Yale Bulletin and Calendar

October 24, 2003|Volume 32, Number 8

A new permanent exhibition at the Peabody Museum traces the history of once-towering conifers that have been transformed over the centuries into petrified wood.

Peabody exhibit showcases
'Rainbows in Stone'

"Petrified Wood: Rainbows in Stone," a new permanent exhibition in the Peabody Museum of Natural History's Great Hall, explores the processes whereby plants become petrified and documents the persistence of an ancient line of conifers that continues to dominate living remnants of a once flourishing Mesozoic ecosystem.

All of the specimens in the exhibit, whether fossil or recent, belong to a single long-lived family of giant conifers or cone bearing trees, known as the Araucariaceae, or araucarians. The colored logs and cross-section that are the centerpiece of the exhibit were collected near the Petrified Forest National Park in east central Arizona and date to the Late Triassic Period, about 225 million years ago. At that time, a great forest dominated by these towering trees extended from Texas into Utah.

Today, survivors of the araucarians are restricted to a portion of the warm-temperate to sub-tropical parts of the southern hemisphere, reaching into the northern hemisphere only in the Philippines. The fossil record shows that araucarians were abundant in moderate-to-warm climate areas of the northern hemisphere throughout the Mesozoic era. However, they disappeared abruptly 65 million years ago -- presumably, speculate scientists, as a result of the same catastrophe that caused the extinction of dinosaurs.

Judging from the size of their trunks and the stature of their descendents, araucarian trees grew upwards of 150 to 200 feet in height with trunks four to five feet in diameter. Older specimens of modern araucarians had an umbrella-shaped silhouette -- a characteristic that some paleobotanists think may have once served to protect the trees' foliage from grazing by gigantic sauropod dinosaurs such as Apatosaurus. Other scientists maintain, however, that the constraints of blood-pressure physics would have prevented such giant sauropods from ever raising their heads so high.

Once these trees died they were transported by swollen streams from their original growth site to swampy lowland regions. As they tumbled or jostled one another, their branches, bark and small roots either broke off or were worn away. Over time they were buried in silt, mudstone or volcanic ash. This blanket of sediment sealed the logs away from oxygen, slowing their decay. At the same time, ground waters containing petrifying minerals seeped through the logs encasing much of the original cell wall.

Altogether, some 40 minerals have been associated with the process of petrification, but silica (silicon dioxide) in the form of quartz is the most abundant. As the cell walls filled with silica, the logs began to harden. During this process of petrification, mineral impurities mixed with silica, producing the prized "rainbow wood" featured in this exhibit. The brilliant array of colors comes from manganese oxides (blues, black, purples), gypsum (white), and the iron oxides limonite (yellow) and hematite (red, rust).

One highlight of the exhibit is a slab of hardened volcanic ash from the Jurassic period of Argentina. Approximately 160 million years old, it contains over 40 araucarian cones, many still attached to the branches on which they grew. Cones of Araucaria typically disintegrate within a few weeks of ripening.

This specimen was preserved when a volcanic eruption tore down the forest canopy and buried the cones just as they had finished ripening. Because the parts of plants normally occur widely dispersed in the fossil record, with foliage, cone and tree specimens gathered from different localities and times, such an integral specimen is rare.

Also displayed in the exhibit are individual petrified cones of Araucaria mirabilis from the same fossil forest of Argentina viewed next to a nearly identical modern cone of Araucaria angustifolia from southern Brazil.

"Petrified Wood: Rainbows in Stone" was made possible through the support of Ruth R. Lapides, Alex G. Nason Foundation Inc., and Ralph Thompson.

The Peabody Museum of Natural History, located at 170 Whitney Ave., is open 10 a.m.-5 p.m. Monday-Saturday and noon-5 p.m. Sunday. Admission is $7 for adults, $6 for seniors age 65 and over, and $5 for children ages 3-18 and students with I.D. Admission is free to all Thursdays 2-5 pm. Museum members, Yale community members with a valid I.D. and children under 3 are admitted for free. The museum is wheelchair accessible.

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Campus Notes