La Brea del Sur

Los Angeles’ Rancho La Brea is one of the world’s most famous fossil-bearing sites. The tar pits there have yielded more than 1 million fossils representing 50 mammal species, 125 types of birds, and dozens of reptiles, insects, and other invertebrates. But L.A.’s claim to fossil fame could someday soon be equaled or surpassed by any of several spots far south of the U.S. border.

Thousands of miles from Rancho La Brea, hundreds of similar petroleum seeps, or menes, dot the landscape in Venezuela. Although explorers have known of these animal-trapping sites since the 16th century, only in the past decade or so have paleontologists begun to conduct serious digs. So far workers have removed less than two truckloads of sediment at the most thoroughly studied site, yet the number and diversity of fossils found already suggest that the South American pits may eventually rival Rancho La Brea.

For one thing, the Venezuelan menes are found in a variety of ecosystems, so their sediments may hold a wider variety of creatures. Also, because these sites are located near where the Isthmus of Panama joins South America, their fossils may shed light on the migrations of creatures to and from the continent. Finally, excavations at one site suggest that its oil-rich sediments—and the creatures that they’ve trapped—have accumulated over a much longer period of time than those at Rancho La Brea. Thus fossils entombed there might yield insights into how and when climate changed as Earth slipped in and out of recent ice ages.

Sticking points

At thousands of sites in southern California, crude oil seeps up from petroleum-rich strata to Earth’s surface through cracks in the overlying layers of rock. The more-volatile components of that sludge quickly evaporate, leaving behind asphalt-rich goo that can mire even the strongest creature. Yet only a handful of such places in the region—Rancho La Brea, two other sites near the Pacific coast, and two more farther inland—have yielded substantial numbers of fossils.

The same sort of stratigraphy can be found in northern South America, says David M. Orchard, a geologist with ConocoPhillips in Houston. Tar pits occur in a swath from Trinidad, a Caribbean island off the northeastern coast of Venezuela, westward to Peru and Ecuador, he notes. In the oil-rich Maracaibo Basin of northwestern Venezuela alone, a small area about one-tenth the size of California, there are more than 200 menes. There, crude oil from the La Luna formation—an organic-rich shale whose sediments were laid down about 75 million years ago—seeps to the surface through overlying sediments as much as 6 kilometers thick. Orchard and Ascanio D. Rincón, a paleontologist at the Venezuelan Institute of Scientific Investigations in Caracas, described the geological context of the menes in October in Austin, Texas, at a meeting of the Society of Vertebrate Paleontology.

So far only a few South American tar pits have been studied in depth or yielded fossils, says Orchard. For example, Peru’s Talara site or Ecuador’s La Carolina site, both of which were studied by Canadian researchers in the 1950s, proved exceptionally rich in the remains of birds and canids, a group of mammals that includes dogs, wolves, and foxes. The subsequent dearth of attention paid to South America’s menes is coming to an end, however. In particular, paleontologists are clamoring about fossils found in a tar pit near Inciarte, in northwestern Venezuela.

That mene, more than 1 km long and 500 meters wide, is at least 10 times the size of the tar pits at Rancho La Brea, says Orchard. Nevertheless, paleontologists have removed only about 1.5 cubic meters of sediment, or around 15 wheelbarrow loads, from the site. That fieldwork—which involved carefully digging down into the oily muck 10 centimeters at a time, screening the material, and then tallying the fossils—took place in the summer of 1998.

In that single field season at the Inciarte site, researchers dug no deeper than 1 m into the mene, says Rincón. Despite that limited excavation, he and his colleagues identified the remains of more than 100 species, including 43 mammals, 56 birds, 11 lizards, and 4 frogs. One of the mammal species, a type of bamboo rat related to species that live in the Amazon Basin today, represents a new genus.

As at Rancho La Brea, most of the vertebrate fossils from Inciarte show up as disarticulated bones. Even so, the tar can preserve fragile fossils like those of beetles, snails, and the delicate bones of bats. Of the million or so bones found at Rancho La Brea, only four come from bats, says Nicholas J. Czaplewski, a paleontologist at the Oklahoma Museum of Natural History in Norman. However, he notes, a dozen or so of the fossils from Inciarte—including skulls, jaws, teeth, and a humerus—belonged to bats.

The Inciarte bats may have fallen victim to the mene when they tried to snap up an insect floating on a thin veneer of water that masked its tar, says Czaplewski. Or, he suggests, the flying creatures may have been snagged by the underlying tar while taking a drink as they skimmed the surface of the water—another common behavior among modern bats. The presence of a shallow pond atop the tar during some seasons also may explain why many of the birds found in the mene represent aquatic species, says Orchard.

Along with the fragile fossils of bats and insects, Venezuelan menes have yielded the tar-soaked remnants of plants, says Gregory McDonald, a paleontologist with the National Park Service in Fort Collins, Colo. Such fossils usually aren’t preserved in sediments in warm environments and could help scientists track long-term changes in the area’s climate. “Who expects wood to be preserved in the tropics?” he asks. “It typically gets recycled when it rots.”

Rincón and his colleagues have carbon-dated some of the Inciarte fossils, which range between 25,000 and 27,000 years of age—a few millennia before the height of the last ice age. Because researchers don’t know the depth of the tar in the mene there, it’s not yet possible to predict whether older fossils may be found.

Gone to the dogs

The species tally at Inciarte renders that site the most fossiliferous in northern South America. In general, says Rincón, tar pits can yield the remains of species that aren’t commonly preserved in other types of rocks. Therefore, menes probably offer a more thorough chronicle of the creatures that lived or foraged nearby than do other types of sediments. Compare, for instance, the 43 species of mammals found at the Inciarte mene with the 24 excavated from the tarfree sediments at Taima-taima, an archaeological site along the eastern coast of Venezuela. At that location, most of the fossils represent species eaten by humans who had lived or hunted there.

The Inciarte excavations have produced several revelations about the region’s ancient canids. Researchers have identified fragments of the skull of Urocyon cinereoargenteus, the gray fox, for example. Fossils of that species have never before been reported from South America, says Rincón.

Also, teeth and jaw fragments of Protocyon troglodytes, an extinct species known as the cave wolf, have turned up at Inciarte. This creature is known only from South America and previously has been found only at sites at least 1,500 km away and toward the south, says Rincón. He and colleague Francisco J. Prevosti, a paleontologist at the La Plata Museum in Buenos Aires, reported their finds in the September Journal of Paleontology.

The presence of P. troglodytes at Inciarte raises the possibility that members of the species could have migrated north into Central America during the last ice age, says Rincón. So far, paleontologists have no evidence that ancient Panama had the open grasslands and forest-edge environments that this creature apparently preferred. However, he notes, such ecosystems could have existed along the isthmus’ ancient coastlines, which later became flooded when sea levels rose.

“At Inciarte, there’s a commingling of what are typically thought of as North American and South American species,” says Christopher A. Shaw, a vertebrate paleontologist at the Natural History Museum of Los Angeles County. There, the ground sloths, camels, and other species characteristic of ancient South America lived alongside the dire wolves and saber-toothed cats typically associated with Rancho La Brea and other North American sites. “It’s a good spot to look at the interchange,” Shaw notes.

Rincón agrees: “We know a lot about the species in North American and in Argentina. In the middle, we know almost nothing about what happened.”

Riches of the orient

Like western Venezuela’s Maracaibo Basin, the Maturín Basin in the eastern part of the country is dotted with scores of menes. In 2006, workers digging a pipeline there unexpectedly began to excavate oil-rich sediments that were chock-full of bones. Work at the site quickly ceased, and researchers were called in to conduct what Orchard of ConocoPhillips describes as “salvage paleontology” before the walls of the waist-deep trench collapsed. Although the fieldwork at the site, dubbed El Breal de Orocual, was rushed and wasn’t as systematic as it could have been, early results are promising.

Paleontologists have so far identified the fossils of more than two dozen species, says Rincón. The presence of semiaquatic creatures such as caimans and tapirs suggests that the area once was a floodplain or river delta, he notes. However, the remains of creatures such as llamas and glyptodonts-armored armadillos, some species of which grew to the size and weight of Volkswagen Beetles—hint that the area also hosted a savannah. Probably, says Rincón, the mene sat in a swampy area where water helped disguise the deadly tar.

As at Rancho La Brea, predators at Orocual dramatically outnumber the prey. Most of the herbivores trapped in the mene, such as tapirs, camels, horses, and the like, were young animals—inexperienced creatures much more likely than adults to be trapped in the tar. However, most of the predator fossils, such as those of wolves and saber-toothed cats, came from adults, who probably attempted to partake in an easy feast before they themselves became entrapped. Analyses of the Orocual sediments suggest that they were deposited between 1 million and 500,000 years ago, Rincón reported in October at the vertebrate paleontology meeting.

Orocual “is the first area I’ve seen that can rival Rancho La Brea,” says Shaw. “The preservation is just wonderful down there, and it’s amazing how well even delicate things like centipedes have been preserved.” The diversity of fossils, including plants and invertebrates, is “incredible,” he adds. “The fossil record there is as rich as, if not richer than, Rancho La Brea.” Considering the extent of the site, Shaw speculates that it could easily yield hundreds of thousands of fossils.

Although each tar pit may be active for only a few thousand years, just a small sampling of the vast number of menes in Venezuela could provide paleontologists with a fossil record for the region that could extend back more than 2 million years, to an era before Earth began its regular cycle of ice age and interglacial periods. In contrast, notes Orchard, the fossil record at Southern California’s Rancho La Brea stretches back only 40,000 years or so.

“The combination of that age range and the diversity of ecosystems that could be represented in these menes,” Orchard says, “is an extraordinary opportunity for science.”

“The Inciarte site is so rich, there’s more [paleontology] than anyone can do in a lifetime,” says McDonald. “We’re just now beginning a very long and very exciting process.”