This is a very, very simple calculation. About 2 minutes should cover it. It is directly useful in the discussions at hand.
If you need a long time I will give it to you as a 2 day suspension. Whould that help?
You have told us about the large number of Basiosauri fossils. Are you know saying that they are divided into separate groups and there aren't so many fossils of each type after all?
Why don't we just stick with the larger grouping that has produced the, I think you called it, "HUGE" numbers of fossils? That is where you started.