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An apex predator is a predator at the top of the food chain. An apex predator has no other predators, except for perhaps other members of its own species. Apex predators have been around for a long time, but not forever. Although there is some cursory evidence of limited predation during the Ediacaran period, about 570 million years ago, modern predation did not begin until the Cambrian period, about 542 million years ago. During this period, animals evolved hard shells and other signs of defense against predation. Obvious predators begin to appear in the fossil record.
The first known apex predator was the very odd invertebrate, Anomalocaris (meaning "anomalous shrimp"). For the time in which it lived, when most organisms were no more than a few inches in length, Anomalocaris was huge, ranging up to a meter (3.3 ft) in size. Anomalocaris had a large head, with two eyes on stalks, and two barbed feeding "arms" in front of its mouth which were 7 inches in length when fully extended. Its mouth, one of the oddest in nature, consisted of 32 overlapping plates arranged in a circle, resembling a slice of pineapple in appearance. These plates were tipped with barbed teeth, and extended all the way down the gullet.
Anomalocaris had a mode of locomotion similar to a cuttlefish -- it had two flexible lobes on either side of its body, which it undulated to move around or hover in place. This apex predator had a large lobster-like, fan-shaped tail. The fossil was initially very difficult to identify as three parts of Anomalocaris were discovered separately, and were all thought to be associated with different organisms until a complete body cast was found.
At the time, the majority of prey were affixed to the sea floor or wandered along it, although there were various cnidarians (relatives of jellyfish) and small trilobites swimming above the sea bottom. Anomalocaris would have used its huge eyes, among the first in the fossil record, to snatch various small animals and shove them into its cavernous mouth. Anomalocaris is thought to be an apex predator because of various adaptations that indicate its extreme specialization as a predator, and its comparatively large size. It would have consumed other, smaller predators, such as its fellow anomalocarids, and close relatives like Opabinia.