In the hills outside the small village of Sexi, Peru, a fossil forest holds secrets about South America’s past millions of years ago.
When we first visited these petrified trees more than 20 years ago, not much was known about their age or how they came to be preserved. We started by dating the rocks and studying the volcanic processes that preserved the fossils. From there, we began to piece together the story of the forest, starting from the day 39 million years ago when a volcano erupted in northern Peru.
Ash rained down on the forest that day, stripping leaves from the trees. Then flows of ashy material moved through, breaking off the trees and carrying them like logs in a river to the area where they were buried and preserved. Millions of years later, after the modern-day Andes rose and carried the fossils with them, the rocks were exposed to the forces of erosion, and the fossil woods and leaves again saw the light of day.
This petrified forest, El Bosque Perificado Piedra Chamana, is the first fossil forest from the South American tropics to be studied in detail. It is helping paleontologists like us to understand the history of the megadiverse forests of the New World tropics and the past climates and environments of South America.
By examining thin slices of petrified wood under microscopes, we were able to map out the mix of trees that thrived here long before humans existed.
To figure out the types of trees that had been growing in the forest before the eruption, we needed thin samples of the petrified wood that could be studied under a microscope. That was not so easy because of the volume and diversity of fossil wood at the site.
We tried to sample the diversity of the woods by relying on features that could be observed with the naked eye or with small hand-held microscopes, things like the arrangement and width of the vessels that carry water upwards within the tree or the presence of tree rings. Then we cut small blocks from the specimens, and from those we were able to prepare petrographic thin sections in three planes. Each plane gives us a different view of the tree’s anatomy. They allow us to see many detailed features relating to the vessels, the wood fibres and the living-tissue component of the wood.