The Ediacaran biota
For many years, the fossil record of life on Earth was thought to be limited only to the Phanerozoic, with the oldest known fossils coming from rocks of Cambrian age (back to 541 million years). Fossils were not found in older rocks (which were thus termed 'Azoic', meaning 'without life'). This fact troubled Charles Darwin as he wrote On the Origin of Species, since he realized that for his theory of evolution by natural selection to be correct, the history of life must have extended considerably further back in time than the Cambrian. The apparent absence of fossils in older strata became known as "Darwin's Dilemma".
However, even as Darwin was writing The Origin, palaeontological discoveries were being made that would ultimately lead to a revolution in our understanding of ancient life. John Salter found impressions in Azoic rocks from Shropshire, England, that he considered to reflect the preserved remains of seaweeds, worm burrows, and a trilobite (Salter 1856). The Canadian geologist Elkanah Billings discovered discoidal specimens in downtown St. John's, Newfoundland, that he recognized as ancient organisms (Billings 1872; see Gehling et al. 2000). These findings were met with skepticism by the geological community, but both have in recent years been recognized as evidence for pre-Cambrian life (albeit in Salter's case, of microbial organisms and not of the complex lifeforms he envisaged; Callow et al. 2011; Menon et al. 2016).
Complex organisms prior to the Cambrian
Discoveries in Namibia and Russia in the 1930s and 1940s, and most notably in South Australia (Sprigg 1947), revealed fossils of complex, soft-bodied organisms that looked very unlike anything previously described from the Cambrian. Although they occurred in rocks that lay below the Cambrian trilobite-bearing horizons, they were still largely considered to be early Cambrian in age, since it seemed too great a leap to suggest complex organisms prior to the Cambrian. The situation changed in 1958, with the publication of a paper describing the fossil Charnia masoni by Trevor Ford. Charnia is a frondose fossil from Charnwood Forest in England, and was found in rocks that lay 100s of metres below the oldest Cambrian strata, which unquestionably pre-dated the Cambrian Period (Ford 1958). Following this find, reassessment of the Australian, Namibian and Russian material recognised that they jointly comprised a global assemblage of unique, Precambrian complex organisms. These forms were named the 'Ediacara biota', after the Ediacara Hills locality in South Australia from which some of the most diverse assemblages were reported (see Glaessner 1985).
However, even as Darwin was writing The Origin, palaeontological discoveries were being made that would ultimately lead to a revolution in our understanding of ancient life. John Salter found impressions in Azoic rocks from Shropshire, England, that he considered to reflect the preserved remains of seaweeds, worm burrows, and a trilobite (Salter 1856). The Canadian geologist Elkanah Billings discovered discoidal specimens in downtown St. John's, Newfoundland, that he recognized as ancient organisms (Billings 1872; see Gehling et al. 2000). These findings were met with skepticism by the geological community, but both have in recent years been recognized as evidence for pre-Cambrian life (albeit in Salter's case, of microbial organisms and not of the complex lifeforms he envisaged; Callow et al. 2011; Menon et al. 2016).
Complex organisms prior to the Cambrian
Discoveries in Namibia and Russia in the 1930s and 1940s, and most notably in South Australia (Sprigg 1947), revealed fossils of complex, soft-bodied organisms that looked very unlike anything previously described from the Cambrian. Although they occurred in rocks that lay below the Cambrian trilobite-bearing horizons, they were still largely considered to be early Cambrian in age, since it seemed too great a leap to suggest complex organisms prior to the Cambrian. The situation changed in 1958, with the publication of a paper describing the fossil Charnia masoni by Trevor Ford. Charnia is a frondose fossil from Charnwood Forest in England, and was found in rocks that lay 100s of metres below the oldest Cambrian strata, which unquestionably pre-dated the Cambrian Period (Ford 1958). Following this find, reassessment of the Australian, Namibian and Russian material recognised that they jointly comprised a global assemblage of unique, Precambrian complex organisms. These forms were named the 'Ediacara biota', after the Ediacara Hills locality in South Australia from which some of the most diverse assemblages were reported (see Glaessner 1985).
A cast of the holotype specimen of Charnia masoni, first described by Trevor Ford from Charnwood Forest, England, in 1958.
Characteristics of the Ediacaran biota
There are currently around 200 species of macro-organisms that have been described from the Ediacaran Period, an interval of time 635-541 million years ago that was only formally erected in 2004 (Knoll et al. 2004). The earliest macro-organisms only appear in the late Ediacaran Period, ~575 million years ago, with specimens from Newfoundland currently thought to be amongst the oldest. The term 'Ediacara biota'/'Ediacaran biota' has been widely used to describe those soft-bodied, macroscopic fossils in the key localities of the White Sea (Russia), South Australia, Namibia, and Newfoundland. More recently, biomineralized forms (e.g. Cloudina), tubular forms, and some algal fossils have been found within Ediacaran fossil assemblages in China, Spain, South America and Siberia, and the term 'Ediacara biota' has become slightly confused in its inclusion of these forms, and the temporal and geographic connotations it may invoke. As such, its use has been questioned (MacGabhann 2014), but we use the term here in its broadest sense, to encompass all large, complex organisms of the late Ediacaran Period, most of which were soft-bodied.
Many members of the traditional 'Ediacara biota' possess chambered or 'quilted' body plans, with bodies constructed by segments or quilts of soft tissue. These forms have historically been called the Petalonamae (Pflug 1970) or the Vendobionta (Seilacher 1992; Buss & Seilacher 1994), to recognise their apparently unique body plans.
There are currently around 200 species of macro-organisms that have been described from the Ediacaran Period, an interval of time 635-541 million years ago that was only formally erected in 2004 (Knoll et al. 2004). The earliest macro-organisms only appear in the late Ediacaran Period, ~575 million years ago, with specimens from Newfoundland currently thought to be amongst the oldest. The term 'Ediacara biota'/'Ediacaran biota' has been widely used to describe those soft-bodied, macroscopic fossils in the key localities of the White Sea (Russia), South Australia, Namibia, and Newfoundland. More recently, biomineralized forms (e.g. Cloudina), tubular forms, and some algal fossils have been found within Ediacaran fossil assemblages in China, Spain, South America and Siberia, and the term 'Ediacara biota' has become slightly confused in its inclusion of these forms, and the temporal and geographic connotations it may invoke. As such, its use has been questioned (MacGabhann 2014), but we use the term here in its broadest sense, to encompass all large, complex organisms of the late Ediacaran Period, most of which were soft-bodied.
Many members of the traditional 'Ediacara biota' possess chambered or 'quilted' body plans, with bodies constructed by segments or quilts of soft tissue. These forms have historically been called the Petalonamae (Pflug 1970) or the Vendobionta (Seilacher 1992; Buss & Seilacher 1994), to recognise their apparently unique body plans.
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The Ediacaran macro-organisms seemingly dominated their ecosystems for around 40 million years, reaching maximum diversity worldwide around 560 million years ago, and then dwindling back to only a handful of taxa by the end of the Ediacaran. Frondose forms dominate the earliest assemblages, while towards the end of the Period tubular fossils predominate, along with the first biomineralizers, which formed the first carbonate reefs (e.g. Penny et al. 2014). With a few rare exceptions (Narbonne et al. 1997; Shu et al. 2006; Jensen et al. 1998; Hagadorn et al. 2000) the Ediacaran macrobiota disappear from the fossil record at the base of the Cambrian. The cause of their disappearance is an area of intense study, with various suggestions including a mass extinction; out-competition by Cambrian mobile animals; and preservational artefacts all discussed as possible explanations (Laflamme et al. 2013; Darroch et al. 2015).
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Dickinsonia costata, showing the classic 'quilted' body plan of a Vendobiont. This specimen resides in the public gallery at the South Australia Museum, Adelaide.
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Subdivision of the Ediacaran biota
There have been many attempts to classify or divide the Ediacaran macrobiota into smaller groups, on the basis of morphology or perceived biological affinities. Many of the earliest attempts (summarised in Fedonkin et al. 2007) classified most of them as belonging to various modern animal groups, but that view has largely fallen by the wayside following Seilacher's criticism of the animal hypothesis (see discussion below).
In recent years, two schemes have emerged that attempt to sub-divide the Ediacaran biota along purely morphological lines, both of which state specifically that the groupings are not 'clades', but simply groups of organisms linked by a common body plan (Grazhdankin 2014; Laflamme et al. 2013 - first proposed in Erwin et al. 2011). The groups proposed by these studies are broadly consistent, but do differ in several important details (see discussion in Liu et al., 2016).
On this website, we group organisms broadly following the scheme outlined by Marc Laflamme (in Erwin et al. 2011), though also accounting for some of Dimitriy Grazhdankin's suggestions for alternative groups. However, following the recent recognition that Dickinsonia is bilaterally symmetrical (Gold et al. 2015), it has become clear that Dickinsonia no longer reflects the group to which it was suggested to belong (the Dickinsoniomorphs, which are suggested to share a glide plane of symmetry). As such, we have opted to include all former Dickinsoniomorph taxa (except Dickinsonia) within a group termed 'former Dickinsoniomorphs' until this issue is resolved.
What were these organisms?
There is broad consensus in the research community that the Ediacaran groups include some of the earliest animals, but this has not always been the case. When they were first described, Ediacaran fossils were almost all considered to be extinct members of various animal groups, with, for example, frond-like forms being considered ancient members of the Cnidaria (the group including the corals, sea pens and the jellyfish), and organisms like Spriggina and Dickinsonia thought to be annelid worms (Wade 1972).
However, in a series of papers from 1984 to 1994, and building on previous suggestions by Pflug and Fedonkin, the German ichnologist Adolf Seilacher changed the field completely by suggesting that these groups were not animals at all, but a group of organisms that were more closely related to each other than to anything else. This idea culminated in the erection of a new phylum-level group, the Vendobionta (Buss & Seilacher 1994). The Vendobionta were considered to be sister group to the Cnidaria, but lacking the cnidae containing nematocytes that define the modern cnidarian group.
These ideas, radical at the time, provoked a reflective period in Ediacaran science where a host of non-metazoan affinities were proposed (discussed in Seilacher et al. 2003) but it now seems as if the argument has gone full circle, with these groups once again being considered animals. However, unlike previously, it is widely held that the Ediacaran groups constitute now extinct lineages within the animals (e.g. Budd & Jensen 2015), but further work is needed to confirm this.
There have been many attempts to classify or divide the Ediacaran macrobiota into smaller groups, on the basis of morphology or perceived biological affinities. Many of the earliest attempts (summarised in Fedonkin et al. 2007) classified most of them as belonging to various modern animal groups, but that view has largely fallen by the wayside following Seilacher's criticism of the animal hypothesis (see discussion below).
In recent years, two schemes have emerged that attempt to sub-divide the Ediacaran biota along purely morphological lines, both of which state specifically that the groupings are not 'clades', but simply groups of organisms linked by a common body plan (Grazhdankin 2014; Laflamme et al. 2013 - first proposed in Erwin et al. 2011). The groups proposed by these studies are broadly consistent, but do differ in several important details (see discussion in Liu et al., 2016).
On this website, we group organisms broadly following the scheme outlined by Marc Laflamme (in Erwin et al. 2011), though also accounting for some of Dimitriy Grazhdankin's suggestions for alternative groups. However, following the recent recognition that Dickinsonia is bilaterally symmetrical (Gold et al. 2015), it has become clear that Dickinsonia no longer reflects the group to which it was suggested to belong (the Dickinsoniomorphs, which are suggested to share a glide plane of symmetry). As such, we have opted to include all former Dickinsoniomorph taxa (except Dickinsonia) within a group termed 'former Dickinsoniomorphs' until this issue is resolved.
What were these organisms?
There is broad consensus in the research community that the Ediacaran groups include some of the earliest animals, but this has not always been the case. When they were first described, Ediacaran fossils were almost all considered to be extinct members of various animal groups, with, for example, frond-like forms being considered ancient members of the Cnidaria (the group including the corals, sea pens and the jellyfish), and organisms like Spriggina and Dickinsonia thought to be annelid worms (Wade 1972).
However, in a series of papers from 1984 to 1994, and building on previous suggestions by Pflug and Fedonkin, the German ichnologist Adolf Seilacher changed the field completely by suggesting that these groups were not animals at all, but a group of organisms that were more closely related to each other than to anything else. This idea culminated in the erection of a new phylum-level group, the Vendobionta (Buss & Seilacher 1994). The Vendobionta were considered to be sister group to the Cnidaria, but lacking the cnidae containing nematocytes that define the modern cnidarian group.
These ideas, radical at the time, provoked a reflective period in Ediacaran science where a host of non-metazoan affinities were proposed (discussed in Seilacher et al. 2003) but it now seems as if the argument has gone full circle, with these groups once again being considered animals. However, unlike previously, it is widely held that the Ediacaran groups constitute now extinct lineages within the animals (e.g. Budd & Jensen 2015), but further work is needed to confirm this.
Further reading
This overview is a very brief introduction to the Ediacaran biota. There is a vast literature on this subject, which charts the changing opinions of how these fossils fit into the history of life. Some good introductory reviews include:
The Dawn of Animal Life. Glaessner 1985.
Martin Glaessner was one of the early giants within this field, working primarily on material from South Australia. He considered most of the Ediacaran organisms to be early animals. This book not only outlines his opinions regarding what the organisms were, but also includes the first descriptions of many key taxa.
The Rise of Animals. Fedonkin et al. 2007.
Several of the leading researchers in Ediacaran palaeontology combined to produce this popular book. It reviews the history of discovery and research into the Ediacaran biota, and nicely introduces many of the scientists who have contributed to the development of knowledge in this field. A photographic atlas at the back is a valuable resource.
Darwin's Lost World. Brasier 2009.
Martin Brasier's summary of the Cambrian Explosion, and the position of the Ediacaran biota within it, is presented in an approchable narrative format, and provides numerous entertaining personal insights into the politics and nuances of research at the forefront of a scientific field.
First Life. Kaplan & Attenborough 2011.
This book is the accompaniment to a two-part television series in which Sir David Attenborough reviews current understanding of the early fossil record, and particularly the evolution of the first animals in the Ediacaran and Cambrian periods.
Fossil Focus: The Ediacaran biota. Dunn & Liu 2017.
A brief introduction to the Ediacaran biota written for a general audience by some of the authors of this website. It outlines some of the major unresolved scientific questions in this area of research.
This overview is a very brief introduction to the Ediacaran biota. There is a vast literature on this subject, which charts the changing opinions of how these fossils fit into the history of life. Some good introductory reviews include:
The Dawn of Animal Life. Glaessner 1985.
Martin Glaessner was one of the early giants within this field, working primarily on material from South Australia. He considered most of the Ediacaran organisms to be early animals. This book not only outlines his opinions regarding what the organisms were, but also includes the first descriptions of many key taxa.
The Rise of Animals. Fedonkin et al. 2007.
Several of the leading researchers in Ediacaran palaeontology combined to produce this popular book. It reviews the history of discovery and research into the Ediacaran biota, and nicely introduces many of the scientists who have contributed to the development of knowledge in this field. A photographic atlas at the back is a valuable resource.
Darwin's Lost World. Brasier 2009.
Martin Brasier's summary of the Cambrian Explosion, and the position of the Ediacaran biota within it, is presented in an approchable narrative format, and provides numerous entertaining personal insights into the politics and nuances of research at the forefront of a scientific field.
First Life. Kaplan & Attenborough 2011.
This book is the accompaniment to a two-part television series in which Sir David Attenborough reviews current understanding of the early fossil record, and particularly the evolution of the first animals in the Ediacaran and Cambrian periods.
Fossil Focus: The Ediacaran biota. Dunn & Liu 2017.
A brief introduction to the Ediacaran biota written for a general audience by some of the authors of this website. It outlines some of the major unresolved scientific questions in this area of research.
