The Cambrian explosion
Between 520 and 550 million years ago, a sudden explosion of animal types appear in the fossil record. This example of rapid evolution is known as the Cambrian explosion. Theories of how and why it occurred range from the notion that it didn’t happen at all, to a spike in oxygen levels, to the advent of the sense of vision.
Here’s an article that includes an animation of some of the oddball animals that appeared and disappeared during the Cambrian period.
Here’s an article that describes the link between oxygen and predators.
And here are a couple more articles with more information on the Cambrian explosion.