As the Flood of 10,500 years ago drained off North America it passed over degraded oil from a major spill along the coast. Herbivores, with methane-filled bellies, tended to float past, while many more carnivores sank into the oil.
This illustrates the fantasy version of how so many animal bones ended up in the tar deposits at La Brea. The reality is that sea water from the most recent oceanic flood running off the continent, brought with it huge numbers of drowned animals. Until then, North America had been a veritable zoo. Axis change and the resultant oceanic flood is the biggest secret in geology, and martiangrandpa.com has the complete story.