Conditions of Fossilization

Bones that were preserved by the asphalt are stained in different shades of brown. Even though the asphalt is an amazing preservative for bones, the fossils of Rancho La Brea are not always perfectly preserved. Because the bones took between several weeks and two years to be completely buried in the asphalt seeps, the bone surfaces may show:

Surface Weathering - Bones were often subjected to physical processes while on the surface. These included heat, cold, wind and rain.

Rodent Gnaw Marks - Rodents gnawed on the bones while on the surface, and their teeth marks were preserved on the fossils.

Water Wear - Because of the climate during the last Ice Age, larger streams and rivers were year-round in the Los Angeles area. The bones of animals that died near these streams sometimes eventually ended up in the water. The bones would then tumble downstream, bouncing off rocks and the streambed, creating the type of damage called water wear. While traveling downstream, the bones would sometimes become stuck in the asphalt that had seeped into the streambed.

Pit Wear - Bones that have rubbed against a rock or one another within the asphalt display a condition called "pit wear." Caused by earthquakes or by animals trampling on the bones stuck in the asphalt, this condition takes the form of holes and long grooves cut in the bones.

		Foot Note! The fossils at Rancho La Brea are preserved so well that it is possible to detect healed bone fractures and subsequent infections, as well as degenerative forms of osteoarthritis and other bone diseases.

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