Darwin's dust, dumbells and mini-volcanoes

Fossils are formed when organic structures are gradually replaced by rock. In one scenario, groundwater, loaded with minerals in solution, seeps into the dead tissues. Gradually, the minerals are deposited, filling the dissolving plant or animal structures. The process takes thousands or millions of years, and only a tiny minority of living creatures end their lives in the right conditions for it to happen.

However, many plants jump the gun, starting the replacement of organic material with minerals while they are still living. The structures they form from silica are opal phytoliths, "plant stones", and they are recoverable from the plant remains whether they are newly harvested, or tens of thousands of years old.

They can be baked at 1000 degrees without losing their shape; they are released in the ash of forest fires. They resist dissolving in groundwater, unless it is extremely acidic or alkaline.  They can be powdered without being destroyed; the phytoliths themselves are tiny enough to survive, at 5 to 100 micrometeters long, or about the width of an average human hair.

Relative sizes, in microns, or micrometres. Image from the EPA.

Phytoliths can be found in the plaque on your teeth (or the teeth of a long-extinct dinosaur), in gardens ancient and modern, in old fire pits and cookware, in the stomach of fossilized animals. Paleoarchaeologists look for them in coprolites, fossilized animal dung. Darwin collected dust from the sails of the HMS Beagle, anchored off the Cape Verde Islands: this dust contained phytoliths.

And here's the beauty of it all; phytoliths come in many shapes and sizes, and each plant family produces its own pattern. Sometimes the phytoliths of the leaves are different from those of the stem or roots, so that a palaeobotanist can identify a plant from a handful of ancient ashes. This helps to trace the development of some of our modern plants: for example; where did our beans come from? Phytoliths help to discover the varieties used in a prehistoric Great Plains (US) village.

Another example: what did the dinosaurs eat? Grass, among other things. Paleoanthologists in India have discovered grass phytoliths in dino coprolites.

A few sample phytolith shapes:

From Waynes' World, Palomar.edu

 Microscopic view of the leaf epidermis of crabgrass (Digitaria sanguinalis) showing a row of dumb-bell shaped phytoliths just below the scabrous (minutely toothed) margin. The toothed margin is why grass leaves can cut your skin. Photo taken of a fresh leaf at 400x magnification. One phytolith is only 32 micrometers in length.

Family Poaceae, the true grasses. From U of Bohemia.

Phytoliths typical of Cucurbita sp. (Squashes) This one is Cucurbita ficifolia, AKA pie melon. From Science Direct.

These are from modern plants; lawn clippings from Washington State. The reddish bits are ash. And you can recognize the grass phytolith by now. From MicroLab NW.

Banana phytoliths are shaped like active volcanoes, with a flat bottom and a crater at the top. Rice leaves have "volcano" phytoliths, but these have round bottoms, like those roly-poly children's toys.

More info is available at:

• Wikipedia, of course.
• About.com
• Sandstone Archaeology, answering the question, "What questions can be answered using phytoliths?"
• A sample of work being done:  Using plant microfossils from dental calculus to recover human diet: a case study from Tell al-Raqā'i, Syria: Abstract