Some of the most interesting evidence for ancient life does not look like a fossil. It can look like a wrinkle.
Researchers studying Jurassic rocks in Morocco have identified unusual sedimentary structures that may record microbial communities living on the seafloor roughly 180 million years ago.
That would not, by itself, be particularly surprising. Microbial life was already widespread in Earth's oceans.
The strange part is where these structures formed. The sediments appear to have accumulated in relatively deep water — around 200 metres below the ancient ocean surface.
That raises an important question.
If sunlight was severely restricted at that depth, what was providing the energy for these organisms?
One possibility is that they were not using sunlight at all. They may have been running an ecosystem on chemistry.
And if that interpretation is correct, the discovery provides another reminder that the ancient ocean was probably far more biologically diverse — and far stranger — than the conventional fossil record suggests.
The strange "wrinkles" in the rock
The story began with an unusual texture preserved in Jurassic rocks in Morocco. The surface of the rock contains irregular ridges and depressions that resemble wrinkled skin.
Geologists refer to structures of this general type as "wrinkle structures". They are important because some wrinkle structures are associated with microbial mats.
Microbial mats are dense communities of microorganisms that grow across sediment surfaces.
They can trap and bind sediment, alter the chemistry immediately above and below the surface, and leave physical structures behind.
Over geological time, the organisms disappear while the structures they created can become preserved in rock.
But there is a problem. Wrinkle structures are not automatically fossils. Some similar structures can be produced by entirely physical processes.
That means the Moroccan discovery requires considerably more investigation than simply identifying a strange pattern and declaring it biological.
The real question is whether the geological evidence supports a biological explanation.
Why the depth matters
The rocks are thought to have formed in a relatively deep marine environment. That matters because sunlight becomes increasingly restricted as it passes through seawater.
Photosynthesis is therefore progressively more difficult as depth increases.
The exact amount of light available depends upon factors including water clarity, suspended sediment and the wavelength involved.
But a microbial community living hundreds of metres below the surface cannot simply be assumed to have been powered by sunlight.
Something else may have been happening. And Earth already provides us with an example of an alternative.
Life without sunlight
In the modern deep ocean, some ecosystems operate almost independently of sunlight.
Hydrothermal vents are perhaps the most famous example. Chemical-rich fluids emerging from the seafloor provide microorganisms with usable sources of energy. Other organisms then feed directly or indirectly on those microbial communities.
Cold seeps provide another example. There, chemicals escaping from geological structures and sediments can support specialised microbial ecosystems.
The fundamental principle is simple. Life does not necessarily require sunlight. It requires an available source of energy.
For some microorganisms, chemical reactions provide that energy. This raises the possibility that the Jurassic organisms responsible for the Moroccan structures were using a similar strategy.
Rather than photosynthesis, their metabolism may have depended upon chemical gradients in the sediment or surrounding water.
An ecosystem powered by geology
This is where the story becomes particularly interesting. The ancient seafloor was not necessarily a chemically uniform environment. Sediments were constantly being deposited, buried, disturbed and transported. Underwater landslides and turbidity currents could move enormous quantities of sediment downslope.
A turbidity current is essentially an underwater avalanche — a dense flow of sediment suspended in water.
Such events can radically alter the seafloor. They bury existing communities, they move organic material, they introduce minerals and nutrients into new environments, and they can create fresh surfaces for microorganisms to colonise.
The Moroccan researchers have proposed that such sediment flows may have helped create conditions in which microbial communities could flourish.
In other words, an event that looks destructive from one perspective could have created an ecological opportunity from another.
The seafloor may have been repeatedly reset
Imagine the environment 180 million years ago. There is no sunlight reaching the seafloor in useful quantities. The environment is relatively dark. Then a massive sediment flow moves down the continental slope.
It deposits a new layer of material. The chemistry of the newly deposited sediment changes. Microorganisms colonise the surface and they begin exploiting chemical energy.
As the microbial community grows, it interacts with the sediment. It stabilises some areas, and it can modify the chemistry.
It produces the characteristic textures that eventually become preserved. Then another geological event buries the community. The process starts again elsewhere.
If something like this happened repeatedly, the seafloor could have supported a specialised microbial ecosystem despite being a very different environment from the sunlit waters above.
But there is a catch
The biological interpretation is fascinating, but it should not be overstated. Wrinkle structures are not uniquely biological.
Scientists have long recognised that sedimentary processes can produce structures that resemble microbial textures.
That means morphology alone cannot establish that ancient microbes created the Moroccan structures.
The strength of the interpretation comes from the combination of evidence:
- the physical appearance of the structures
- their sedimentary setting
- the depth at which the sediments formed
- the chemistry of the environment
- and comparisons with known microbial systems
Even then, the conclusion remains an interpretation rather than an observation of the organisms themselves.
We cannot look at the rock and see the microbes. We are reconstructing their existence from the changes they may have made to their environment.
The predators create another problem
There is another reason microbial mats are interesting. They can be extremely vulnerable to grazing.
If larger organisms constantly feed on a microbial mat, it becomes difficult for a substantial structure to develop.
This may help explain why extensive microbial mats are particularly common in some ancient environments where grazing pressure was limited.
Chemically unusual environments can potentially provide another form of protection.
Some chemosynthetic microbial communities are associated with sulphur-rich chemistry and other compounds that can be unsuitable for many animals.
That creates an unusual ecological possibility. The same chemistry that provides energy for microorganisms may also help prevent them from being consumed.
The microbes effectively create an environment in which they can thrive while making it less hospitable to potential grazers.
Why this matters beyond one Moroccan rock
The real significance of the discovery may not be the Jurassic period. It is the possibility that similar ecosystems have been hiding in the geological record all along.
Geologists know that the fossil record is incomplete. But it is not simply incomplete because fossils get destroyed. The problem also begins with recognition.
If scientists expect microbial communities to occur mainly in particular shallow-water environments, they may interpret similar structures found in deeper sediments differently.
A texture interpreted as a physical sedimentary structure might instead deserve investigation as a possible biological structure.
That changes what researchers know to look for.
The fossil record has a built-in bias
Our picture of prehistoric life is dominated by organisms that fossilise well. Bones survive, shells, teeth, and hard mineralised structures survive.
Microbial communities are different. They can leave no obvious body fossil whatsoever.
Their existence may instead be recorded through:
- sedimentary textures
- mineral deposits
- chemical changes
- isotope signatures
- microscopic structures
- altered minerals
- or the geometry of the sediment itself
That means an ecosystem can disappear completely while leaving behind only an environmental fingerprint.
And recognising that fingerprint is not always straightforward.
The ancient ocean was more than dinosaurs
The Jurassic is usually imagined through its spectacular animals. Dinosaurs walked across the continents, marine reptiles hunted beneath the waves. Ammonites occupied the oceans.
But beneath all of that was another biological world. Microorganisms were transforming chemicals. Sediment was constantly being moved. Oxygen levels varied.
Much of this world left little obvious evidence. Yet it was fundamental to the functioning of the planet.
The spectacular fossils may actually represent only a small fraction of the ancient biosphere.
There is a much bigger question
If microorganisms could establish ecosystems in dark Jurassic seafloor environments, how widespread were such ecosystems?
Were they rare? Or were they common wherever geological processes created the right chemical conditions?
The answer could change our understanding of how much of the ancient ocean was biologically active.
It also raises another possibility. Perhaps some environments that have traditionally been considered relatively barren were actually highly productive at the microbial level.
We tend to judge environments using the organisms that are easiest for us to see. That may be a dangerous assumption when dealing with ancient ecosystems.
The implications extend beyond Earth
This is also why research into Earth's deep environments matters to astrobiology. If life requires sunlight, the number of potentially habitable environments beyond Earth becomes considerably smaller.
But if microorganisms can survive by exploiting chemical energy, the situation changes.
Europa has an ocean beneath its icy exterior. Enceladus appears to have a subsurface ocean and releases material into space through plumes. Mars has a long history of water interacting with rocks and minerals.
None of these observations demonstrates that life exists on those worlds. But they show why scientists are interested in environments where chemical energy could potentially exist without sunlight.
Earth provides the only confirmed example of life we have.
Understanding the full range of environments in which terrestrial life can operate gives scientists better models for searching elsewhere.
The uncomfortable possibility
There is an interesting irony here. Humans tend to search for life by looking for places that resemble the environments in which familiar organisms thrive.
We look for water, suitable temperatures, organic molecules, energy sources. But we can also unconsciously assume that life needs conditions similar to those supporting plants and animals on the surface.
The deep ocean demonstrates otherwise.
An ecosystem does not need forests, sunlight or even an abundance of oxygen. It can operate in darkness, it can be driven by chemistry, it can exist around geological processes.
And it can leave behind a geological signature rather than a recognisable fossil.
What the Moroccan rocks really tell us
The discovery does not prove that every wrinkle in an ancient rock represents a microbial ecosystem. Nor does it overturn our understanding of Jurassic life.
What it does is provide a plausible example of how microorganisms may have occupied environments that would previously have seemed unlikely.
That is scientifically more interesting than a simple fossil discovery. It expands the list of places where researchers can search for ancient life.
And it demonstrates once again how easily biological evidence can be mistaken for geology.
A rock may appear to record nothing more than sediment moving across an ancient seabed. But sometimes the sediment itself may have been altered by organisms.
The organisms are gone, the chemistry is altered and the community has vanished. Only the footprint remains.
Life leaves clues
Perhaps that is the most important part of the story. The ancient ocean was not divided neatly into "habitable" and "uninhabitable" zones.
It contained gradients, chemical reactions, temporary energy sources, volcanic activity, sediment flows. And countless microscopic organisms exploiting whatever energy was available.
Some of those ecosystems may have existed in places where sunlight was almost irrelevant.
The Moroccan structures offer one possible window into that hidden world.
They are not a photograph of an ancient ecosystem; they are something subtler. A trace, a geological clue, a possible fingerprint left behind by organisms that lived in darkness nearly 200 million years ago.
And that raises a question that extends far beyond Morocco:
How much of Earth's ancient biosphere are we still failing to recognise because we are looking for fossils, when we should be looking for the footprints that life left behind?
Read more on:
What happened to the lost forests of Earth?
The hidden world under Antarctica
Are there lost civilisations beneath the worlds deserts?
The 5 million year old whale graveyard
The missing civilisations of the Ice Age
Comments
Post a Comment