Trees of stone tell tropical tale

Peruvian petrified forest offers insight into low-latitude conditions of millions of years ago

The fossilized remains of a South American forest buried by volcanic ash
are providing scientists with a detailed look at a tropical forest ecosystem from millions of years ago.

FOSSILS WITH A VIEW A petrified forest now high in the Andes (including fossilized leaves, inset) provides scientists with a rare and detailed look at an ancient tropical forest ecosystem. Woodcock

The petrified forest, known as El Bosque Petrificado Piedra Chamana, covers a 1-by-0.5-kilometer area in northwestern Peru, Deborah Woodcock, a paleoclimatologist at Clark University in Worcester, Mass., and her colleagues report in the July-August GSA Bulletin. Fossils at the site include both large tree trunks and leaves — a rarity for low-latitude forests, since tropical heat dramatically accelerates the decomposition of organic matter. Analyses of leaf size and shape hint that the average annual temperature at the site around 39.4 million years ago, when the forest was growing, was above 25° Celsius.

Some of the fossils are 1-meter-tall stumps, still standing upright with their petrified roots preserved in the ancient, now-solidified soil. Many of the other fossils are logs that lie oriented along a northwest-southeast direction. Together, these data suggest that the forest was buried waist-deep in volcanic ash from a nearby eruption and then smothered by a flow of ash that snapped the trees off and entombed them, Woodcock says.

Now 100 kilometers from the coast and about 2,400 meters high in the Andes, the site sat near sea level at the time of the killer eruption, Woodcock and her colleagues report. “There’s a lot of biological interest in tropical forests, both those of today and of the past,” Woodcock says. South American forests “are the most diverse forests in the world, but we don’t know a lot about their history,” she notes.

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