to a firm systematic placement, such as Microdictyon, which is now known to be an onychophoran.

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Extinctions are proposed to have affected even life's earliest organisms. About 650 million years ago, seventy percent of the dominant Precambrian flora and fauna perished in the first great extinction. This extinction strongly affected and , and was also the predetermining factor that encouraged the diversification of the Ediacarans.

However, this distinct fauna may also have perished in a second extinction event at the close of the Ediacaran. This event may have been responsible for the ensuing diversification of the Cambrian shelly fauna.

The Ediacaran extinction, occurring near the close of the Ediacaran period, is currently under debate as to whether an extinction event occurred or not. Many paleontologists believe that the Ediacaran fauna were the progenitors of the Cambrian fauna. However, others believe that the Ediacaran fauna have no living representatives. Under this latter hypothesis, the Ediacaran fauna is believed to have an undergone an extinction, after which the Cambrian fauna evolved. Until more information can be collected, details on the Ediacaran extinction event will remain open to debate.

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. Some believe this is evidence of a mass extinction.

Moreover, 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).

One school of thought holds that Ediacarans may have been largely wiped out by a supposed nutrient crisis – ‘Kotlin Crisis,’ see Brasier (1992) – immediately prior to the Ediacaran-Cambrian boundary.

However, 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 favorable to their preservation with the arrival of more numerous and more diverse scavenging and bioturbating organisms. Indeed, the lower boundary of the Cambrian is now defined by the occurrence of a distinctive horizontal burrow trace fossil, Trichophycus (formerly Phycodes) pedum in the reference section at Fortune Head, southeastern Newfoundland.

"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: 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. and (extinct forms thought to be related to mollusks), true mollusks and, perhaps, 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 , , , and 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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