The La Brea Tar Pits in Los Angeles are one of the most famous fossil sites in the world, but they are not dinosaur fossil sites. Instead, they preserve animals and plants from the late Pleistocene Epoch, often called the Ice Age. Sticky asphalt seeped to the surface and trapped animals such as mammoths, mastodons, saber-toothed cats, dire wolves, ground sloths, birds, insects, and many smaller organisms.
The site matters because it gives scientists an unusually detailed view of an ecosystem from tens of thousands of years ago.
The tar pits formed when natural petroleum seeped upward through cracks in the ground and thickened into sticky asphalt at the surface. Animals became trapped in the asphalt, and predators or scavengers were sometimes trapped while approaching the struggling animals. Over time, sediment covered the remains, slowing decay and helping preserve bones, teeth, plant material, and microfossils.
Paleontologists study these fossils to reconstruct Ice Age food webs, climate conditions, extinction patterns, and how life changed near the end of the Pleistocene.
Key Facts
- La Brea Tar Pits preserve late Pleistocene fossils, not dinosaur fossils.
- The Pleistocene Epoch lasted from about 2.58 million years ago to 11,700 years ago.
- Natural asphalt forms when petroleum seeps to the surface and loses lighter chemicals, leaving a thick sticky residue.
- Relative abundance can be estimated with percentage = (number of specimens of one type / total specimens) x 100.
- Fossils at La Brea include mammals, birds, reptiles, amphibians, fish, insects, plants, pollen, and other microfossils.
- Radiocarbon dating can estimate ages of once-living material up to about 50,000 years old.
Vocabulary
- Asphalt seep
- An asphalt seep is a place where natural petroleum reaches the surface and becomes thick, sticky asphalt.
- Pleistocene
- The Pleistocene is the geologic epoch from about 2.58 million years ago to 11,700 years ago, known for repeated ice ages and many large mammals.
- Megafauna
- Megafauna are large animals, especially the big mammals of the Ice Age such as mammoths, mastodons, and giant ground sloths.
- Microfossil
- A microfossil is a tiny fossil, such as pollen, spores, insect parts, or microscopic shells, that often requires a microscope to study.
- Radiocarbon dating
- Radiocarbon dating is a method for estimating the age of once-living material by measuring the remaining amount of carbon-14.
Common Mistakes to Avoid
- Calling La Brea a dinosaur fossil site is wrong because the fossils are mostly Ice Age organisms from long after dinosaurs went extinct.
- Assuming the tar preserved only large mammals is wrong because insects, plants, pollen, seeds, birds, reptiles, and microfossils are also important parts of the fossil record there.
- Thinking every fossil found in one layer has exactly the same age is wrong because layers can contain material deposited over time or disturbed by later movement.
- Using one fossil species to describe the entire ecosystem is wrong because paleontologists need many fossil types to reconstruct climate, food webs, habitats, and extinction patterns.
Practice Questions
- 1 A fossil layer contains 240 dire wolf bones and 60 saber-toothed cat bones. What percentage of these predator bones are dire wolf bones?
- 2 A sample from the tar pits is dated to 38,000 years ago. The end of the Pleistocene was about 11,700 years ago. How many years before the end of the Pleistocene was the sample formed?
- 3 Explain why the La Brea Tar Pits can teach scientists about Ice Age ecosystems even though they do not contain dinosaur fossils.