Apex predators are thought to have existed since at least the Cambrian period, around 500 million years ago. Extinct species cannot be directly determined to be apex predators as their behaviour cannot be observed, and clues to ecological relationships, such as bite marks on bones or shells, do not form a complete picture. However, indirect evidence such as the absence of any discernible predator in an environment is suggestive. "Anomalocaris" was an aquatic apex predator, in the Cambrian. Its mouthparts are clearly predatory, and there were no larger animals in the seas at that time.