Cambrian

The Cambrian is a major division of the geologic timescale that begins about 542 ± 1.0 Ma (million years ago).

The Cambrian is the earliest period in whose rocks are found numerous large, distinctly fossilizable multicellular organisms that are more complex than sponges or medusoids. This sudden appearance of hard body fossils is referred to as the Cambrian explosion.

Cambrian in "Worlds Enough, and Time"
A family of space-travelers from another galaxy visited Earth during the Cambrian. While here, they released animals from a failing aquarium, causing the Cambrian explosion.