The pits are composed of heavy oil fractions called gilsonite, which emerged from the Earth as oil. These deposits had become topped with water, dust, and leaves. Animals would venture in and become trapped, luring predators who come to eat the animals and become trapped themselves. The thick tar then encases the bones, preserving them remarkably well.
The preserved organisms in it are mainly from the Pleistocene epoch, although a great deal of finds date from prehistory (10 kya — 4 kya). Saber-toothed cats, mastodons, ground sloths, bison, western horse, and camelids are . Dire wolves are the most common large land mammal found there.
The La Brea Tar Pits Museum has made a beautiful timeline of the creatures found in the tar pits here.
Learn More about the Tar Pits and the Museum here, and watch Will Ferrell speak sabertooth.