Badlands, good finds

Once an ancient sea, the Anza-Borrego Desert is still awash in fossils

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.

© Copyright 2007 Union-Tribune Publishing Co. ? A Copley Newspaper Site