The Cambrian began 540 million years ago, and was when things decided EXOSKELETONS ARE AWESOME YO and the fossil record really got going. (Before that we have, um, bacterial mats. And not a lot else. But they're awesome because we can use them to track how long Earth days & for that matter years were back then!)
The Cretaceous, on the other hand, followed the Jurassic and Triassic and ended 65 million years ago with the KT (Cretaceous-Tertiary) boundary, noted in the geological record around the world for the impact glasses that provide evidence for a major meteor strike ;)
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The Cambrian began 540 million years ago, and was when things decided EXOSKELETONS ARE AWESOME YO and the fossil record really got going. (Before that we have, um, bacterial mats. And not a lot else. But they're awesome because we can use them to track how long Earth days & for that matter years were back then!)
The Cretaceous, on the other hand, followed the Jurassic and Triassic and ended 65 million years ago with the KT (Cretaceous-Tertiary) boundary, noted in the geological record around the world for the impact glasses that provide evidence for a major meteor strike ;)