Royal ferns are "living fossils," unchanged since the Jurassic

497 Royal ferns are "living fossils," unchanged since the Jurassic
Cross section through the stem of the fossilized fern / Benjamin Bomfleur
Scientists have discovered a 180-million-year-old fossil fern that is so pristinely preserved, its chromosomes are still clearly recognizable. Not only that, but its various microscopic parts closely match that of today’s ferns, suggesting that their DNA content or genome size hasn’t changed for hundreds of millions of years. 
Discovered in Early Jurassic lahar deposits at Korsaröd in southern Sweden, the fossil fern strengthens the reputation of royal ferns -- those belonging to the Osmundaceae family -- as “living fossils,” or the close, modern relative to something we only know about from the fossil record. In particular, the ancient fern closely resembles the cinnamon fern (Osmundastrum cinnamomeum), which can be found throughout the eastern U.S. and Canada today. 
The specimen, which is only about two inches long, might have been preserved in volcanic rocks by hydrothermal brines while it was still alive. Hot dissolved minerals solidified in the plant so rapidly that it preserved several subcellular structures including: cytoplasm, nuclei, and even chromosomes in various stages of cell division. You can see the tiny dots of nuclei below. Delicate organelles rarely fossilize so well.

Let’s step back for a minute. You might remember the various stages of mitosis (prophase, metaphase, and so on), but what we often forget is that about 90 percent of the time, cells are in interphase. That’s when it copies its chromosomes (which are kept within the nucleus) in preparation for cell division, or reproduction. 
Using at least three different types of microscopy, a team led by Benjamin Bomfleur from the Swedish Museum of Natural History analyzed the fossil stem, measuring the dimensions of interphase nuclei in the fern’s fossilized pith (the core) and the cortical parenchyma cells (a type of unspecified cell that differentiates into other plant cell types). 
They found that they match those of its living relative -- O. cinnamomeum -- very closely. The ancient Korsaröd fern has essentially the same number of chromosomes and DNA content in the Early Jurassic period as Osmundaceae ferns do today. Talk about evolutionary stasis.
The work was published in Science this week. 
Images: Benjamin Bomfleur 


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