I like to ask them if grasses and flowering plants also ran for high ground, because they are only ever found in the uppermost layers.
No, no. They were cut loose from the earth in sod packets by salt water tidal waves, then carried by those same tidal waves to the calm fresh water pools in which fresh water fish were surviving, nourished there, and then gently set down on the top layers after the tidal waves stopped.
Surely that's obvious.