ASU Research E-Magazine: Crunchy Rock-and-Fossil Jello Salad

Although the history of plants is a more subtle interest than that of dinosaurs, it is the plants that make up the environment.

What a difference a couple hundred million years make.

Today, scenic but desolate badlands dominate the Petrified Forest National Park in northeast Arizona. But during the Triassic Period, sea-floor spreading and plate tectonics had not yet begun to push the region 1,700 miles northward from its original location near present-day South America.

Giant reptiles and the earliest dinosaurs crept through a tropical swampy forest of 200-foot-tall, pinelike trees and among more than 60 other types of seed plants and ferns. Tourists who visit the park now marvel at the fossil bones, petrified logs, and leaf compressions found in the park’s Chinle Formation.

Tourists rarely see another type of fossil that Kathleen Pigg finds much more interesting. The Petrified Forest contains fragmentary seeds, leaf pieces, and bits of fern fronds. Three-dimensionally preserved, these bits of old stuff tell her about plant evolution and diversity during the Triassic Period.

acorn fossils
Ancient fossil acorns surrounded by their modern descendants

“Small nodules of rarely preserved plant fragments contain some of the first evidence we have for internal structures of some of these plants,” says Pigg, a paleobotanist at Arizona State University. “The large petrified logs aren’t going to help you figure out the evolution of many different kinds of plants because they only represent a few types.”

To date, Pigg has found in these new fossils the remains of extinct seed-bearing plants that may be related to modern pines and several extinct tropical ferns. The ASU researcher specializes in fossils preserved in this mode. Think of it as a sort of prehistoric compost. She compares them to a rather crunchy rock-and-fossil jello salad with fruit cocktail.

The carbonate nodules from the Petrified Forest and similarly preserved plant fragments in cherts were deposited in lakes or swamps. There they became saturated by water containing a concentrated solution of calcium carbonate or silica and other minerals. Petrified cherts formed in this manner often are of volcanic origin.

“These minerals infiltrated the plant’s internal cellular spaces, crystallized, and encased the plant’s cell walls in a rock matrix, preserving the original three-dimensional organization of tissues,” Pigg explains.

Pigg specially prepares specimens for analysis either through a chemical procedure that leaves organic fossil plant material embedded in hundreds of cellulose acetate peels, or she grinds rock sections thin enough for light to pass through.

cross section image of fossil stem
Kathleen Pigg studies thin sections of fossil samples which preserve fine internal details. Above, a cross section of a fossil stem shows good views of vascular structure. Below, a progressive series of thin sections from a fossil twig.
several sections of a fossil twig

“Both types of sections can then be reassembled with the aid of computerized three-dimensional reconstruction programs to understand the organization of a plant structure,” she says.

When used on anatomically preserved plant fossils, these techniques provide plant scientists with the most detailed structural data obtainable from the fossil record.

They allow Pigg to compare fossil plant anatomy directly to that of living forms, yielding crucial data about the origin, diversity and evolution of major plant groups.

“Plants are a fundamental component of the environment and can be indicative of climate,” Pigg says. “The detailed reconstruction of fossil plants in the context of a particular geological time and type of deposition provides a unique window into understanding Earth’s ecological and climatic history.”

Prehistoric Garden
The Petrified Forest contains one of the largest assemblages of petrified logs in the world. Fossil cherts, however, occur at select locations all over the world. Pigg thus tends a garden of research projects that includes fossils from Australia and Antarctica to western Canada, Washington state and Nevada.

In the late 1960s, researchers at Ohio State University discovered petrified cherts in Antarctica that contained anatomically preserved Glossopteris leaves. Similar fossils also turned up in Queensland, Australia.

Glossopteris fossils from Australia are more than 250 million years old.

“The glossopterids are an extinct group of plants that have sometimes been considered to be ancestors of the flowering plants,” Pigg says.

Botanists based this assessment on the glossopterids’ unusual reproductive structures, and on the net-like veins of their leaves, both of which bear some similarity to flowering plants.

A seed plant, Glossopteris dominated the Southern Hemisphere during the Permian Period 250 million years ago. Not only did plants look vastly different then. So did the globe. Scientists generally believe that during the Permian Period, Australia, Antarctica, India, Africa and South America all made up one large landmass they call Gondwana.

“For many years, paleobotanists knew about the glossopterids only from compressions and impressions—fossils that show only the external form of the plants and tell us little about the internal structure. This limited our ability to interpret the relationship of these plants to other seed plants,” Pigg says.

Pigg studied Antarctic fossils as a doctoral student at Ohio State in the 1980s. She discovered that their internal structure is much more variable than studies of compressions and impressions had shown.

Her research suggested that plants bearing Glossopteris leaves were a diverse group that thrived throughout the temperate forests of the Southern Hemisphere, but that they probably left no living descendants.

More recently, Pigg also documented the first leaf-stem attachment for petrified Glossopteris. Paleobotanists now have a context in which to view the numerous isolated glossopterid wood and leaf fossils that occur throughout the Southern Hemisphere.

In 1991, postdoctoral researcher Mary Louise Trivett accompanied Pigg to Australia to collect additional Glossopteris fossils to compare to the Antarctic remains.

“Along with an Australian collaborator, we are trying to understand the distribution of different types of glossopterids. While some anatomically preserved leaves and seeds occur in several basins in Antarctica and Australia, others are apparently restricted to a single locality,” Pigg says.

With another collaborator in Japan, Pigg is studying the anatomical structure of Glossopteris reproductive organs. Their goal is to determine the evolutionary relationship of this group to other seed plants, and to continue to document the unique plant history of the Southern Hemisphere.

Were the glossopterids different groups of plants that just happened to have similar features? Or, were they one major group that diversified to fill available ecological niches?

“We don’t really know the answer to that,” Pigg says. “They all seem to be based on the same major game plan, but they’re doing a variety of different things. I think they’re all probably related to one another somehow.”

The eucalyptus trees of Australia could illustrate a modern analogue of diversity, Pigg explains. Eucalyptus come in hundreds of species that vary subtly from one another, depending on the soils and climates they live in.

Flowering Research
The other major study that is beginning to blossom in Pigg’s laboratory focuses on the much more recent flowering plants yielded from the sediments of northwestern North America.

“These fossils are preserved in much the same way as the Glossopteris fossils and can be studied using the same techniques,” Pigg says. “But we are looking at plants that are considerably more closely related to modern families of flowering plants, as well as conifers (such as evergreens), and ferns.”

Pigg has used the fossils to document the evolution and distribution of modern flowering plants, as well as the changing climate of northwestern North America, in rocks of Eocene and Miocene age.

She works the 40-million-year-old Eocene sediments with the University of Alberta’s Ruth Stockey. The remains they find indicate the climate was milder in western Canada than it is today.

“Tropical and subtropical plants such as palms, numerous aquatic plants, and plants closely related to modern guavas grew near ponds and small lakes,” Pigg says. “In more upland habitats of the same age, members of the pine, birch, and rose families flourished.”

Paleobotanists generally consider the Northwest plant life during the Miocene Epoch 20 million years ago to be more modern by contrast.

“By then the climate was changing and becoming increasingly similar to the climates that occur in these areas today,” Pigg says. Even though Miocene plants look modern, they grew in quite different geographical regions than their modern kin.

The Columbia River area of central Washington state, for example, is now essentially treeless and covered with sagebrush. But it once was occupied by a combination of plants that have relatives now found living only in Asia, the eastern United States, and other areas of the west.

Asian relatives that once lived in Washington include the Ginkgo or “maidenhair tree,” and the Metasequoia, or “dawn redwood.” “Even plants like oaks that have modern relatives may not necessarily be the same,” Pigg says.

Pigg has initiated a new anatomical study of Miocene fruits and seeds from central Washington state with ASU graduate student Sandra Borgardt and Wesley Wehr, a curator of paleobotany at the University of Washington’s Burke Museum.

“These anatomically preserved fossils, which we are studying for essentially the first time, will provide an independent means of testing the hypothesis of just how modern these Miocene plants were.”—Steve Koppes