In rock layers, jellyfish fossils are found lower than trilobite fossils, and trilobite fossils are found lower than ammonite fossils. What can be determined from this information?

In undisturbed rock layers, lower layers formed earlier than the layers above them, so their fossils are older. Jellyfish fossils are lowest, followed by trilobite fossils and then ammonite fossils. Therefore, jellyfish existed before trilobites and ammonites, making choice D correct.

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