Vendian Period

Abstract

This page describes the stratigraphy and fossil record of the Vendian Period, including a brief introduction to the Ediacaran fauna. Some famous Ediacaran lagerstätten are briefly discussed, followed by a sketched outline of some of the major evolutionary events.

Keywords: Vendian, Vendian biota, fossil record, evolution, Varangian, Varanger-Marinoan, snowball earth, Ediacaran, Metazoan radiation

Introduction

The Vendian System and Period were first proposed by Sokolov 1952, from drill core sequences on the Siberian Platform (sokolov & Fedonkin 1984). Although the Vendian was not embraced quickly, and was not adopted by the Subcommission on Precambrian Stratigraphy, it came into almost universal de facto usage. A new stratotype in Belarus has been recently proposed (Makhnach & Veretennikov 2001).

There is much confusion over the status of the Vendian, so it is worth further detailing its status, particularly in relation to the "new" Ediacaran Period (Knoll et al. 2004). The Vendian is an Eurasian geochronologic and chronostratigraphic unit, just as the Sinian is recognised in China. It was never recognised by the Subcommission on Precambrian Stratigraphy which had, in 1991, opted instead for a "Neoproterozoic III" interval defined "strictly by chonometric age, without reference to events recorded in sedimetary rocks" (Knoll et al. 2004, p. 621). The recent recognition of a properly (GSSP) defined Ediacaran System and Period by the Subcommission on Precambrian Stratigraphy is certainly a step in the right direction, and it probably means that new publications will increasingly refer to the Ediacaran rather than the more recent part of the Vendian (see fig. 2). However, it does not mean, as some commentators have suggested, that the Vendian is suddenly invalidated as a legitimate geochronologic and chronostratigraphic unit, or that it has "gone away" in any sense.

As with the better-known Cambrian Period, the absolute age constraints on the Vendian interval have ebbed and flowed over the past few years; perhaps the best current estimate is from ~605 to 543 million years (Ma) ago.

The Precambrian era was a period in earth history before the evolution of hard-bodied and complex organisms. Throughout the extent of both periods, dominant Precambrian and Vendian 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 "small shelly faunas" appeared.

Stratigraphy

Type Section/Sections

The Vendian Period and System were first proposed by Sokolov 1952, from drill core sequences on the Siberian Platform. A new stratotype in Belarus has been recently proposed (Makhnach & Veretennikov 2001).

The Lower Boundary

...

The Upper (Vendian–Cambrian) Boundary

Since 1947, when H.E. Wheeler initiated debate with the suggestion that the Precambrian-Cambrian boundary should be based upon the first appearance of trilobites, 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 appearence date (FAD) of Trichophycus pedum (fig. 1) in the reference section at Fortune Head.

Current Chronology of the Vendian

Varangian ...

The name 'Ediacaran,' in its geochronologic sense, provides 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' to any Vendian (or Ediacaran) age macrofossil, whereas others restrict the term to the unique and distinctive assemblage of enigmatic organisms best known from the Ediacara Hills of South Australia.

Paleogeography

Major Tectonic Events

The Precambrian supercontinent usually known as Rodinia (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 Laurasia. Subsequently - perhaps ~540 Ma - West and East Gondwana merged in the mountain-building event known as the Pan-African Orogeny. (After Rogers 1996.)

Land and Sea

(RCB paleogeographic reconstruction)

Climate

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 strong negative excursions.

Paleontology

General Characteristics

Trace fossils ...

Body fossils typically of cnidarian 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 embryos

Lagerstätten

Lagerstätten (sing. lagerstätte) are fossil localities which are highly remarkable for for either their diversity or quality of preservation; sometimes both. Some of the classic Vendian fossil localities are described below, in approximate order from oldest to youngest.

610 to 600 Ma: McKenzie Mountains

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, p. 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. 1990 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, p. 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, p. 1202).

The principal significance of this occurrence of cnidarian-grade metazoans is their stratigraphic position below Varangian glaciomarine tillites (Aitken 1988, 1989). 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 (e.g. see 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 leiosphaerids.

575 Ma: Ediacarans of the Drook Formation

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, 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 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.

~570 Ma: Weng'An

Soft-tissue fossils preserving cellular structures, notably including the earliest record of sponges (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 the age of 570 Ma for the Doushantuo fossils (proposed in Martin et al. 2000 and adopted here) 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 bilaterians – 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.

(Read more.)

565 Ma: Mistaken Point

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.

555 Ma: Zimnie Gory

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 [® sidebar], 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).

555 Ma: Ediacara Hills

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, p. 844). It is the assemblage from this site that is most widely associated with the Ediacaran biota. Although best known for the 'classical' body fossils, the region also provides interesting traces. One ichnotaxon, similar to that from Zimnie Gory, has been interpreted as the rudula scratchings of a mollusc (possibly Kimberella).

549 to 543 Ma: 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. Moreover, a second, unnamed, shelly taxon ("goblet-shaped shelly fossils") coexists with Cloudina from at least 545 Ma through into the Cambrian (Grotzinger et al. 1995, fig. 1).

Major Evolutionary Events

• 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 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.

Major Taxa

Stromatolites

Cyanobacteria

Acritarchs

Olivoiides?

Extinctions

from http://park.org/Canada/Museum/extinction/venmass.html

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 stromatolites and acritarchs, and was also the predetermining factor that encouraged the diversification of the Ediacarans.

The first extinction of the Precambrian, which largely affected stromatolites and acritarchs, has been correlated with a large glaciation event that occurred about 600 million years ago. This event was of such severity that almost all micro-organisms were completely wiped out.

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

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

Conclusion

...

References

Cowie, J.W.; Brasier, M.D. (eds.) 1989: The Precambrian-Cambrian Boundary. Clarendon Press.

Fedonkin, M.A.; Waggoner, B. M. 1997: The Late Precambrian Fossil Kimberella is a Mollusc-Like Bilaterian Organism. Nature 388: 868-871.

Grotzinger, John P.; Watters, W.; Knoll, Andrew H.; Smith, O. 1998: Diverse Calcareous Fossils from the Ediacaran-Age Nama Group, Namibia. Abstracts with Programs, Geological Society of America, 20, 7, pp. A-147.

Knoll, Andrew H.; Walter, Malcolm R.; Narbonne, Guy M.; Christie-Blick, Nicholas 2004: A new period for the geologic time scale. Science 305: 621-622.

Rogers, J.J.W. 1996: A history of the continents in the past three billion years. Jl. Geol. 104: 91-107.