Meet the ancient water moulds that were already devastating plants 330 million years ago
Fossil oomycetes from Carboniferous cherts reveal that these notorious plant pathogens had established both saprotrophic and parasitic lifestyles long before the first dinosaur walked the Earth.
This post is adapted from a talk given at The Sainsbury Laboratory, Norwich, by Christine Strullu-Derrien (Natural History Museum, London): “Early terrestrial microorganisms revealed through advanced imaging techniques .”
If you work on oomycetes, you know the frustration. You tell someone you study plant pathogens and they ask: “So, fungi?” No. Not fungi. Not even close. Oomycetes — the water moulds — are Stramenopiles, more related to brown algae than to any true fungus. But the misidentification is understandable: they look like fungi, they behave like fungi in many ways, and they cause some of the most devastating plant diseases on the planet.
On April 9th, 2026, at The Sainsbury Laboratory, Christine Strullu-Derrien gave a talk that stopped us in our tracks. The title was deceptively understated: “Early terrestrial microorganisms revealed through advanced imaging techniques”.
What followed was a tour through 330 million years of oomycete history, reconstructed from Carboniferous chert fossils using some of the most sophisticated microscopy techniques available today.
The question is this: how deep do oomycete–plant interactions go? When did these organisms first establish themselves as plant colonisers, decomposers, and parasites? Strullu-Derrien’s work gives us a surprisingly clear answer.
The fossil record of oomycetes is not a footnote. It is a story about the deep evolutionary roots of some of the most destructive organisms in agriculture today.
Cherts and coal balls: nature’s time capsule
Most fossils are compressions — flattened impressions of organisms pressed between rock layers. Useful, but they rarely preserve three-dimensional cellular detail. Cherts are different. When silica-rich fluids permeate through peat before decay sets in, they can mineralise tissues at the cellular level, locking microorganisms in three dimensions with extraordinary fidelity. It is also the case in coal balls that are calcareous permineralized peats).
Two Carboniferous chert formations have proven particularly productive for fossil oomycetes: the Esnost Chert and Grand’Croix Chert of the Massif Central in France (~330 Ma and ~300 Ma respectively), and the coal balls of Yorkshire in England (~315 Ma). Together, they give us a ~30-million-year window into Carboniferous swamp ecosystems dominated by giant lycophyte trees like Lepidodendron — club mosses that grew to 30 metres tall.
What researchers have found inside these fossils has been steadily rewriting our understanding of oomycete evolution.
Combresomyces cornifer: the saprotroph that started it all
Let us start at 330 Ma, in what is now the Massif Central of France. Here, the lycophyte Lepidodendronrhodumnense was colonised by an oomycete named Combresomyces cornifer — described by Dotzler and colleagues in 2008 (PMID: 18692373).
This species was a saprotroph — feeding on dead or decaying plant material rather than attacking living cells. You can see the hyphae threaded through the plant tissue in the images above, with the arrows pointing to branching filaments, and on the right, that striking spherical structure with ornamentations radiating outward like a biological sunburst. If you showed me this image without context, I would have a hard time telling it apart from a modern oomycete.
C. cornifer is now one of the reference fossils of the entire lineage, and for good reason: it is beautifully preserved, morphologically diagnostic, and stratigraphically well-constrained.
Oochytrium lepidodendri: deep inside the xylem
Still at 330 Ma, but now looking at the Esnost Chert specifically, we find something even more interesting. Oochytrium lepidodendri (Strullu-Derrien et al., 2021, PMID: 33651837) was living inside the primary xylem of a waterlogged Lepidodendron branch as the tree began to decay in the swamp.
What I find particularly exciting about this paper is the methodology. The authors did not stop at brightfield microscopy — they applied confocal scanning laser microscopy (CSLM) and three-dimensional tomographic reconstruction using SPIERS software. The result is a 330-million-year-old organism rendered in full 3D, with internal anatomy visible that no conventional thin section could reveal.
This is where palaeomycology is going. Not just cataloguing organisms but reconstructing them. The technology is catching up with the questions.
Combresomyces williamsonii: the first fossil parasite
Now we move to Yorkshire, UK, at 315 Ma. This is where things get really interesting.
Combresomyces williamsonii (Strullu-Derrien et al., 2011, PMID : 20843846) was not decomposing dead material. It was colonising the living seed fern Lyginopteris oldhamia — a parasite.
Look at the diagnostic diagram in the centre of the image above. That is an oogonium containing the oosphere — the female gamete attached to an antheridium containing a fertilization tube, the structure through which the male nucleus is delivered during sexual reproduction. These structures are morphologically indistinguishable from those seen in living oomycetes today. We are looking at a 315-million-year-old reproductive apparatus, frozen in stone, that still matches the modern blueprint.
The fertilization tube of Combresomyces williamsonii pushes the origin of oomycete parasitism firmly into the Carboniferous. This is not an early relative exploring a new lifestyle — this is a fully formed parasite.
This matters enormously for our understanding of oomycete evolution. Plant parasitism did not emerge gradually in the modern era. It was already a perfected strategy over 300 million years ago.
Galtierella biscalithecae: life inside a reproductive organ
Jumping to 300 Ma and the Grand’Croix Chert, we encounter Galtierella biscalithecae (Krings et al., 2010). This species was found preserved inside a partially degraded reproductive organ — a saprotroph making the most of decaying tissue in the swamp ecosystem.
The panel of images here is particularly rich — you can see oogonia, oospheres, and stalks in various stages of development and preservation. The Grand’Croix Chert is remarkable for exactly this kind of completeness. It is as though the silicification process captured an entire life-history snapshot: the organism at multiple developmental stages, in one place, at one moment in time.
Kamounia striata: the most sophisticated imaging yet
And finally, we arrive at the most recent addition to the fossil oomycete catalogue — and the one with the most technically impressive characterisation to date.
Kamounia striata was described in 2026 (Strullu-Derrien et al., 2026, PMID: 41660263) — a saprotroph on plant stem remains from the Grand’Croix Chert at 300 Ma. The paper deploys the full modern imaging toolkit: brightfield microscopy, CSLM maximum intensity projections, and 3D reconstruction via SPIERS software. The results are extraordinary — a 300-million-year-old microorganism rendered in nanoscale three-dimensional detail.
What you are looking at in the right-hand panels is not a cartoon reconstruction. It is derived directly from optical sectioning of the fossil material, with fluorescence signals mapped onto a 3D volume. This level of structural resolution for a Carboniferous microfossil was unthinkable even a decade ago.
What does it all mean?
Let me try to synthesise what these five species — spanning 330 to 300 Ma — are telling us.
First: Oomycetes had diversified ecologically by at least the Carboniferous. Saprotrophic and parasitic lifestyles were already well established. This is consistent with molecular clock estimates that place the oomycete crown group origin deep in the Palaeozoic.
Second: The core reproductive morphology has been remarkably conserved. The oogonia, oospheres, and fertilization tubes we see in these fossils are recognisable to anyone working on modern Phytophthora or Pythium. Evolution has tinkered at the margins without dismantling the fundamental toolkit.
Third: Permineralized preserved fossils are an underexploited resource. The application of CSLM and 3D tomography to material like this is still in its early days. I would not be surprised if many more oomycete (and other eukaryotic) fossils are hiding in museum collections, waiting for the right imaging technology to reveal them.
These fossils are not just curiosities for palaeontologists. They are evolutionary anchors — fixed points in deep time against which we can calibrate the molecular history of one of the world’s most destructive groups of plant pathogens.
Why should we care — beyond the academic interest?
Because oomycetes are not history. Phytophthora infestans caused the Irish Potato Famine. Plasmopara viticola devastates European vineyards. Phytophthora ramorum is actively destroying oak and larch populations across the UK and North America right now.
Understanding when and how oomycetes evolved their plant-colonising strategies — including when parasitism first emerged, when key virulence mechanisms appeared, how host range diversified — is essential background for fully understanding the biology of this unique group of eukaryotic microbes. The fossil record, imperfect as it is, provides calibration points that complements with data the molecular clock estimates.
So next time someone asks you whether you study fungi: no. We study something stranger and more enigmatic. And the evidence for just how old goes back more than 330 million years, preserved in the chert and coal forests of ancient Europe..
Acknowledgements
I would like to sincerely thank Christine Strullu-Derrien for inspiring this blog post, for sharing her presentation slides and photos, and for revising the draft. I also thank Sophien Kamoun for his revision and helpful feedback. This blog was written with assistance of Claude cowork.
Cite as: Boukteb, A, C. 2026. Meet the ancient water moulds that were already devastating plants 330 million years ago. Zenodo. https://doi.org/10.5281/zenodo.20704777
References
Dotzler N, Krings M, Agerer R, Galtier J, Taylor TN. Combresomyces cornifer gen. sp. nov., an endophytic peronosporomycete in Lepidodendron from the Carboniferous of central France. Mycol Res. 2008 112:1107–14. doi: 10.1016/j.mycres.2008.03.003. PMID: 18692373.
Krings M., Dotzler N., Taylor T.N., Galtier J. A Late Pennsylvanian fungal leaf endophyte from Grand-Croix, France. Rev. Palaeobot. Palynol. 2009; 156:449–453. doi: 10.1016/j.revpalbo.2009.04.010.
Strullu-Derrien C, Kenrick P, Rioult JP, Strullu DG. Evidence of parasitic Oomycetes (Peronosporomycetes) infecting the stem cortex of the Carboniferous seed fern Lyginopteris oldhamia. Proc Biol Sci. 2011 278(1706):675–80. doi: 10.1098/rspb.2010.1603. PMID: 20843846.
Strullu-Derrien C, Gèze M, Spencer ART, De Franceschi D, Kenrick P, Selosse MA, Knoll AH. An expanded diversity of oomycetes in Carboniferous forests: Reinterpretation of Oochytrium lepidodendri (Renault 1894) from the Esnost chert, Massif Central, France. PLoS One. 2021 16(3):e0247849. doi: 10.1371/journal.pone.0247849. PMID: 33651837.
Strullu-Derrien C, Spencer ART, Gèze M, Martos F, De Franceschi D, Kenrick P, McLoughlin S, Selosse MA. A new occurrence of saprotrophic oomycetes from the ca 307–303 million-years-old Grand’Croix Chert (Massif central, France). iScience. 2026 29(2):114634. doi: 10.1016/j.isci.2026.114634. PMID: 41660263.
