Build a Better [Carnivore] Trap

Science News Online has an interesting article this week about the La Brea Tar Pits in Los Angeles. The pits are famous for the large number of well preserved Ice Age fossils, especially of large mammals, including saber-toothed cats, giant ground sloths, and mammoths.

However, as Science News points out, the reason so many large mammals have been found in the pits is that fossil hunters in the early 1900s were focused on getting big, complete bones out of the tar with no regard for context or the many smaller bones and other fossils. In 1969, when more careful excavations were begun in Pit 91, a lot more began to be understood about predation at the tar pits. Of the 40,000 specimens unearthed from the site between 1969 and 1980, 18,498 are individual bones from mammals weighing more than 5 kilograms.

More than 95 percent of the mammal bones that the researchers studied came from just seven species. Three were herbivores -- the western horse [Equus occidentalis], the ancient bison [Bison antiquus], and the 2-meter-tall Harlan's ground sloth [Paramylodon harlani] -- and four were predators -- the dire wolf [Canis dirus], the saber-toothed cat [Smilodon fatalis], the North American lion [Panthera atrox], and the coyote [Canis latrans]. Except for the coyote, all these herbivores and predators are now extinct.

The article went on to note that "the bones of predators were almost seven times as common in Pit 91 as were those of prey."

Overall, an estimated 80 percent of the mammals were carnivores, and 60 percent of the birds were birds of prey. That's a surprise, says [John] Harris [curator of the tar pits' George C. Page Museum], since the number of herbivores in a stable ecosystem always outnumbers the predators by a wide margin. The disparity arises because the tar pits acted as predator traps, says [Blaire] Van Valkenburgh [of the University of California, Los Angeles]. After an herbivore stumbled into sticky asphalt, which may have been masked by shallow water or leaves, its struggles attracted meat eaters. Each herbivore entrapment probably triggered a feeding frenzy that resulted in up to a dozen predators being trapped as well, says Van Valkenburgh.

The article goes on to talk about the different predation rates suffered by herbivores versus different carnivores, for example, saber-toothed cat carcasses were more likely to be scavenged than those of dire wolves. Using carbon-isotope ratio analyses, they also determined that dire wolves foraged as far away as the Pacific coast for seals and other marine mammals. The wolves exhibited elevated isotopes that were similar to isotopes in the remains of pit-trapped eagles that had eaten ocean fish.

One final factoid from the article: " scientists have identified remnants from every mammal species that lives in the Los Angeles Basin today -- with the curious exception of opossums."