Herbivores are consumed by omnivores or carnivores. Dogs have been described as carnivores or omnivores. Heterotrophs can be classified by what they usually eat as herbivores, carnivores, omnivores, or decomposers. Carrion is an important food source for large carnivores and omnivores in most ecosystems. Today wolves (carnivores) and bears (omnivores) are endangered. Furthermore, this allowed the dinosaurs to succeed them as the dominant terrestrial carnivores and herbivores. These nutritional requirements place the dog part-way between carnivores and omnivores. They are preyed upon by large mammalian carnivores and birds of prey. Archaic mammals filled the world such as creodonts (extinct carnivores, unrelated to existing Carnivora). King mackerel are voracious, opportunistic carnivores. Today the larger carnivores (wolves and bears) are endangered. Nevertheless, some fall prey to large carnivores, especially the spotted hyena. American marten are vulnerable to predation from raptors and other carnivores. The corneas of cats, dogs, wolves, and other carnivores only have four. Examples are humans, rats, crabs and many carnivores.