This time period is known for: first hard parts, a huge evolutionary explosion of diversification, first chordates. The abundance of fossils starting here is so different from older rocks, that the older rocks are all lumped together. What is the name of this time period?

Cambrian

Explanation:

It occurred around 55 million years ago and created an overall lowering of the photosynthesis levels and lead to the evolutionary process of the first chordates and the abundance of the life forms that were so different from the older rocks and the rocks that were slumped together.

There’s no picture or character?