Learning from the Present

Several meters away, through the wavering heat of a desert afternoon, a paleontologist spies what looks like a thumb-size chip of bone. As he approaches the relic, he wonders what it will be: A piece of leg bone? A fragment of skull? A chunk of a vertebra? What sort of creature does this remnant represent? The paleontologist reaches the find, kneels, and whips out a whisk broom. Delicately, he brushes away loose grains of sand to reveal the fragile skull of a nine-banded armadillo. “Jackpot!” the scientist thinks. From the bits of flesh still on a few bones, he knows that this animal roamed the Earth, oh, maybe a couple of months ago.

A jackpot indeed. Increasingly, paleontologists are concerned not only with creatures that lived, died, and fossilized millions of years ago. Bone hounds today are broadening their investigations to include modern times. They scout remote, undisturbed areas to survey and identify unfossilized bones lying about on the ground and then compare the resulting list of species with the known inhabitants of that ecosystem. These analyses of the earliest steps in the fossilization process are providing scientists with insights into how complete–or, in some cases, how incomplete–Earth’s fossil record may be.

Walk in the park

Not every organism that dies becomes a fossil. In fact, fossilization is the exception, not the rule. Only certain combinations of biological materials, environmental conditions, and fate will preserve a recently dead organism and give it a chance at fame in a museum display. Many ancient species are known only from a single set of often-fragmentary remains. In other cases, plants or animals are known just by the traces they’ve left behind, not from their actual remains (SN: 6/9/01, p. 362: Beyond Bones).

Scientists have begun to study the fossilization process to understand how likely various species were to be preserved. That information could revise some estimates of the relative abundance and dominance of various animal species in the fossil record.

In one long-term investigation, researchers have been studying the bones littering the landscape in Kenya’s Amboseli National Park, a 392-square-kilometer reserve just northwest of Mount Kilimanjaro. During the dry season, wildlife flocks to the park’s spring-fed marshes. Amboseli also contains woodlands, grasslands, and a low area that becomes a lake during rainy spells, says Anna K. Behrensmeyer of the Smithsonian Institution in Washington, D.C. She and her colleagues have systematically scoured certain paths across the plain and through the woodlands of the park since the 1970s, recording the bones they find. Most they leave in place and revisit during later surveys, but some they take back to the lab for identification and analysis.

In the past few decades, the park has experienced an ecological shift that has influenced the quantity of bones there. The park in the 1970s and 1980s hosted a diverse set of predators–including lions, hyenas, cheetahs, and jackals. The bones of their prey, large and small, were abundant. However, climate change and other factors transformed a large part of Amboseli’s woodlands to open grasslands by the 1990s. As a result, populations of hyenas skyrocketed.

Unlike most of the predators in the park, scavenging hyenas use their massive jaws to crush all but the largest bones of big carcasses. Therefore, Behrensmeyer and her colleagues now find few bones from prey weighing less than around 400 kilograms, the size of a Cape buffalo. That change has consequences for the future fossil record.

Scientists could use detailed analyses of the remains in the park to infer predator-to-scavenger ratios in ancient ecosystems, Behrensmeyer says. She described the team’s findings last fall in Norman, Okla., at the annual meeting of the Society of Vertebrate Paleontology.

The soil in Amboseli ranges from neutral to alkaline, conditions that can encourage fossilization, says Behrensmeyer. In the 3 decades of the team’s observations, many bones have already absorbed minerals from soil and groundwater, setting the stage for long-term preservation.

Bones that don’t end up in the gullet of scavengers often fall prey to environmental degradations. For example, exposure to harsh sunlight tends to quickly break down bones, which in living animals are made of up to 30 percent protein by weight. Quick burial of a carcass can slow such weathering, says Behrensmeyer, and even bones resting atop the soil in shady areas can endure if they absorb mineral-rich water from just below the ground’s surface.

Highly porous bones, such as many of those from birds, are especially effective at wicking up groundwater. Behrensmeyer and some of her colleagues recently published a detailed analysis of some modern avian remains they collected in Amboseli in 1975. The new study, which appeared in the Winter 2003 Paleobiology, suggests that paleontologists can do a good job of reconstructing some aspects of ancient ecosystems even if the fossils they’ve found represent only a small proportion of the species in an area.

Behrensmeyer’s team analyzed 126 bones and fragments from 54 bird carcasses, 25 of which were ostriches. The bones had sat in museum drawers for more than a quarter century. Behrensmeyer and her colleagues identified 16 modern bird species as sources of the specimens, 3 of which the researchers couldn’t classify.

Ostrich remains were especially prominent in the scientists’ sample both because these birds are common at Amboseli and because they’re so large, says Behrensmeyer. The bones’ size makes them more resistant to weathering and more likely to be spotted in a survey, she notes.

That bias also shows up among the other avian remains that the team identified. None of the species’ members typically weighs less than 100 grams, even though nearly 60 bird species that have been spotted in Amboseli fall into that size range.

Even though the 16 species identified by their bones make up only 4 percent of the bird species at Amboseli, those species represent 9 of the 10 categories of feeding habits among birds there, says Behrensmeyer. Only fish-eating birds weren’t represented in the bone tally, perhaps because those species die along swampy shores, where their bones quickly sink out of sight.

The researchers concluded that despite such limitations, if they had been examining these specimens as fossilized remains a million years from now, they could have surmised that the park contained open grasslands, lakes, and swamps inhabited by enough large animals to nourish scavenging birds.

Stroll on the beach

Nearly half a world away from the African savanna, scientists have tallied the clutter of marine mammal bones in a desolate ribbon of Mexican coastline on the Colorado River delta at the northern end of the Gulf of California. They’ve surveyed an area that’s largely undeveloped because it experiences tides that can rise and fall through a range of 8 m. Because people rarely visit the area, beached remains there are seldom disturbed.

Eighteen species of marine mammals have been recorded swimming in the northern portions of the gulf, says Karl W. Flessa, a paleontologist at the University of Arizona in Tucson. Four of them live there year-round: the California sea lion, the common dolphin, the bottle-nosed dolphin, and the vaquita, which is also known as the Gulf of California harbor porpoise. Ten of the species, including the fin whale and the false killer whale, migrate through the region, and four other species are spotted there only rarely.

In December 2001, Flessa and his team surveyed two sections of shelly beach that together stretched 4 km. They found 470 bones at 112 sites within 18 m of the shoreline. On average, that’s a bone or group of bones every 36 meters or so, he notes.

The abundance of remains depends in part on the scarcity of both people and scavenger animals in the area. The only scavengers are coyotes, but they usually don’t discover carcasses before the region’s arid conditions turn them into an inedible material that Flessa wryly terms “dolphin jerky.” Although the researchers discovered several remains that were obviously fresh, some other bones in the count may have been on or partially buried in the sand for 40 years or more, says Flessa.

Excluding the three relatively intact carcasses found during their bone census, the researchers tallied 26 skulls, 315 vertebrae, and 65 ribs. Although vertebrae were found most frequently, they didn’t turn up in the expected proportion to the number of skulls, notes Flessa. A California sea lion has 44 vertebrae, and the common dolphin boasts 74–numbers that indicate that the beachcombing scientists should have found between 1,000 and 2,000 vertebrae along with the 26 skulls. Smaller bones, such as those from limbs and flippers, turned up even less often than did vertebrae and ribs.

This result suggests that by simply counting skulls, paleontologists studying disarticulated remains from a single fossil site can get their best estimate of the number of animals those fossils represent. Flessa and his colleagues report their analysis in the April Palaios.

The team found skulls representing all 4 of the year-round marine-mammal residents, 3 of the 10 migrant species, and 1 of the 4 infrequent visitors. About 17,000 sea lions live in the northern Gulf of California, says Flessa, and their skulls were most common. Least common of the year-round species’ skulls were those from the vaquita, an endangered species found only near the Colorado River delta. A 1997 aerial survey counted only around about 225 vaquita in the region. Threats to the vaquita include incidental capture in fishing nets. Indeed, during their survey, Flessa and his colleagues found the remains of a vaquita tangled in a net.

The survey’s results may be of interest to modern-day marine biologists, as well as paleontologists, because the numbers of found skulls reflect relative populations of year-round resident marine mammals in the region. Flessa and his colleagues suggest that modest surveys of beached remains in other remote regions may be a cost-effective supplement to aerial or nautical surveys of living marine mammals.

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After the Dig

All these data from fossils . . . what can they teach us?

Many factors affect the quality of Earth’s fossil record. Some organisms simply live in environments that aren’t conducive to fossilization, such as ocean areas with rocky bottoms rather than carcass-covering sediments. Some gaps in the fossil record occur because few fossil-carrying rocks of a particular age are available at Earth’s surface (SN: 7/6/02, p. 5: Available to subscribers at Into the Gap: Fossil find stands on its own four legs). However, one of the largest influences on what paleontologists find is how thoroughly they sample the rocks that are available to them. For instance, for a long period of paleontology’s history, scientists generally conducted their field trips close to home. As a result, North America and Europe have been studied more thoroughly than, say, Africa and Australia.

One way to compensate for these biases is to construct comprehensive databases that reveal what’s lacking. By recording as much information as they can about fossils and the circumstances of their discovery, scientists may be able to identify factors that have shaped the fossil record locally, regionally, and globally. For instance, researchers led by John Alroy at the University of California, Santa Barbara are compiling a detailed paleobiology database that identifies fossils discovered worldwide according to their species, where they were unearthed, how they were excavated, the name and age of the rock formation in which they were found, how abundant the various species were at that site, and the fossils’ condition.

Such efforts can pay off, says James S. Crampton, a paleontologist at the Institute of Geological and Nuclear Sciences in Lower Hutt, New Zealand. Scientists there have been compiling a paleontological database, called the Fossil Record File, since 1946. The researchers have computerized approximately two-thirds of the whole file by randomly selecting sets of records. This electronic catalog contains information on more than 76,000 species that have been collected at more than 56,000 sites in the country.

Crampton and his colleagues recently analyzed the portion of the fossil database related to marine mollusks that have lived during the past 60 million years. That subset includes more than 5,200 species that have been discovered at more than 6,200 New Zealand sites. The researchers confirmed the notion that the more rock that’s available for scientists to excavate, the more species they’re likely to find. This trend has already been observed in North America and Europe, says Crampton, and it’s certainly a factor that can influence estimates of ancient biodiversity. The scientists report their findings in the July 18 Science.

The researchers also found that the number of species identified in a region of New Zealand didn’t correlate well with the number of distinct layers of rock in that region, each of which preserves a separate ecosystem. In North America, fossil diversity in a region generally follows the number of rock layers there. The discrepancy may stem from New Zealand’s volcanic activity, which can add layers of ash to the landscape but also can lift up a region and cause faster erosion, thereby erasing rock layers.

“Only when you look at the data do you realize how complicated it is,” says Crampton. The fossil database that New Zealand paleontologists have compiled, he adds, “has enough raw material to keep scientists occupied for decades to come.”

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