A carnivore is an animal that eats flesh and meat. As long as there have been animals, there have been carnivores. The first carnivores were single-celled, such as dinoflagellates, that fed on smaller organisms. The carnivores were microscopic and single or multi-celled until the Cambrian period. The Cambrian Explosion transformed the carnivores from microscopic prokaryotes and eukaryotes to sponges, worms, mollusks, echinoderms, tunicates, etc. When an animal, probably worms, evolved into the worm-like hagfish during the Silurian, fish followed shortly afterwards. Fish then became the largest carnivores in the seas and fed by filter-feeding. The introduction of fish probably forced small herbivores, like millipedes, onto land. The millipedes probably fed on Cooksonia and were safe from the fish and orthocones, but they then had to deal with sea scorpions. Fish evolved jaws, large incisors, and the first sharks appeared during the Devonian and fish were the largest animals on the planet, like Dunkleosteus. Smaller fish, like Panderichthys, evolved into fish that could crawl onto land for breif periods of time to escape predators. Tiktaalik eventually evolved into Ichthyostega by the end of the Devonian and amphibians lived on land. Acanthostega and Eogyrinus would have fed on fish and smaller amphibians as they patrolled the shallow coasts. By the late Carboniferous, the first amniotes had evolved and were small omnivores. Hylonomus and Protoclepsydrops were the first diapsids and synapsids, respectively, and gave rise to the largest carnivores of the Permian. Permian carnivores, such as the diapsid Archosaurus and the synapsid Eotitanosuchus, probably fed on the contemporary herbivorous anapsids. The Permian-Triassic Extinction wiped out all of the large carnivores and made way for the diapsids to take over. They quickly diversified into the dinosaurs, Rauisuchians, plesiosaurs, nothosaurs, placodonts, etc. and became the new large carnivores. By the Jurassic the crocodiles had formed and some became so evolved to hunt in the oceans, that they evolved flippers instead of legs, like Metriorhynchids. The dinosaurs were the carnivores on land, pterosaurs were the carnivores of the air, and plesiosaurs were the carnivores of the sea. Birds were evolving but had yet to reach large sizes. When the Cretaceous-Paleogene Extinction occurred, it wiped out all of the large diapsids and established the birds as the new predators. The birds ruled during the Paleocene because the mammals were small and rat-like, but they were replaced in the Eocene by the Mesonychids. By the late Eocene, the dogs, cats, and mustelids were the new carnivores of the land and whales like Basilosaurus were the predators of the sea. Megalodon had evolved by the Miocene and became the new ruler of the seas, while the mammalian predators were being replaced on land. When the Pleistocene arrived, the primates like Homo sapiens had developed tools and complex hunting techniques to bring down the large herbivores and carnivores.