All the previously described lagerstatte are deposits of organisms from a marine environment. The fifth and final pick for "greatest fauna" is something completely different: The fossils encased in the La Brea tarpits were land-dwellers who became entrapped in a natural asphalt spring, and if for no other reason, the unique environment and fossilizing agent earn the La Brea tarpits a place on our list of top 5 fossil faunas.
How can an animal be so stupid as to fall into an asphalt pool? These were probably not steaming or bubbling pits like the mudpots and geysers of Yellowstone, but still pools of dense oil muck camoflaged by plant debris and perhaps even holding a pool of water.
Animals that came to take a drink got stuck, their distress cries were heard by predators who themselves became ensnared, and finally scavengers that came along for the easy pickings became mired in the muck—leaving a record of the whole food chain, from plants to insects to mammoths to saber tooth tigers and vultures—650 species of plants and animals so far identified, on the basis of 3.5 million fossils recovered, an astonishing diversity of animals that gives us an unprecedented window into an ice-age ecosystem.
Photo from www.papermag.com/blogs/2008/11/mr_mickeys_mustsee_guide_to_la_1.php
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