Ediacaran fossils
A recent paper published in the journal Geology by three geologists from Yale, led by Dr Lidya G. Tarhan, explains how the soft-bodied Ediacaran biota came to be preserved in such exquisite detail, while the transition to the (mostly) soft-bodied Cambrian biota and the Cambrian diversification are so poorly represented in the fossil record. It turns out that this was due to the particular chemistry of Ediacaran seawater, which enabled dead organisms to be coated and encased in a fine layer of clay that protected and preserved their structure. As ocean chemistry changed, this fortuitous process became progressively less effective.
Creationists love few things more than a gap in scientific knowledge as somewhere to position their favourite god, presumably having been fooled into believing a false dichotomy — either science can currently explain it or God did it — or at least expecting their target audience to be fooled by it. Sadly for creationists, this has created an ever-decreasing number of places in which to force-fit their ever-shrinking little god, as science, with relentless, unstoppable efficiency, fills one gap after another.
One such gap which creationists regularly trot out and misrepresent is the so-called “Cambrian Explosion”, which they have been fooled into believing was a sudden event occurring at an instant in time, before which there were no multicellular organisms and after which a myriad diverse body plans all appeared overnight without ancestry.
This conveniently ignores two important facts: the pre-existing Ediacaran biota, and the fact that the Ediacaran biota transitioned into the Cambrian biota over a period of some 30 million years.
In reality, of course, there is no such gap — it exists only in the minds of those ignorant enough to believe the misrepresentation. However, there is, or rather was, a gap, and one which creationists would probably prefer not to think about. It was the lack of a good explanation for how the soft-bodied Ediacaran biota came to be preserved in the fossil record in such exquisite detail, while the Cambrian “explosion” only looks like a sudden event because so few of the (soft-bodied) transitional forms were preserved.
Certainly, once it began, the Cambrian was a period of exponential diversification during which hard body parts evolved as defensive structures such as shells, spines, and hard exoskeletons; offensive structures such as jaws; and organs of mobility such as limbs and fins. Also evolving were sense organs and nervous systems. It would have been astonishing almost beyond credibility if every step of a rapid diversification of initially soft-bodied organisms had contrived to leave a fossil record of every stage, so all we really have is an infrequent series of snapshots at discrete locations, each capturing a brief moment in a global evolutionary history lasting about 55 million years.
From the Ediacaran Biota to the Cambrian Diversification. The Ediacaran biota lived between about 635 and 541 million years ago and represent the earliest known communities of large, complex multicellular organisms. Most were soft-bodied, lacked hard skeletons, and were probably sessile or only weakly mobile. These organisms included forms such as Dickinsonia, Charniodiscus, and Spriggina. Some show clear links to later animal groups, while others appear to represent extinct evolutionary experiments. Far from being a biological dead end, the Ediacaran biota provided the ecological and evolutionary foundation for the Cambrian diversification that followed.The driving force behind this rapid diversification was the evolution of mobility in the mostly sedentary Ediacaran biota. Mobility created the opportunity to start eating other organisms, which in turn initiated an arms race between predator and prey and few things drive evolution more quickly than a rapidly escalating arms race. As more organisms become mobile so the diversification would have increases exponentially. The ultimate losers in this arms race were, of course, the sedentary remnants of the Ediacaran biota, which were exterminated by the mobile winners.
Many Ediacaran organisms also make sense when viewed through the lens of simple physical and biological constraints rather than as precursors to modern animals in the usual sense. Most appear to have been composed of poorly differentiated tissues, lacking specialised organs, internal cavities, or active transport systems for nutrients and gases. Without circulatory systems, guts, or complex tissues, their survival depended on direct diffusion across their external surfaces.
As a result, their body forms tend towards shapes that maximise surface area relative to volume within a minimal amount of biological material — a mathematical outcome of diffusion-limited life in an aqueous environment. Frond-like, quilted, flattened, or radially repetitive forms are precisely what would be expected of multicellular colonies optimised for nutrient and gas exchange directly with seawater. These morphologies are not “failed designs” or evolutionary dead ends, but natural solutions to the constraints imposed by size, chemistry, and physiology at the time.
Once internal transport systems, active feeding, and mobility began to evolve, those surface-maximising forms rapidly became obsolete. Animals capable of ingesting food, circulating nutrients internally, and moving through their environment were no longer constrained to diffusion-dominated geometries. This shift opened up an entirely new region of biological design space, helping to drive the rapid diversification of body plans seen in the Cambrian — while simultaneously eliminating many of the diffusion-limited Ediacaran forms.
The transition from the Ediacaran to the Cambrian did not happen instantaneously. It unfolded over roughly 20–30 million years, during which early animals gradually evolved greater mobility, more complex body plans, sense organs, nervous systems, and eventually hard body parts such as shells, spines, and exoskeletons. Molecular clock and fossil evidence both show continuity between late Ediacaran organisms and early Cambrian animals, even though most of the transitional forms were soft-bodied and therefore rarely preserved.
A crucial driver of this rapid diversification was the emergence of predation. Once some organisms became mobile enough to hunt others, natural selection intensified dramatically. Predators that evolved better senses, speed, and weapons gained an advantage, while prey that evolved shells, spines, toxins, camouflage, burrowing behaviour, or greater agility were more likely to survive. This escalating predator–prey arms race created a powerful feedback loop: each innovation by predators selected for counter-innovations in prey, and vice versa.
Such arms races can accelerate evolution to the point where diversification becomes exponential rather than linear. In geological terms, this can produce a burst of new body plans over a few tens of millions of years — a brief interval by the standards of deep time. When combined with the poor preservation of soft-bodied organisms, this rapid but gradual evolutionary process can misleadingly resemble something approaching instantaneous creation in the fossil record.
In reality, the “Cambrian Explosion” is not a mystery event without ancestry. It is the predictable outcome of ecological innovation, escalating evolutionary arms races, and severe preservation bias acting on a long, continuous evolutionary history that began well before the Cambrian itself.
The research of Dr Tarhan’s group is explained in a news release from The Geological Society of America®.
How Did These Strange, Ancient Organisms Turn into Such Remarkable Fossils?
New Research in Geology reveals why the 570-million-year-old Ediacaran Biota were so exceptionally preserved
In Earth’s fossil record, soft-bodied organisms like jellyfish rarely stand the test of time. What’s more, it’s hard for any animal to get preserved with exceptional detail in sandstones, which are made of large grains, are porous, and commonly form in environments swept by rough storms and waves. But around 570 million years ago, in a geologic time interval called the Ediacaran period, strange-looking, soft-bodied organisms died on the seafloor, were buried in sand, and fossilized in incredible detail.
Now, these fantastical fossils are found in deposits around the world. Scientists want to discover why the Ediacara Biota were so well preserved—in particular, the reasons for their very unusual fossilization as impressions in sandstone—in part to fill in a critical gap in the evolution of macroscopic life on Earth.
The Ediacara Biota look totally bizarre in their appearance. Some of them have triradial symmetry, some have spiraling arms, some have fractal patterning. It's really hard when you first look at them to figure out where to place them in the tree of life.
Dr. Lidya Tarhan, lead author
Department of Earth and Planetary Sciences
Yale University
New Haven, Connecticut USA.
The Ediacara Biota lived at a liminal moment just a few tens of millions of years before what geologists call the Cambrian Explosion, an episode that began about 540 million years ago, marked by flourishing diversity and complexity across nearly all evolutionary lineages of almost all animals living today. But more and more research suggests what Tarhan calls a “long fuse” to the explosion, and the rise of the Ediacara Biota formed a key stage of that slow burn.
Understanding how and why these Ediacara organisms are so exceptionally preserved is central to placing them in their evolutionary context and illuminating the origins of the complex life forms from which many animals, including humans, ultimately descend. A study from Tarhan and her colleagues published last month in a Geology paper titled “Authigenic clays shaped Ediacara-style exceptional fossilization” helps fill in that gap.
If we want to understand the origins of complex life on Earth, the Ediacara Biota really occupies a critical point in that trajectory. It's incredibly important, not just for the Ediacara Biota but for all exceptionally preserved fossil assemblages, that we try to figure out what are the mechanisms behind that exceptional fossilization so we can better gauge to what extent these fossil assemblages provide a faithful reflection of life on the ancient sea floor.
Dr. Lidya Tarhan.
To learn more, Tarhan and her team used a novel approach to determining what processes and minerals were at work when the Ediacara Biota organisms experienced death, burial, and fossilization. They measured isotopes of the element lithium in Ediacara Biota fossils found in Newfoundland and northwest Canada in deposits that are both sandy and muddy. The lithium isotopes helped the scientists determine whether clay minerals participated in fossilization and, in particular, whether these were detrital, meaning they washed off the continents, or authigenic, meaning the clays precipitated in the sea floor.
The researchers found that detrital clay particles were present in the sediments that buried these organisms on the sea floor. These minerals then served as nucleation sites for authigenic clays to form from silica- and iron-rich seawater in the upper sea floor, driven by the unusual chemistry of the Ediacaran seawater. These clays acted like cement, holding together sand particles in the sandstone and preserving outlines and replicas of the soft-bodied forms of the Ediacara Biota.
This counters a longstanding idea that the exceptional preservation of the Ediacara Biota might have occurred because their bodies were made of a uniquely hardy substance. Instead, it was the chemistry of the environment that lent itself to fossilization, according to Tarhan and colleagues.
Going forward, Tarhan wants to apply this lithium isotope technique to more fossils from different locations and geologic ages to see if the same mechanism applies. In the meantime, Tarhan says their findings help fill in the picture of what the world was like at a critical time in the evolution of complex animal life on Earth.
It's hard to overemphasize how dramatic of a change it is from the small and microbial life forms that dominate much of the Precambrian to the big step up in size and complexity seen in the Ediacara Biota and Cambrian Explosion. A clearer understanding of the processes responsible for fossilization across this interval will allow us to more robustly evaluate longstanding hypotheses for drivers of not only the appearance of the Ediacara Biota but also for their subsequent disappearance at the close of the Ediacaran period.
Dr. Lidya Tarhan.
Publication:
What this research neatly demonstrates is that the supposed “mystery” of the Cambrian Explosion is not a failure of evolutionary theory, but a failure of creationist caricature. Once the biases of the fossil record are properly understood, and once preservation, chemistry, ecology, and basic biology are taken into account, the illusion of a sudden, uncaused appearance of complex life simply evaporates. What remains is a long, continuous evolutionary history stretching deep into the Ediacaran, punctuated by periods of ecological innovation and acceleration, not moments of creation.
Creationists rely heavily on the hope that gaps in knowledge will remain permanent, because their arguments collapse the moment those gaps close. The Ediacaran–Cambrian transition was once poorly understood, and so it was pressed into service as evidence for design by default. Research such as that of Dr Tarhan’s team removes yet another refuge for this kind of thinking, showing that what once looked like a discontinuity is in fact an artefact of preservation and taphonomy rather than a genuine biological absence.
There is no requirement here to invoke sudden creation, new genetic information appearing from nowhere, or a designer intervening at a particular moment in deep time. Evolutionary theory not only accommodates the Cambrian diversification, it predicts exactly the kind of pattern we observe when mobility, predation, and arms races begin to dominate ecosystems. The fact that creationist claims grow ever more strained as the evidence accumulates is not a problem for science; it is simply the predictable fate of an idea that depends on ignorance for its survival.
As with so many previous “problems for evolution”, the Cambrian Explosion turns out to be a problem only for creationism — and one more example of how evidence, patiently accumulated, continues to erode the last remaining gaps into which its god can be forced.
Although I have to say, I've often enjoyed watching creationists try to argue that the 'sudden' appearance of the Cambrian biota, 450 million years ago, all of which became extinct long ago, proves Earth is only 6,000 - 10,000 years old and all life was created then without ancestry, just as the Bronze Age myths in the Bible relate.
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