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Section through the rhizome of a fossilized fern that looks for all the world like a living royal fern (A), complete with nuclei, nucleoli, and cell membranes (arrow) still visible (B) Fig. 1 from Bomfleur et al. 2014. Scale bars (A) 500 micrometers (B) 20 micrometers. Click image for source.
It defies belief, but a 180 million year old fern fossil unearthed in Sweden is so exquisitely preserved that it is possible to see its cells dividing.
So pristine is the fossil, reported scientists from the Swedish Museum of Natural History in the journal Science in March, that it is possible for them to estimate its genome size from the size of its cell nuclei -- and that it has remained substantially unchanged from its living descendants since the early Jurassic.
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The ferns were swallowed by a volcanic mudflow called a lahar, in which gas and rocky debris from an eruption mix with water and sediment. After entombment, hot salty water percolated into the coarse sediments around the ferns and acted as a preservative brine that immortalized the hapless plants. Their misfortune was our luck: 180 million years later, we can see details of their macro and micro anatomy so well that we can see how uncannily similar they are to their living descendants, royal and cinnamon ferns. They could be sisters!
Fossilized royal fern (top row) compared to an extant royal fern (G) and living cinnamon fern (H-L). Fig. S4 from Bomfleur et al. 2014. Click for source.
Incidentally, here is what a cinnamon fern looks like today. The brown spikes are the reproductive fronds loaded with spores. You would not be out of place picturing a giant dinosaur foot resting right behind this shrub.
Alive and well. "I wanna live with a cinnamon fern, I could be happy the rest of my life, with a cinnamon fern". "Cinnamon fern" by Physics major - en:File:Cinnamon fern.jpg. Licensed under Public domain via Wikimedia Commons.
Fossils from the family this fern belongs to had already been found from 220 million year-old rocks that were recognizable as the living species Osmunda claytonia -- the interrupted fern -- and other fossils from the Mesozoic have been found that are virtually indistinguishable from other genera and species in the fern's family, the Osmundaceae (the royal ferns). But microscopic preservation of this quality has rarely been seen in any fossils before.
In the picture at the top of this post, you can see at upper left a cross-section through the base of the plant, an underground stem called a rhizome. At right in (C), you can see cells complete with nuclei -- cells' DNA storage compartments -- and the tiny dots inside them of nucleoli, the structures inside nuclei where ribosomes are built (ribosomes make proteins for cells).
In addition to the clearly visible cell walls in C, you can also see at the head of the black arrow the cell's actual cell membrane, which evidently shrank away from the wall during fossilization (normally, plant cell membranes are flush against their cell walls due to a phenomenon called Turgor pressure that holds plants upright when it's operational and causes them to wilt when it isn't).
Preserved inside the cells, in addition to nuclei and nucleoli, are cytoplasm, cytosol granules, and chromosomes caught in flagrante divisio.
Cell division -- or mitosis, as you may recall from high school biology -- is the process by which cells copy and divide their DNA and then physically split the cell in two. In the images above, you can see the various phases of nuclear cell division, as the chromosomes condense and the nucleolus disintegrates (D-F), line up along the cell's equator (G and H), and finally pull apart toward the two future daughter cells (I and J).
In case you'd like a refresher, here is what the same process looks like in a modern lily (with apologies to Ravel).
Although many plants are prone to duplicating or hybridizing their genomes, inflating their size, the ferns in this family all have a identical and low chromosome count, suggesting long term genetic stability. But all evidence for this hypothesis had been based on living plants until now.
To see what light the fossil could shed on the issue, the Swedish measured the diameters, perimeters, and cross-sectional areas of non-dividing nuclei in the preserved cells. They match very closely with the living cinnamon fern Osmundastrum cinnamomeum.
Size of cell nuclei in living cinnamon fern compared to fossilized fern. "Colored box-and-whiskers plots in upper graph indicate interquartile ranges (box) with mean (square), median (solid transverse bar) and extrema (whiskers); dashed colored lines in lower graph indicate linear fits (n=76 versus n=37 measured nuclei for extant O. cinnamomeum versus the fossil). Fig. 2 from Bomfleur et al. 2014. Click image for source.
The results, the authors say, demonstrate that royal ferns and their fossilized relative are comparable with regard to these parameters and probably shared the same chromosome count and DNA content, making them a "notable example of evolutionary stasis among plants."
Although the term "living fossil" has fallen into disrepute among scientists and science writers of late, I have argued for its continued use in the past and I continue to maintain it has value as long as the user explains the term doesn't mean that the plant and its ancestor are literally identical. No plant, animal, or microbe, can remain untouched by 180 million years of evolution, no matter how little their appearance and genome size have changed.
But like horseshoe crabs, sharks, and coelacanths, and many other forms, these ferns have had incredible staying power, and I still believe the vivid and accessible term "living fossil" captures the essence of the phenomenon in a way that justifiably excites the public imagination. If there was ever such a thing as a living fossil, I'd argue based on this stunning hunk of rock that the royal ferns qualify.
Only slightly less amazing than the Korsaröd fern fossil (as the authors have dubbed it) was the assortment of spores and pollen preserved along with it. Incidentally, the analysis of these reproductive cells was how the scientists dated the fern fossil to the early Jurassic. Note the intricate surface textures and, in some cases, sharp bends giving the spores a Mercedes-emblem-esque appearance -- the marks of meiosis -- that reveal where the three sister spores that were the product of the sexual cell division originally lay.
Spores and pollen that may have drifted by a dinosaur. Fig. S2 from Bomfleur et al. 2014. Click image for source.
You will not look this good after 180 million years, even if you are swallowed by a lahar.
Reference
Bomfleur B. & V. Vajda (2014). Fossilized Nuclei and Chromosomes Reveal 180 Million Years of Genomic Stasis in Royal Ferns, Science, 343 (6177) 1376-1377. DOI: http://dx.doi.org/10.1126/science.1249884