The ancient bison, the most common large herbivore recovered from the asphalt deposits, is a direct ancestor of the living North American bison. By examining the bison fossils at Rancho La Brea, paleontologists have discovered that these animals were migratory animals, similar to their modern day descendants. This conclusion was first proposed when scientists discovered that all the juvenile (young) bison within the fossil record fit into specific age groups. By examining the bison jaws for the presence of baby and permanent teeth, paleontologists concluded that all the bison found were either 2-4 months old, 14-16 months old, or 26-30 months old. No bison fossils have been found that do not fit into this annual pattern. This pattern indicates that bison were at Rancho La Brea for only a few months at a time. If the calves of ancient bison were born in early spring like their modern day relatives, then the bison would have traveled through the area each year during late spring.

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