Fossil Ferns Display Preserved Chromosomes

An exceptional example of cellular preservation,
from the Rhynie Chert in Scotland

As an organism dies, it sinks to the bottom of the ocean or slowly buried by sand or ash. Predators wait to either swallow it up in one gulp or take chunks out of its flesh. In the sediment around it bacteria and fungal spores latch on and break it down further into its constituent molecules until there is nothing left.

Even if the remains are somehow preserved, a rough ride through geological time can have adverse effects on a fossil. Trilobites contained within rocks in North Wales have been stretched and warped by continental collisions over 450 million years ago. Some are crushed out of existence.

As a result specimens of exceptional quality are exceedingly rare. Highly specific and infrequent conditions are needed to produce clear fossil preservation. Firstly the body must be buried extremely quickly under conditions hostile to life to prevent the remains from being damaged. Secondly the minerals preserving the fossil must be tiny. The smaller the mineral grain, the higher the resolution of the specimen. Sandstone provides a blurry picture of the past while silica gives a crystal clear, razor sharp view. Finally it must survive burial in the ground for potentially billions of years.

Yet fossils which meet these criteria can preserve structures down to a cellular level and sometimes even further. Recently researchers from the University of Lund and the Swedish Museum of Natural History struck lucky. The fossil, namely a fern, was actually collected in the 1960s near Korsaröd in central Skåne, by farmer Gustav Andersson, who donated it to the Swedish Museum of Natural History. The specimen remained forgotten in the museum's collections for over 40 years before it came to the attention of the researchers.

Ferns are common in the fossil record. I have a number in my collection, ranging from long leaves from the Coal Measures of Pennsylvania to thin, fragile fronds I chipped out of sandstones on the Yorkshire coast. Some show more detail than others. Indeed my Pennsylvanian specimens preserve the original water-transporting veins of the leaves. Yet this Swedish fern contained an unprecedented level of detail for one simple reason: it was engulfed by a lava flow. This is itself highly unusual. In almost all cases any biological matter which comes into contact with molten rock is reduced to carbon and ash. This is why geologists do not bother searching igneous rocks for fossils.

A wonderful cross section of cells from the 180 million year old fern fossil

Yet in rare circumstances a quick burial by molten rock can prevent the breakdown or the combustion of biological matter. A couple of years ago a rhinoceros fossil was found within the rocks of an ancient lava flow. The animal died an horrific death, but its bones were all present if a little charred. This fern is another example of the almost unheard-of igneous preservation. 180 million years ago Europe, including Sweden, existed as a series of volcanic islands. Lava flows found have been common, one of which incarcerated the leaf.

'The preservation happened so quickly that some cells have even been preserved during different stages of cell division,' said Vivi Vajda, Professor of Geology at Lund University. Invariably cellular fossils are found in glassy silica deposits yet these cells somehow survived burial in coarse grained, superheated liquid rock. Already this is a seemingly impossible specimen, a soft bodied plant with individual cells which were preserved in molten lava; but there was more.

A close up of some of the cell's nuclei on the left. The dark spots are the
chromosomes. On the right are modern equivalents stained to highlight
the nuclei and chromosomes

Different cells were frozen at different stages of the cell cycle, meaning that the organelles were preserved at different stages of replication. It is for this reason that some cells contained multiple nuclei, some of which actually contained fragments of the chromosomes, DNA packaged around proteins to form a dense encoding structure: a truly stunning revelation.

While it will be almost certainly impossible to extract genetic information from the structures, by examining the size of the cell nuclei the researchers were able to show that the plant was a member of the Royal Fern family and closely related to the cinnamon fern Osmundastrum cinnamomeum.

'Royal Ferns look essentially the same now as they did during the Jurassic Period, and are therefore an excellent example of what we call a living fossil,' said Vajda. By studying pollen grains trapped within the lava rocks, the researchers found that the fern's ecosystem was incredibly diverse. 'This naturally leads us to think that there must be more to discover. It isn't hard to imagine what else could be encapsulated in the lava flows at Korsaröd in Skåne,' concluded Vajda.

The ability to study ancient cells and their structure stretches the bounds of our knowledge far beyond what we thought possible even just a few years ago. We can reconstruct the colour of dinosaurs, the origins of complex life and even find the relationships between the organisms which inhabit and once inhabited the Earth. Some people say that fossils are of limited use and that molecular biology is the way forward. Yet as the analysis of Osmundastrum cinnamomeum shows a new age of paleontology begins.