The Ediacaran Biota

On the emergence of higher life on earth

Artistic representation of life in the deep sea as fossilized at the "Mistaken Point" site in Newfoundland, Canada. (Screenshot from video MYSTERIOUS EDIACARAN ORGANISMS - Rangeomorphs; © Oleg Kuznetsov / 3D Epix Inc.)

Darwin's Dilemma

Charles Darwin 1868. (Source: Julia Margaret Cameron; Original on Wikimedia Commons; Public Domain)

For a long time, trilobites and brachiopods were considered the oldest surviving animal fossils. Fossils of these species are abundantly documented in Cambrian sediments, and are now dated to be up to 540 million years old. They appear in the fossil record of geologic history practically out of nowhere together with countless other species. For a long time, no predecessors of these species were known.

The incredible diversity of species and forms that was to be shown in the Cambrian posed a problem for Charles Darwin, as the fossil record of his time seemed to contradict his notion of a gradual evolution. Where were the anticipated transitional forms? According to his theory of the origin of species by natural selection, an increasing diversity of species was to be expected over time. However, the fossil record of the Cambrian indicated that the basic body plans of all known species arose suddenly. So suddenly that today we speak of the "Cambrian explosion of species."

"It is very difficult to find a good reason for the absence of vast, fossil-rich beds beneath the Cambrian system.

Charles Darwin in the book "On the Origin of Species" from 1859

The apparent absence of transitional forms in the fossil record long hindered the acceptance of his theory of evolution. But Charles Darwin also knew that the knowledge of his time was still too incomplete to solve this dilemma. Today, we have a better understanding of the origins of species than Charles Darwin, and a better understanding of the causes of gaps in the fossil record. Transitional forms could have been relatively short-lived, and thus much rarer to be fossilized and preserved.

Whether this dilemma can be considered solved is still debated. However, a significant contribution to the understanding of the fossil record has been made over the past 150 years by a multitude of pre-cambrian fossil finds. Today we know that complex, multicellular life did not arise only in the Cambrian, as there was already an era with a diversity of strange organisms that inhabited the seafloor: the Ediacaran.

Ediacaran - The discovery of a new era

In research, new ideas are often not accepted by established scientists, but instead they are accepted only when young scientists, who are familiar with new theories, replace their predecessors. Retirements have often contributed more to the scientific progress than persuasion.

Abraham Gottlob Werner, the "father" of German geology, mistakenly believed until his death that all rocks were sedimentary rocks and engaged in a decades-long bitter dispute with his opponents over it, which went down in German scientific history as the "Plutonism-Neptunism Controversy". It should therefore not be surprising that the discovery of the oldest complex multicellular life forms is a lesson on the persistence of an established but obsolete scientific consensus.

The Disc Fossils by Alexander Murray and Elkanah Billings (1868/1872)

Drawing of Aspidella terranovica, made by Elkanah Billing, published in the journal "The Canadian Naturalist" (1872; Volume VI; page 478)

Aspidella discs on a surface layer of the Fermeuse Formation near Ferryland, Newfoundland. (Photo: Martin Smith; via Wikimedia Commons; CC BY-SA 4.0)

Right: Reconstruction of a Charnia masoni (Matteo De Stefano/MUSE; Original; CC BY-SA 3.0)
Left: Cast of a Charnia masoni in the Wrexham Museum, Wales. (Photo: User Rept0n1x at Wikimedia Commons; Original image; CC BY-SA 3.0)

The first discoveries on the path to completing our picture of early life were made in the 19th century in Newfoundland, Canada. The Scottish geologist Alexander Murray discovered disc-shaped fossils in 1868, which were described four years later by his colleague, the paleontologist Elkanah Billings in an article for the journal "The Canadian Naturalist" as fossils of the species Aspidella terranovica. They ranged in size from a few millimeters to 18 centimeters and had concentric rings and/or rays radiating out from the center.

They were the first finds of larger fossils in rock formations older than the Cambrian period [3]. However, their discovery was doubted and the fossils were misinterpreted as the result of natural geological processes, such as residues of bursting gas bubbles. The existence of macroscopic life before the Cambrian period seemed too far-fetched at that time.

The Ediacara Hills in Southern Australia (1946)

Reginald Sprigg was an Australian geologist and paleontologist who, in 1946, investigated the abandoned mines in the Ediacara Hills on behalf of the South Australian Mining Department. These are a series of flat hills on which copper and silver were mined since the 19th century. The aim was to find out whether the abandoned mines could be operated using modern technology on an economic basis.

The hills consist of flat layers of sandstone that used to form the seabed. In them, Sprigg found some disc-shaped imprints that apparently came from mollusks. He mistakenly thought they were jellyfish and realized that his finds had to be very old, possibly even from the Precambrian era. So he wrote an article about his discoveries, which he sent to the magazine "Nature". The article was rejected. Two years later, in 1948, he presented his results at the International Geological Congress, where he did not receive much approval. Precambrian life still seemed too far-fetched.

However, the Ediacara Hills were to prove to be a rich source of fossils, including fossils of the Charnia masoni, Kimberella, Dickinsonia species, and Spriggina which was named after Sprigg. In this region, in 2005, the first evidence was found that Dickinsonia and Kimberella were able to move. [5]

Discoveries in the Charnwood Forest (1956)

Tina Negus was a 15-year-old student with an interest in geology in 1956. She had convinced her family to visit the Charnwood Forest, where she searched for underwater volcanic deposits from the Precambrian era in a remote quarry. She discovered the imprint of a strange fern-like fossil in the rock and made a print on paper. When she showed it to her geography teacher later, he did not believe that it could be a fossil or that it could be one from the Precambrian era. Her own research also yielded no information on this find. But the find did not leave her alone, and in the following year, she wanted to see "her" fossil again. It was gone... [4], [12]

It was discovered by 16-year-old Roger Mason, who wanted to go mountaineering with friends in the Charnwood Forest in northwestern Leicestershire, England. He also made an imprint and showed it to geologist Trevor D. Ford, who finally recognized its significance. His publication in the "Journal of the Yorkshire Geological Society" received international attention. The species found by Negus and Mason is now known as "Charnia masoni" and belongs to the Rangeomorpha, one of the most important groups of what we now call the Ediacaran biota. Charnia masoni was a unique find that left no room for interpretation. The fractal, fern-like structure made it clear that it could not be a geologically formed structure. Whatever it was, it had lived.

It wasn't until 1957 that the realization came that the history of higher life on Earth didn't start in the Cambrian period. Since 2004, this geochronologic interval in the geological time scale has been known as the Ediacaran. It is the last era of the Neoproterozoic, which began about 635 million years ago and ended approximately 541 million years ago. We date the oldest known animal fossils in geological history to this period today.

Earth in the Neoproterozoic

Global Ice Ages in the Cryogenian

The Ediacaran follows the Cryogenian. This was a period in Earth's history during which the planet was nearly completely covered in ice for millions of years. These were ice ages of superlatives. In the last 500 million years, Earth has not experienced anything comparable. The ice cover extended to the equator and the Earth appeared as a "snowball".

After the theory of continental drift was established in the mid-20th century, it was recognized that the boulder clay deposits found on Spitsbergen and Greenland must have been formed when these regions were still in the tropics. On the one hand, the glacier deposits interrupted sediment layers that are normally found in tropical regions. On the other hand, geomagnetic studies also suggested a tropical origin. [6]

The emergence of biodiversity on Earth began with the Ediacaran Biota in the Ediacaran period, which followed the Cryogenian, during which the Earth repeatedly completely froze over. However, the newly emerged life forms died out either with or shortly after the Cambrian explosion of species.

Once a planet is frozen, the bright snow reflects most of the sunlight directly back into space. A geologically less active planet might have remained in an ice age for billions of years. However, Earth managed to free itself from the ice again and again through volcanic activity. Immediately before the emergence of the Ediacaran Biota, the Gaskiers Ice Age came to an end. It was the last and shortest of three ice ages in the Neoproterozoic. Its duration is estimated to be between 340,000 and one to two million years. The extent of glaciation is disputed; it is possible that the planet did not completely freeze over, and a few latitudes remained ice-free.

The end of this ice age was accompanied by a temperature rise, which probably led to the formation of larger shallow water zones on the continental shelves. In addition, after the global glaciation, the input of nutrients from the rivers into the sea was probably increased, which likely led to an increase in the bioactivity of photosynthetic unicellular organisms. As a result, there was a rapid increase in oxygen concentration in the sea and atmosphere, which for the first time in Earth's history provided the basis for the emergence of larger living organisms.

The Earth in the middle of the Ediacaran period, 600 million years ago. The ice had cleared the mid-latitudes, and many shallow shelf areas were formed in the oceans. (Source: Christopher Robert Scotese; PaleoMap Project; License Information)

Seabed as a Habitat

The organisms of the Ediacaran period belong to the oldest known large multicellular animals (Metazoa). They have no known descendants today. The group mainly consisted of shell and non-skeletal mollusks, which fed on nutrients dissolved in water and biomass present at the seabed.

The fact that some of the layers in which fossils were found came from depths of around 1000 meters helped to classify fossil findings. An example of this are the rich finds from "Mistaken Point" in Newfoundland. Light does not penetrate to these depths, thus it was clear that the species found there were not plants, which rely on photosynthesis.

The Mistaken Point fossil site in Newfoundland, Canada is now on the UNESCO World Heritage List. More than 10,000 fossils provide insight into the Ediacaran period 580 to 560 million years ago. (Source: Barrett&McKay Photography; Mistaken Point Ambassadors Inc.; Source 1; Source 2; CC-BY-ND)

Imprints of Charniodiscus in the fossilized Ediacaran era seabed at Mistaken Point. (Photo: Dr. Alex Liu, University of Cambridge)

From the multitude of pre-Cambrian sites known today, we know that life in the Ediacaran period was not restricted to the deep sea. It also spread on the floor of the shallow shelf areas of the seas. However, the seabed during the Proterozoic and early Cambrian periods was not covered, as it is today, with soft sand and mud, but with bacterial mats. These mats created a hard surface that was so tough and thick that at many pre-Cambrian sites the seabed was fossilized. This fossilized seabed is also referred to as "Elephant Skin".

Fossil communities of the Ediacaran Biota

Fossil communities are accumulations of fossils of different organisms that lived together in a specific geological period and in a specific environment. These communities provide insight into the composition of habitats and ecosystems in the past and enable paleontologists and geologists to reconstruct and understand the evolution and environmental changes over long periods of time.

The Ediacaran biota is divided into three fossil communities. These are the Avalon, White Sea, and Nama communities. They represent different time and habitat ranges within the Ediacaran biota and reflect the evolution of pre-Cambrian life within the Ediacarium.

Temporal occurrence of Avalon, White Sea and Nama communities. (after [10])

Avalon community - life in the deep sea

The Avalon community (579-559 million years) is the oldest of the three fossil communities from Ediacarium. It follows immediately after the Gaskiers ice age. With the Avalon community, early life on Earth took root on the hard, lightless bacterial mats of the primordial deep sea. The species of the community are shell-less organisms with a soft-bodied structure. They are essentially species with fractal, fern-like structure from the rangeomorphia group. This group is the best-known and longest-lived group of the Ediacaran biota.

Rangeomorphs

The Rangeomorpha are one of the oldest groups of organisms in the Ediacarum. They dominated the ecosystems of the primordial deep sea. Representatives of this group were detected in layers whose age was determined to be 565 to 555 million years old. Optically, they resemble the present-day sea fans. They were immobile and connected to the seafloor with a suction pad. Their structure was fractal. This means that their structure is self-similar. Starting from a central axis, the organism branches out and forms lateral branches on which structures form that look like smaller copies of the entire organism.

Charnia Masoni, Fractofusus and Bradgatia are representatives of the Rangeomporha. This is what the seabed could have looked like in the deep sea at Mistaken Point in the Ediacarium. (Screenshot from the video MYSTERIOUS EDIACARAN ORGANISMS - Rangeomorpha; © Oleg Kuznetsov / 3D Epix Inc.)

Such a body structure could suggest a strategy for maximizing the total surface area, which could speak for a nutrition by absorption of dissolved nutrients from the water over the entire surface of the organism. Charnia masoni and Fractofusus are among the best-known representatives of this class, where individual specimens of the species were connected to each other by thin filaments. These tendrils could have served reproduction. It is assumed that they were neither plants nor animals, that they were not closely related to any species known today and that they are probably extinct. In addition to the rangeomorphia, sponges such as the conical 2.6-16.5 cm large, Thectardis avalonensis, are also fossilized from this time.

Arboreomorphs

Links: Fossil imprint of Charniodiscus; Right: A morphologically similar present-day sea pen. An example of convergent evolution, the two species are not related.
(Photo Left: Tina Negus; via Flickr; CC-BY 2.0; Foto Right: Nick Hobgood; via Wikimedia commons; CC-BY-SA 3.0)

Also belonging to the class of Arboreomorphs are fern-like life forms. They have a disk-shaped or bulbous anchorage on the seafloor, a central stem, and branches. The "branches" were smooth, tubular structures that were often swollen and branched, and combined into a leaf-like structure. Arboreomorphs differ from the Rangeomorphs by the lack of a self-repeating, modular branching pattern. A significant representative of Arboreomorphs was Charniodiscus, which, like the Rangeomorphs, probably fed by filtering seawater.
(Note: In German sources, this class is often attributed to the Rangeomorphs)

Locations

Known locations include Canada (Newfoundland; Mistaken Point) and England (Charnwood Forest). These locations were situated in relatively cool, polar regions at the time. The Avalon community is known only in deep-sea deposits at depths of 1,000 to 1,500 meters and is absent in sedimentary deposits of the continental shelves of the same age. Trace fossils, such as those left by mobile organisms, are not known or are very rare. This suggests that macroscopic life on Earth originated in the deep sea and was initially non-mobile. [8], [9]

White Sea Community - Life becomes mobile

The fossil community of the White Sea (558 to 550 million years ago) is a relatively short-lived subgroup within the Ediacaran biota. It is named after the region of the White Sea in northwestern Russia, where many of its fossils were found. It is also characterized by a variety of multicellular organisms with soft bodies. The fossil record indicates a global distribution of species as well as an increase in colonization of new ecosystems. Many species, such as the Erniettomorphs, Dickinsoniamorphs, Kimberellamorphs, Bilateralomorphs, and Tetraradialomorphs, first appeared while the diversity of Rangeomorphs declined.

For the first time, trace fossils are found that suggest directed movement of organisms, and fossilized feeding traces indicate that more complex behavioral patterns were soon to emerge. So, some of the trace fossils suggest that organisms were already able to actively avoid areas of the seafloor that had already been "grazed" by them before.

Evolution experimented with geometry and produced for the first time organisms with bilateral symmetry, a basic form in the blueprint of life. Bilateral symmetry organisms can be divided into two mirror identical halves by a cut along their longitudinal axis. Representatives of Bilateria are considered the earliest representatives of animals. The diversity of fossils is also reflected in a new variety of shapes. Now, the fossils show disk-shaped, fern-like, and tubular organisms.

Rangeomorphs and Arboreomorphs

Diversity of species amongst the Rangeomorphs diminished during the age of the "White Sea Community." However, with Rangea, a new species with hexaradial symmetry appeared for the first time. Fossil specimens have been found ranging from a few centimeters to several decimeters in size. Charniodiscus, on the other hand, has been identified by various findings throughout the entire White Sea Community era.

Dickinsoniamorphs (also Dipleurozoa)

Representatives of this class have a flat, oval body. They are segmented and have a bilateral symmetry at first glance. Upon closer examination, a discrepancy between the left and right body structure can be observed. Representatives of this class have been fossilized in sizes ranging from a few millimeters to 1.4 meters and frequently occur in combination with traces left behind on the fossilized seafloor by individual specimens. The most well-known representative of this class is the eponymous Dickinsonia. Since the discovery of cholesterol molecules in fossils, this species has been classified as part of the animal kingdom.

The snail-like Kimberella belongs to the Bilateria and feeds on microbial mats on the seafloor.

Ikaria, a worm-like organism 2-7 millimeters in length, and a representative of Bilateralomorpha. For a long time, it was only known in the form of its half-tubular burrowing traces.

Dickinsonia is fossilized as imprints ranging in size from a few millimeters to 1.4 meters. Since the discovery of cholesterol molecules in some fossils, this species has been classified as part of the animal kingdom.

Bilateralomorpha

This class encompasses the organisms of the Ediacaran biota with bilateral symmetry. Together with the Dickinsoniamorphs, they are counted among the first mobile animal life forms. They too probably lived on the seafloor. It is still unknown whether some of them were also capable of swimming. Some representatives of this class include Spriggina, a bilaterally symmetrical, worm-like organism with a segmented body. It is possible that it is closely related to early arthropods or annelids. Parvancorina was a small (1-2 cm), shield-shaped organism that may be an early arthropod, possibly even an ancestor of trilobites. Kimberella was a snail-like organism with a soft, non-mineralized dorsal plate and a frilled edge. It could grow up to 15 cm long and lived in shallow to 10-meter deep water, where it shared habitat with Dickinsonia, Yorgia, and Charniodiscus.

Locations of discovery

The Flinders mountain chain in Australia is an important site for fossils of the White Sea community. (Photo: Iain Whyte; via Wikimedia Commons; CC-BY-SA-2.5)

Significant sites of discovery are the region around the White Sea in Russia and the Flinders mountain chain in Australia. During the Ediacaran era, they were located at approximately 15 degrees north latitude (equatorial region) and 60 degrees south latitude (polar region), respectively. Similar fossil genera were found at both sites, suggesting their global distribution even though not all species were represented everywhere.

Nama community - Time of Worms

The Nama community of the Ediacara biota was a group of primitive, multicellular organisms that lived on Earth about 550 to 540 million years ago during the late Ediacaran period. Fossils of the classes Rangeomorphs, Erniettomorphs, Arboreomorphs, and sponges have been preserved from the Nama community. Biodiversity declined and most species of the White Sea community disappeared. Possible causes of the decline in species diversity are mass extinction by environmental catastrophes or displacement of established species by more highly specialized ones, with most authors considering displacement as more likely.

During the time of the Nama community, even previously successful lineages such as the Rangeomorphs were expected to become extinct. It was a time of worms. First hints of the development of hunting and defensive behavior were found in the form of boreholes in organisms such as Cloudina. Cloudina were millimeter-sized organisms composed of stacked calcium cones. They are among the oldest preserved shell-forming organisms. It is believed that the formation of shells represented a defense reaction against predatory species. Some Cloudina specimens from China carry the traces of multiple attacks, suggesting that they survived at least a few of them. [10], [11]

Locations

The Nama community is known from fossil deposits found in shallow waters in Namibia, Oman and China. However, it has also been found in deep-water deposits of the Khatyspyt Formation in East Siberia.

The end of the Ediacaran biota - A failed experiment of evolution?

The Ediacaran biota represented the first macroscopic life forms on Earth. Although they were alien in appearance, they were clearly alive. It was the time of the first animal life forms, the time when life discovered mobility. It was also the time when symmetry and fractal structures first appeared in the blueprint of life. The exact nature of the Ediacaran biota and how they fit into the tree of life is still subject to research. Were they precursors to existing species or do they represent an unknown group of organisms that are now extinct? Was it, as often suspected, simply a "failed experiment" of evolution?

Perhaps an experiment, but certainly not a failure. If the Ediacaran biota was only a failed experiment, then trilobites and non-flying dinosaurs are also just failed experiments. Then, when the book of life is fully written, humanity could also appear as a failed experiment. Using this kind of accounting, all life forms on Earth except for archaea are failed experiments over a period of 500 million years.

When the tree of life is represented, LUCA (the Last Universal Ancestor) - the last common ancestor of all life on Earth - is typically located at its center. Even if we cannot yet precisely classify it, the Ediacaran biota is part of the tree of life. It is not a curiosity or an insignificant side branch; it dominated the planet for 30 million years. Whatever our direct ancestor was at this time, it existed alongside the Ediacaran biota or was a part of it. It was the evolutionary response to new possibilities created by a newly emerging habitat. The life forms were just as complex as they needed to be in order to be successful, and there is no doubt that they were successful. [7]

At the end of the Ediacaran period, life began to gear up. In the emergent animal kingdom, the race between defense and attack began. There would now be predators and prey. The time when defenseless mollusks dominated was coming to an end. Evolution loves competition and has no favorites. After 4.1 billion years of Earth's history, everything was now prepared for the next chapter. The age of life had begun and it would forever change the Earth. This change came with such radicality and speed that we now speak of the Cambrian explosion of species. The fuse for this was lit in the Ediacaran.

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