Since the early 20th century, excavations at the La Brea Tar Pits in Los Angeles have yielded of fossils from 600 species of flora and fauna, including several Columbian mammoths. Many of the fossils are the remains of animals that became stuck in asphalt puddles that seeped to the surface of the pits, 40,000 to 11,500 years ago. Dust and leaves likely concealed the liquid asphalt, which then trapped unwary animals. Mired animals died from hunger or exhaustion; their corpses attracted predators, which sometimes became stuck, themselves. The fossil record of the tar pits is dominated by the remains of predators, such as large canids and felids. Fossils of different animals are found stuck together when they are excavated from the pits. The tar pits do not preserve soft tissue, and a 2014 study concluded that asphalt may degrade the DNA of animals mired in it after an attempt was made to extract DNA from a Columbian mammoth.