What does this mean? Rephrase please?

I am working on a school project about extinction events. I found the following on Wikipedia:

The rebound of life's diversity with the sustained re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving orders.

What's the subject of "saw"? Is it "The rebound"? And if so, how can rebound - the return into sth normal, see something?

Also, isn't using sustained and re-flooding in the same sentence pleonasm?

I did not intend to be sarcastic or anything, I'm genuinely curious about all of this.

Solution 1:

The Silurian is a geologic period. The complicated sentence is basically saying this. There was a lot of flooding at the start of the Silurian. This flooding saw an increase in biodiversity. The species that survived diversified as a result of it.

The subject of saw is indeed rebound. Sometimes, impersonal things can be used as the subject of see. For example, the 20th century saw a lot of war.

I don't think that sustained re-flooding is in fact pleonasm, as re-flooding on its own does not indicate frequency. Sustained re-flooding happens over and over again.