Large, diverse, multicellular organisms first appeared in the fossil record approximately million years ago during the

600, paleozoic

Explanation:

The paleozoic era is the oldest era of the Phanerozoic eon. The paleozoic era spans from 541 to 251.902 million years ago. It is the stage where multi cellular organisms came into being. The widespread emergence of life was called the cambrian explosion.

The modern phyla's first emergence was in the paleozoic era.

Hence, the question is referring to the a time period of approximately 600 million years ago called the paleozoic era.