There are a series of asphalt pits called La Brea asphalt pits in Hancock Park in downtown Los Angeles For tens of thousands of years, natural asphalt has been flowing out of underground cracks, forming ponds in the park  The surface of this asphalt pond is long covered with a layer of water animals that go in and get trapped in it and eventually drown When other predators saw the trapped animals, they tried to hunt and were unable to extricate themselves
eventually, they were all turned into a pile of fossils and preserved, thus forming an animal graveyard
Animal cemeteries were formed tens of thousands of years ago