Fossil fern showcases ancient chromosomes

Near-perfect 180-million-year-old sample shows the plants haven't changed much

WELL PRESERVED  A 180-million-year-old fern fossil discovered in Sweden preserves the plant’s intricate interior architecture. A micrograph of a cross-section of the stem exposes a central cylinder surrounded by outer layers of tissue.

Courtesy of B. Bomfleur

After 180 million years buried in volcanic rock in the southern tip of Sweden, a recently discovered fern fossil looks almost as good as new.

The matchbox-sized fossil is among the best ever preserved: Thin slices viewed under a microscope reveal rounded cells jam-packed in the stem, like water balloons stuffed in a barrel. And inside the cells, within tiny dots of nuclei, the shadowy squiggles of chromosomes appear.

Such exquisite detail was probably preserved when minerals dissolved in hot salty water rapidly solidified within the entombed, living plant, paleobotanist Benjamin Bomfleur of the Swedish Museum of Natural History in Stockholm and colleagues report in the March 21 Science.

Because the fossil’s nuclei closely resemble those of the modern cinnamon fern, Osmundastrum cinnamomeum, the researchers suggest that the plants’ genomes probably haven’t changed much since Early Jurassic dinosaurs prowled the planet.

Slicing lengthwise through the stem of a fossilized fern reveals cells packed closely together, with tiny dots of nuclei inside. These nuclei, shown in a micrograph, look just like those of the modern cinnamon fern, suggesting that the fern’s genome has stayed mostly the same over the last 180 million years. Courtesy of B. Bomfleur

Meghan Rosen is a staff writer who reports on the life sciences for Science News. She earned a Ph.D. in biochemistry and molecular biology with an emphasis in biotechnology from the University of California, Davis, and later graduated from the science communication program at UC Santa Cruz.

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