Badlands, good finds
Once an ancient sea, the
Anza-Borrego Desert is still awash in fossils
By Susan Brown
November 16, 2005
SCOTT LINNETT / Union-Tribune
Paul Remeika points to the
3.5-million-year-old footprint of an
extinct, elephantlike gomphothere. The
ceature(shown in this drawing) browsed
on leafy plantsno longer found in
Anza-Borrego.
|
|
ANZA-BORREGO DESERT STATE
PARK – Flash floods have eroded the walls of a dusty
wash in Anza-Borrego Desert State Park, leaving ledges
and overhangs of rock. If you stand beneath one ledge
and look carefully, you can see footprints cast in
stone. A three-lobed center pad marks the print of a
lynx-sized cat, and claws tip the toes of a coyote-sized
dog.
These animals walked by
more than 3 million years ago, and their feet pressed
into an ancient soil. Not long after, sediments covered
the footprints and hardened into stone. The surface they
walked upon has long since eroded, leaving a cast of
their paws protruding from the rock above.
Smack in the middle of
all of this, obliterating the trail of the small
predators, is a single print of a much larger animal –
easy to overlook because it is so unfamiliar. Its big,
round foot sank 5 inches into soft mud, leaving a clear
impression of three of its five toes, compelling
evidence that a gomphothere once walked here.
Gomphotheres were
elephantlike ancestors of mammoths, according to George
Jefferson, head paleontologist for the Colorado Desert
District of California State Parks.
"They were low slung,
hippo or rhino-sized animals," Jefferson said. "They had
a long jaw and teeth designed primarily to eat leafy
vegetation." The spiny ocotillo and prickly cholla
widely scattered across the desert floor today would
never have kept a gomphothere alive, just one of many
clues that this was once a very different place.
As many as two dozen
researchers visit Anza-Borrego each year to study the
fossils found here, as they continue to untangle the
events of the past 7 million years. And now, for the
first time, they have assembled their knowledge in the
first book on the fossil history of the area, to be
published at the end of the year.
Ancient
sea
Paul Remeika, then a park
ranger, discovered the first stony footprints while on
patrol in the Fish Creek basin in 1981. But fossils have
intrigued visitors to this parched landscape for at
least 230 years.
Fray Pedro Font,
traveling in 1775 with Spanish explorer Juan Bautista de
Anza, found piles of sea snail and oyster shells, which
led him to surmise that the sea must once have covered
the region.
Font was right, as it
turns out. The Baja peninsula began to tear away from
mainland Mexico 10 million to 15 million years ago,
opening up the space now filled by the Gulf of
California. The waters of this ancient gulf, often
called the Imperial Sea, inundated the land as far north
as present-day Riverside County.
The rocks hold more than
shells. Giant shark teeth, bones of baleen whales and
the fossil remains of an extinct, toothless walrus that
sucked its molluscan prey right out of their shells
paint a picture of a thriving marine ecosystem.
Erosion continually
exposes new fossils in the Vallecito badlands and other
areas of the park. And paleontologists continue to find
marine fossil remains of corals and spiral casts of the
inside of shells that filled with cemented sand then
leached away.
The shapes help them
identify the animals that formed these structures. In
some cases, they are clearly recognizable – nearly
identical to sea biscuits and conchs you might find
today, if you were diving in the Caribbean.
The similarity between
the fossils and modern oceanic creatures suggests the
waters of this ancient sea must have been like those in
the modern Caribbean – warm and clear – in order for
these tropical species to have thrived.
Corals, for example,
depend on symbiotic plants, which need sunlight to
photosynthesize and cannot survive in murky water, said
Thomas Deméré, curator of paleontology at the San Diego
Natural History Museum.
Back then, ocean waters
flowed freely between the Caribbean and the Imperial Sea
because the Isthmus of Panama didn't exist.
Drawing by Pat Ortega
The extinct, elephantlike gomphothere
browsed on leafy plants no longer found in
Anza-Borrego.
|
"South America was an island
continent, separate from North America," Deméré said.
The tropical fossils are
found in the deepest and oldest layers of rock today.
They are bound in a coarse rock formed by shore sands
that have cemented together over time.
Rather suddenly, the
tropical fossils disappear from the rock layers, and
oysters and clams, things found in brackish, muddy
waters, emerge. These more recent fossils are encased
within fine, silty stone that formed of mud – evidence
of an entirely different sort of environment.
Around 5 million years
ago, the Colorado River began slicing through Arizona to
form the Grand Canyon. The rock it carved away was
carried downstream and deposited in a wide delta at the
head of the gulf. The turbid, silty waters buried corals
but formed a shallow muddy bottom so suitable for
oysters that they formed thick reefs – the piles that
Font found millions of years later.
Between 3 million and 2.5
million years ago, Panama rose up from the seafloor and
halted the flow between the tropical Atlantic and
Pacific oceans. That shut down a tropical current, and
changed ocean circulation patterns – and climates –
throughout the world.
Life on land changed
"If you look at the older
assemblages of fossils, they have a distinctively
tropical flavor to them," Jefferson says.In the late
Pliocene epoch,tropical terrestrial creatures gave way
to things that roam savannas.
Fossil bones, including
skulls from a mammoth and a giant zebra, are displayed
in the newly remodeled visitors center in Borrego
Springs. Most others are assembled nearby at the Stout
Research Center, where visiting scientists can study
them. The menagerie includes giant llamas and three-toed
horses, fossil flamingos and a tortoise the size of a
bathtub.
Paleontologists are still
ironing out the details of when and how these extinct
animals lived. For example, fossil hunters have found
bones from at least nine different camels so far.
"We can ID these critters
by the size and shape of their foot bones," Jefferson
said. "The way in which these animals moved about a
playa (lake shore) is different from how they moved
about a hilly, rocky area." Jefferson suspects the feet
might change with changes in climate, but that work
hasn't been done yet.
"We're painting with a
broad brush right now," Jefferson said.
Misty past
Remeika has been
collecting fossil wood since he began working at the
park nearly 25 years ago. When he started, scientists
were calling most specimens of fossil trees desert
ironwood, the only tree that could grow in the present
climate. Remeika drew on his stint as a ranger near
Monterey to recognize wood he was finding.
"I compared the internal
anatomy of the fossil wood to forms that are living
today along Big Sur."
Instead of desert
species, he discovered the fossils were ancient forms of
California walnut, laurel, Pacific madrone and other
species that thrive in a cool,foggy climate.
In cool weather today,
fog drifts inland right up to the rim of the Laguna
Mountains that form the western boundary of the park.
Those mountains are relatively new – they lifted up just
over a million years ago. Now they hold back clouds
blowing in from the west, creating a rain shadow that
dried out the land to form the desert of Anza-Borrego
today.
Modern visitors are
leaving a different sort of track behind as they drive
across the desert and up the washes. A bit like the
gomphothere, they can erase the traces of what came
before.
"We recently
reconstructed the remains of a fossil horse that was run
over by a Jeep," Jefferson said.
Apart from sorting out
the prehistory of the desert, park officials are charged
with conserving this vast resource, the only continuous
fossil record in North America of the past 7 million
years. They hope protecting this ancient landscape will
maintain our connection to the changing landscapes of
the distant past.
 Susan
Brown is a science intern at the Union-Tribune.
|
