C R U N C H Y
ROCK-AND-FOSSIL
JelloSalad
Although the history of plants is a more subtle interest than that of dinosaurs,
it is the plants that make up the environment
b y
S t e v e
K o p p e s
Ancient fossil acorns surrounded by their modern descendants.
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ASU Research —Fall/Winter 1994
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.
“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.
JOHN C. PHILLIPS PHOTOS
W
hat 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, seafloor spreading and plate tectonics had not yet
begun to push the region 1,700 miles northward from its original location near presentday South America.
Giant reptiles and the earliest dinosaurs
crept through a tropical swampy forest of 200foot-tall, pine-like 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.
“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, an Arizona State University paleobotanist. “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.
Glossopteris fossils
“These minerals infiltrated the plant’s
from Australia
internal cellular spaces, crystallized, and
are more than 250
encase the plant’s cell walls in a rock matrix,
million years old.
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.
“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.
ASU Research —Fall/Winter 1994
21
PHOTO COURTESY KATHLEEN PIGG, PH.D.
Fossil collecting site in Yakima Canyon, Washington.
INTELLECTUAL RESOURCES
JOHN C. PHILLIPS PHOTOS
T
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Kathleen Pigg
ASU Research —Fall/Winter 1994
H E M E M B E R S of the Southwest Paleontological Society are accustomed to finding rare
fossils during field trips and expeditions taken
under the flag of the Mesa Southwest Museum. But when
they discovered paleobotanist Kathleen Pigg at ASU, they
had encountered a rare intellectual resource as well.
There are probably about 300 active paleobotanists
in the United States, not counting those who study
pollen. She is the one of the few in Arizona.
The society now counts nearly 300 dues-paying
members. Almost all of them are amateur paleontologists. Before Pigg came along, few of them knew much
about fossil plants. Now, club members are eager to join
Pigg in her fossil-plant research endeavors, the dinosaur
frenzy spawned by Jurassic Park notwithstanding.
“She really has been helpful in
contributing to our club,” says Brian
McClelland, who doubles as the
society’s president and associate
curator of the museum. “We can
assist her and in turn she can lend
us her expertise. I’m really thrilled
to have her help because there
aren’t any people in our club with
her background.”
Pigg’s collaborations with the
society began in the fall of 1992,
when she became one of its first
guest speakers. The next month she
led the group on a trip to a fossilplant site near Payson.
Since then, she has held a couple
of fossil-plant workshops in her laboratory for the society’s members.
In November 1993, Pigg presented
a paper at the museum’s first
“Fossils of Arizona” symposium.
She was also coauthor of another
paper presented by the society’s
Buck Tegowski. Pigg commended the symposium as an
exceptional job in bringing together interested professional and lay people to discuss topics of regional paleontological interest.
“There has always been a strong interaction
between professional paleobotanists and amateurs who
are interested in fossils,” she says. “Tremendous collections have been amassed and important discoveries
made by amateurs, some of who become quite actively
involved in research.”
Pigg is not the society’s only significant connection
to ASU. McClelland graduated from ASU at about the
time that Pigg arrived as a faculty member. Also a graduate of Pennsylvania State University, McClelland
earned a second bachelor’s degree—in geology—from
ASU in 1988.
He was graduated with a master’s degree from
Texas Tech, where he conducted research on the skull
of Allosaurus, an earlier cousin of the famous
Tyrannosaurus rex.
He currently is analyzing a set of tracks from northern Arizona, possibly left by a mammal-like reptile during the Triassic Period about 200 million years ago. The
tracks are on extended loan to the museum.
“They may be really significant,” McClelland says,
because scientists know of only one other set of mammal-like reptile tracks from North America.
Under McClelland’s leadership but with a great deal
of help from a hard core group of volunteers, the society
has burgeoned from 40 to 280 members in little over a
year. The film Jurassic Park probably didn’t hurt recruiting, either.
“I think people have a fascination with the geological
past, that past plants and animals and environments
could be so different from how they are today,” Pigg says.
“Although the history of plants is a more subtle interest than that of dinosaurs, it is the plants that make up
the environment. Public interest in both should be
encouraged.”—Steve Koppes
Her research suggested plants bearing Glossopteris leaves were
a diverse group that thrived throughout the temperate forests of the
Southern Hemisphere, but they probably left no living descendants.
More recently, Pigg also documented the first leafstem 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 in which they live.
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
At left, a modern hickory nut is shown
in cross section. Above is a similar view of a prehistoric
ancestor from a collecting site in western Washington.
JOHN C.
PHILLIPS
PHOTOS
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 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 western United States.
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.”
ASU’s fossil-plant studies are supported in part by the National Science
Foundation. For more information, call Kathleen B. Pigg, Ph.D.,
Department of Botany, 602.965.3154.
Kathleen Pigg studies
thin sections of fossil
samples which preserve
fine internal details.
At left, a cross section of
a fossil stem shows good
views of vascular structure. Right, a progressive
series of thin sections
from a fossil twig.
ASU Research —Fall/Winter 1994
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