Listening to the monologue of thousand-year-old fossils - Unlocking the code of plant "stones"

Zhang Xinmu (Postdoctoral Fellow, China Science Popularization Research Institute)

"A long time ago, on Earth..." What a familiar opening line.

When you watch documentaries about ancient times, have you ever been curious: How do scientists know these convincing commentaries that replay stories from tens of millions or even hundreds of millions of years ago?

In addition to studying conventional fossils, researchers also use a method commonly used in archaeology, paleoclimate and paleovegetation research - phytolith analysis.

What are phytoliths

In nature, crystals, agates, stones and sand all contain silicon dioxide. A small amount of silicon dioxide is also dissolved in water. The roots of higher plants absorb soluble silicon dioxide while absorbing water and transport it to various parts of the plant. These silicon dioxides settle between and inside plant cells, just like stones in humans, forming plant "stones" in various shapes such as square, dumbbell, cross, and cap - hydrated amorphous silicon dioxide particles, also known as phytoliths.

These "stones" are very small, generally 20-200 microns in size, and are mainly composed of silicon dioxide (70-95%), in addition to adsorbed water (3-12%), organic carbon (0.1-6%), and trace elements such as aluminum, iron, potassium, sodium, and calcium. They are generally colorless or light flesh-red under a microscope, and may also appear brown or brown if there are many impurities.

Why is phytolith analysis useful?

Phytoliths are widely distributed and numerous. Wherever there are higher plants, there are phytoliths. There are 100,000 to 1 million phytoliths in 1 gram of leaves of Poaceae plants.

Phytoliths can be distinguished. The phytoliths produced by different plants and different parts of the same plant are different in appearance and quantity. By studying the size, shape, type and content of phytoliths, it is possible to infer what the vegetation was in the past and what its yield was.

Phytoliths are strong and resistant to acid, alkali and high temperature. They will not decompose even if the plants die and rot, or even if they are eaten by animals. They will eventually seep into the soil and be preserved for a long time along with the original information of many plants. The contents inside them are sealed inside and are not easily affected by changes in the external environment, so they can directly reflect the information of the ancient environment.

It is precisely because of these characteristics that phytoliths, as a micro-plant fossil, are used in various researches.

What can phytolith analysis do?

One of the most puzzling problems in the archaeological process is that some items cannot withstand the aging process and are prone to "rot", such as fabrics. What should we do if we find an incomplete "tattered mat"? During the field archaeological excavation of the Qizhen excavation area of ​​the Zhouyuan site in 2017, researchers from Peking University found some severely degraded fabrics, guessing that they were some kind of straw mats. They extracted phytoliths and found that their morphological characteristics and combinations were completely consistent with the phytoliths in reed stems, finally unveiling the reed mats from more than 3,000 years ago.

Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, dug out "dental calculus" from the teeth of dinosaur fossils and detected phytoliths in it, which turned out to be grass plants. Through comprehensive analysis, this study not only proves that the 100 million-year-old dinosaur was a "vegetarian" dinosaur, but also greatly advanced the key time point of the origin of grass plants from tens of millions of years ago to the Early Cretaceous period more than 100 million years ago.

The fossil of the dinosaur and the phytoliths in its mouth (photo provided by the author)

In addition to archaeology, some scientists use phytolith analysis to reconstruct ancient vegetation and ancient climate. A research team from Northeast Normal University reconstructed the ancient vegetation and ancient climate in the Changbai Mountain area and found that the area has experienced continuous climate change since the late glacial period, from cold and dry to warm and dry, then to warm and humid, and finally to a cool and dry environment. The distribution of forests and grasslands has also fluctuated with climate change.

Finally, it is important to emphasize that any research method has its own limitations. Although phytolith analysis is good, multiple methods are usually used in combination in the research process, such as animal and plant fossils, pollen, biomarkers, radiation or stable isotopes, etc., to prove from multiple angles and provide sufficient evidence to obtain more credible results.

(This article was published in Science Popularization Times on January 28, 2022)

(Typesetting: Zhang Xinmu Editing: Qi Yu Reviewing: Zou Zhen)

Since 2017, the China Science Writers Association has continued to carry out the "Science Popularization and Science Fiction Youth Star Program" project, selecting and cultivating young talents aged 18-45 who are interested in science popularization and science fiction creation from the whole society. In the five years since the launch of the activity, nearly 5,000 trainees have been trained, 1,245 science popularization and science fiction works have been displayed, and hundreds of authors have been awarded the title of "Science Popularization and Science Fiction Youth Star". In 2021, 30 outstanding works were selected from nearly 400 works, including 19 science popularization works and 11 science fiction works. With the active recommendation of the China Science Writers Association, some works have been published in media such as "Science Story Club", "China Youth Daily", and "Science Popularization Times", and the China Science Writers Association WeChat official account has forwarded them through the "Meeting New Stars" column for readers.