The Ediacaran biota is a group of life forms that existed on Earth during the Ediacaran Period, about 635 to 538.8 million years ago. These organisms were mostly immobile and had unusual shapes, such as tubes and fronds. Fossils of these creatures have been found around the world and are considered the earliest known complex multicellular life. Some scientists disagree about the name "Ediacara biota" because it may not be clearly defined and may leave out certain fossils.
The Ediacaran biota may have rapidly diversified during an event called the Avalon explosion, which happened about 575 million years ago. This occurred after Earth’s long ice age, known as the Cryogenian period. Most of these organisms disappeared when the Cambrian explosion brought a sudden increase in biodiversity. Many of the body plans seen in modern animals first appeared in the Cambrian Period, not the Ediacaran. While some Ediacaran fossils were found in Sonora, Mexico, and other locations, they largely vanished by the end of the Ediacaran. Some fossils from the Middle Cambrian suggest a few Ediacaran-like creatures may have survived, but most evidence of these early ecosystems is missing. Scientists have proposed several reasons for their disappearance, such as changes in the environment, the rise of predators, and poor fossil preservation. A 2018 study of Ediacaran rock layers in Baltica suggested these organisms thrived in oceans with low productivity and high bacterial activity.
Scientists struggle to classify Ediacaran organisms because their relationships to modern life are unclear. Some may have been algae, fungi, or simple protists, while others may be unrelated to any living group. Some fossils resemble early animals, such as sponges or jellyfish, but many have no clear connection to later life forms. Most Ediacaran fossils look like discs, tubes, or other unusual shapes, unlike modern animals. Because their evolutionary relationships are hard to determine, some scientists believe these organisms may belong to completely extinct groups. In 1960, a scientist named Adolf Seilacher proposed a special category, Vendobionta, to classify these organisms. A 2018 study found cholesterol in Dickinsonia, a famous Ediacaran fossil, suggesting it may be related to animals, fungi, or red algae.
History
The first Ediacaran fossils found were the disc-shaped Aspidella terranovica, discovered in 1868 by Scottish geologist Alexander Murray. Murray used these fossils to help determine the age of rocks near Newfoundland. However, because these fossils were found below the "Primordial Strata" of the Cambrian period, which was believed to contain the earliest signs of animal life, a 1872 proposal by Elkanah Billings that these simple forms were animal remains was not accepted by other scientists. Instead, scientists thought they were gas escape structures or inorganic concretions. No similar structures were known elsewhere, and the debate about their meaning faded over time. In 1933, Georg Gürich found similar fossils in Namibia and placed them in the Cambrian Period. In 1946, Reg Sprigg discovered what looked like "jellyfishes" in the Ediacara Hills of Australia’s Flinders Ranges, which were then considered Early Cambrian.
The discovery of the iconic Charnia fossil in England changed how scientists viewed the Precambrian period. This frond-shaped fossil was first found in 1956 by a 15-year-old girl named Tina Negus, but she was not believed at first. A year later, a group of three schoolboys, including 15-year-old Roger Mason, also found it. Detailed geological maps by the British Geological Survey confirmed that these fossils were in Precambrian rocks. In 1959, palaeontologist Martin Glaessner connected these findings to earlier discoveries. With better dating methods and renewed interest, more fossils were identified.
Until 1967, all discovered fossils were in coarse-grained sandstone, which made it hard to see fine details. In 1967, S.B. Misra found fossiliferous ash beds at the Mistaken Point assemblage in Newfoundland. These ash beds preserved delicate details that had not been seen before. This was also the first discovery of Ediacarans in deep water sediments.
Poor communication and the difficulty of matching rock formations worldwide led to many different names for these fossils. In 1960, the French name "Ediacarien," after the Ediacara Hills, was added to the terms "Sinian" and "Vendian" for terminal-Precambrian rocks. These names were also used for the life forms. Later, "Ediacaran" and "Ediacarian" were used to describe the geological time period and its rocks. In March 2004, the International Union of Geological Sciences officially named the terminal period of the Neoproterozoic after the Australian locality.
The term "Ediacaran biota" and similar terms such as "Ediacara," "Ediacaran," "Ediacarian," "Vendian," and "fauna" or "biota" have been used in different ways, including geographic, stratigraphic, taphonomic, or biological contexts. The biological use is most common in modern scientific writing.
Preservation
Microbial mats are areas of sediment held together by groups of microbes that produce sticky liquids or bind the sediment particles. These mats seem to move upward when covered by a thin layer of sediment, but this is an illusion caused by the growth of the microbial colony. The individual microbes themselves do not move. If too much sediment is added before the colony can grow or reproduce, parts of the colony may die, leaving behind fossils with a wrinkled ("elephant skin") and bumpy texture.
Some Ediacaran rock layers that show the texture of microbial mats contain fossils. Ediacaran fossils are almost always found in rock layers that include these microbial mats. Although microbial mats were once common before the Cambrian period, the rise of organisms that feed on them greatly reduced their numbers. Today, these communities are found only in harsh environments, such as the stromatolites in Hamelin Pool Marine Nature Reserve in Shark Bay, Western Australia, where salt levels are twice those of the surrounding ocean.
The preservation of Ediacaran fossils is interesting because these organisms had soft bodies, which usually do not fossilize. Unlike later soft-bodied fossils found in places like the Burgess Shale or Solnhofen Limestone, Ediacaran fossils are found globally, not just in special environments. This suggests that the processes that preserved them were widespread and occurred across the world. These creatures may have been preserved when they were quickly covered by ash or sand, trapping them against the mud or microbial mats they lived on. Their preservation may also have been helped by high levels of silica in the ocean before organisms like sponges and diatoms, which use silica, became common. Ash layers can be dated precisely using special methods, but Ediacaran fossils are more often found under sandy layers deposited by storms or in turbidites formed by strong ocean currents. Although soft-bodied organisms today rarely fossilize during such events, the presence of microbial mats likely helped preserve them by stabilizing their impressions in the sediment.
The speed at which the sediment above an organism hardened compared to the speed of the organism's decay determines whether the top or bottom surface is preserved. Most disc-shaped fossils decayed before the sediment hardened, allowing ash or sand to fill the space, leaving a cast of their underside. In contrast, quilted fossils often decayed after the sediment hardened, so their upper surfaces were preserved. Quilted fossils are more resistant, which is why they are sometimes found in storm deposits, where high-energy conditions did not destroy them as easily as the less-resistant discs. In some cases, bacteria helped form a "death mask" by creating minerals that left a positive, cast-like impression of the organism.
Morphology
The Ediacaran biota showed a wide variety of body shapes. Sizes ranged from less than a millimeter to over a meter long. Their structures varied from simple, shapeless forms to complex designs. Some were strong and tough, while others were soft and jelly-like. Many different types of symmetry were found in these organisms. Unlike earlier fossils, which were mostly made of microbes, Ediacaran organisms had organized, multicellular bodies and were at least one centimeter in size.
These different body shapes can be grouped into categories called form taxa.
Classification and interpretation
The classification of Ediacaran organisms is very challenging, so many different theories exist about where they fit on the tree of life.
In 1984, Martin Glaessner wrote in The Dawn of Animal Life that Ediacaran fossils were members of modern animal groups but looked unfamiliar because they had not yet developed the traits scientists use today for classification.
In 1998, Mark McMenamin argued that Ediacarans lacked an embryonic stage, so they could not be animals. He believed they evolved nervous systems and brains independently, suggesting that intelligent life may have developed more than once on Earth.
In 2018, analysis of ancient sterols (certain chemicals found in living things) provided evidence that Dickinsonia, a well-known Ediacaran fossil, was an early animal.
The earliest known multi-cellular animals with tissues are cnidarians, like jellyfish. The first Ediacaran fossil, Charnia, resembled a sea pen, leading early scientists to classify Ediacaran fossils as jellyfish or sea pens. However, later discoveries showed that some circular shapes previously thought to be cnidarian medusae were actually holdfasts—structures that help organisms attach to the ground. For example, Charniodiscus, a circular fossil, was later found to be part of a frond-like organism.
Evidence now suggests that frond-like Ediacarans are not closely related to sea pens. This is because modern sea pens are complex organisms not found in older fossils, and Ediacaran fronds appear to have connected body parts. Some scientists argue that "growth poles" in Ediacaran fossils do not support the idea that they were sea pens.
Adolf Seilacher proposed that Ediacaran organisms replaced giant protists as the dominant life form. Modern xenophyophores are large single-celled organisms found in the deep ocean. Genetic studies show they are a specialized group of Foraminifera.
Seilacher suggested that Ediacaran organisms formed a unique group of related species descended from a common ancestor, which he called the kingdom Vendozoa. Later, he reclassified these organisms as "Vendobionta," a group of cnidarians without stinging cells. Without these cells, he believed Ediacarans may have relied on symbiotic relationships with photosynthetic or chemoautotrophic organisms. Mark McMenamin thought this feeding method applied to all Ediacaran life, calling the period’s marine ecosystem the "Garden of Ediacara."
Greg Retallack proposed that Ediacaran organisms were lichens, a partnership between fungi and algae. He pointed to fossil structures resembling lichen compartments and clay patterns similar to fungal hyphae, and found Ediacaran fossils in what he interpreted as desert soil.
This idea has been challenged by other scientists, who argue the evidence is unclear. For example, Dickinsonia fossils were found on ripple-marked surfaces, suggesting a marine environment, and trace fossils like Radulichnus could not have been caused by ice, as Retallack suggested. Ben Waggoner noted that this theory would push the origin of cnidarians back to 1500–2000 million years ago, conflicting with other evidence. Matthew Nelsen studied the evolution of lichen components and found no support for lichens existing before vascular plants.
Various classifications have been used for Ediacaran fossils, including algae, protozoans, fungi, bacterial colonies, and hypothetical life forms between plants and animals.
In 2014, a new living genus called Dendrogramma was discovered. At first, it seemed to be a basic animal with unknown classification, but it was later identified as a siphonophore, possibly part of a more complex organism.
Origin
It took nearly 4 billion years after Earth formed for Ediacaran fossils to first appear, about 655 million years ago. Some possible fossils are reported from 3,460 million years ago, but the first clear evidence of life is found 2,700 million years ago. Cells with nuclei definitely existed by 1,200 million years ago.
Some scientists believe that no special explanation is needed. Evolution simply required about 4 billion years to develop the necessary traits. Over time, life became more complex. For example, semi-complex life forms like Nimbia were found in rocks about 610 million years old in the Twitya Formation. Older rocks in Kazakhstan date to 770 million years ago.
On early Earth, reactive elements like iron and uranium existed in a form that would react with oxygen produced by photosynthesizing organisms. Oxygen could not build up in the atmosphere until all the iron had reacted (forming banded iron formations) and other reactive elements were oxidized. Donald Canfield found evidence of significant atmospheric oxygen just before Ediacaran fossils appeared. This oxygen increase was thought to trigger the Ediacaran radiation. Oxygen built up in two stages: smaller, stationary organisms appeared during the first stage, and larger, mobile organisms appeared during the second. However, some scientists question the accuracy of these reconstructions, noting that widespread lack of oxygen in the Early Cambrian and Cretaceous periods had little effect on life.
Periods of extreme cold may have blocked the evolution of multicellular life. The earliest known embryos, from China’s Doushantuo Formation, appeared about a million years after Earth emerged from a global glaciation. This suggests that ice and cold oceans might have prevented the development of multicellular life.
In early 2008, a team studied the variety of body structures ("disparity") of Ediacaran organisms from three fossil sites: Avalon in Canada (575 to 565 million years ago), White Sea in Russia (560 to 550 million years ago), and Nama in Namibia (550 to 542 million years ago). While the White Sea site had the most species, there was no major difference in body structure diversity among the sites. Researchers concluded that Ediacaran organisms likely underwent an evolutionary "explosion" before the Avalon period, similar to the Cambrian explosion.
Few Ediacaran fossils are found after the Cambrian period. This may be because conditions no longer favored preserving these organisms, even if they continued to exist. However, if Ediacaran organisms were common, more examples might be found in well-preserved fossil sites like the Burgess Shale and Chengjiang. Some disputed reports of Ediacaran-like organisms in the Cambrian period exist, as well as unconfirmed observations of "vendobiont" fossils in 535-million-year-old rocks in China.
It has been suggested that by the Early Cambrian, organisms higher in the food chain caused microbial mats to disappear. If these grazers appeared as Ediacaran life declined, they may have disrupted microbial mats, leading to the "Cambrian substrate revolution" and ecosystem changes. Alternatively, skeletonized animals may have directly fed on Ediacaran organisms. However, if Kimberella, an Ediacaran organism, was a grazer, this suggests Ediacaran life had already faced some predation.
Increased competition from other groups, possibly due to evolutionary innovations, may have pushed Ediacaran organisms out of their niches. However, the idea that brachiopods were replaced by bivalve mollusks was later found to be unrelated trends.
Major changes occurred at the end of the Precambrian and start of the Early Cambrian. These included the breaking apart of supercontinents, rising sea levels (creating shallow, life-friendly seas), a nutrient crisis, changes in atmospheric composition (including oxygen and carbon dioxide levels), and shifts in ocean chemistry that promoted biomineralization. All these factors may have contributed to the changes in life during this time.
Assemblages
Late Ediacaran macrofossils are found worldwide in at least 52 rock layers and various types of environments. Each rock layer is usually grouped into three main types, called assemblages, named after places where they were first discovered. Each assemblage typically exists in its own time period and region of morphospace, and after an initial increase in diversity (or extinction), changes little for the rest of its existence.
The Avalon assemblage is named after Mistaken Point on the Avalon Peninsula in Canada, the oldest location with many Ediacaran fossils. This assemblage is easy to date because it contains many thin layers of ash, which contain zircons used in uranium-lead dating. These ash layers also preserve detailed fossil impressions. Organisms from this assemblage appear to survive until the extinction of all Ediacaran life at the start of the Cambrian period.
One idea is that these organisms were deep-sea creatures called rangeomorphs, like Charnia, which all have a fractal growth pattern. They may have been preserved in place, though this is not certain. While less diverse than the White Sea or Nama assemblages, the Avalon assemblage resembles Carboniferous communities that filtered food from water, which may suggest these organisms also filtered food, as they are often found in deep water where photosynthesis was not possible.
The White Sea or Ediacaran assemblage is named after the White Sea in Russia or the Ediacara Hills in Australia. It has much higher diversity than the Avalon or Nama assemblages. Most fossils are preserved as imprints in microbial layers, though some are found in sandy layers.
In Australia, these fossils are often found in red, gypsum-rich, and calcareous soils formed from wind-blown dust and flood deposits in a cool, dry climate. Since the early 2000s, about 40 fossil surfaces from the White Sea assemblage have been found in the Ediacara Member of the Rawnsley Quartzite in the Ediacara Hills, located in the Nilpena Ediacara National Park in South Australia. The fossil bed called 1T-F has the highest diversity of Ediacaran fossils discovered so far, showing complex ecological interactions. This bed contains over 400 fossils from 16 different genera.
The Nama assemblage is best represented in Namibia. It is marked by extreme changes in life forms, with extinction rates higher than new life forms appearing. Three-dimensional preservation is common, with organisms found in sandy layers containing internal bedding. Some scientists, like Dima Grazhdankin, believe these fossils represent burrowing organisms, while others, like Guy Narbonne, think they lived on the surface. These layers are between units of sandstone, siltstone, and shale, with microbial mats often containing fossils. The environment is thought to be sand bars at the mouths of river deltas. Pillow-like fossils (Ernietta, Pteridinium, Rangea) in these sandstones form a different group from worm-like fossils (Cloudina, Namacalathus) found in marine dolomite of Namibia.
These fossils are found on all continents except Antarctica, so geography does not seem to affect their distribution. The same fossils are found at all ancient latitudes (accounting for Earth's past positions) and in different sedimentary basins. A study of a White Sea fossil bed showed that specific Ediacaran organisms were linked to different environments, such as continental seabeds, tidal zones, and estuaries. However, while some organisms adapted to different environments, the three assemblages are more distinct by time than by environment. Because of this, the three assemblages are often separated by time periods rather than environmental differences.
The Ediacaran biota represent an early stage in the history of multicellular life, so not all possible life modes were present. It is estimated that of 92 possible combinations of feeding, movement, and habitat, only about 12 were present by the end of the Ediacaran period. Only four of these combinations are found in the Avalon assemblage.