The Cambrian period
In the Cambrian period, roughly 540 million years to 485 million years ago, most of the types of animals that are with us today got their start, even if it’s difficult to recognize them.
Note: I managed to unintentionally imply that the trilobites were the ancestors of the insects. I didn’t mean to, and it’s probably not so.
Here are a couple of websites with more information on the Cambrian.