Fossil fern: not much to show for at least 180 million years of evolution

Royal ferns have long been seen as 'living fossils.' A study of an ancient specimen, unearthed from volcanic deposits in Sweden and perfectly preserved, confirms a cellular match with living ferns.

As ferns unfurl in your garden or along your favorite hiking path this spring, pause for a moment to respect your elders.

Royal ferns have long been seen as living fossils. But researchers have drawn that conclusion by studying today's species. A newly described bit of fossil fern, reported in the current issue of the journal Science, marks the first time researchers can make direct comparisons at the level of individual plant cells and the chromosomes they contain.

It verifies that the genome of royal ferns has remained unchanged for at least the past 180 million years.

The 180-million-year-old specimen – a small underground portion of the stem known as a rhizome – was unearthed from volcanic deposits in southern Sweden near Korsaröd, some 36 miles northeast of the coastal city of Malmo. Its exquisite preservation hints at the possibility that much more remains to be uncovered, including a potential for finding dinosaurs.

"We might have only scratched the surface of a Swedish Jurassic Pompeii," says Stephen McLoughlin, senior curator at the Swedish Museum of Natural History in Stockholm and part of a trio of scientists on the team describing the find.

Royal ferns first emerged in the Southern Hemisphere more than 250 million years ago, researchers say. Fossil specimens dating to 220 million years ago show structural similarities with today's species. In addition, information gleaned from today's plants suggests that their genetic makeup has remained remarkably stable over geologic time.

Using three types of microscopes to analyze the newly described rhizome, Dr. McLoughlin and colleagues Benjamin Bomfluer and Vivi Vajda were able to image cells in a variety of developmental stages, as well as the fossil remains of the gel-like cytoplasm that filled the cells and the chromosomes that emerged in them. Features in the cells are virtually identical to those in living royal ferns, the team found, leading to the conclusion that they have remained evolutionarily dormant for most of their history on Earth.

The discovery is significant if for no other reason than its ability to see into ancient cells, suggests Gar Rothwell, a paleobotanist at Ohio University in Athens, Ohio.

Preservation at the level of a cell's chromosomes "is extremely rare for the fossil record," he writes in an e-mail.

The discovery is due in no small part to the persistence of Gustav Andersson, a local farmer who came across a piece of fossilized wood in the area in the 1960s and sent it to a geologist at nearby Lund University. A follow-up trip by a science team turned up nothing else, so the geologist washed his hands of the farmer's find, McLoughlin explains in an e-mail. So the farmer dug deeper, found more, and sent the new samples to the Swedish Museum of Natural History.

For a variety of reasons, the samples – including the rhizome the team analyzed – languished there nearly half a century. Six months ago, the trio reporting the results found themselves between projects, knew about the samples of the rhizome, and took the time to analyze them.

Their analysis opened a window on more than just the fern. Among their other attractions, the samples bear evidence of interactions with fungi and insects – bits of evidence "that help us to reconstruct the Jurassic ecology of the area," McLoughlin says.

The approach the team used for studying the samples also may pave the way for a new branch of paleoscience – the study of ancient cells, McLoughlin adds in an e-mail. The museum has in its collection a variety of three-dimensional plant and animal fossils to which the team hopes to apply the multi-microscope approach it used on the fern rhizome.

For instance, it should be able to recover information about the reproductive structures of extinct plants, including the architecture of seeds, he writes, adding that this "will greatly aid our understanding of the placement of these ancient plants in an evolutionary context."