Comparing True Diversity Curves to LTT-esque Plots

Here is a plot that Dave Jablonski sent me of generic diversity
in cockles. One curve shows the 'true' diversity (based on fossil
appearances) and the other curve shows the diversity if you only found
the living genera (and thus approximates an extant lineage-through-time plot,
although not quite... but its close enough!). There was discussion
last week of what such a plot would look like, so there it is.

As you can see, the 'extant-only' fails to catch many of the details, particularly a pronounced mass extinction in the KT. The timing of the Cenozoic diversity increase is off by 10 million years. Finally, cockles have actually been decreasing in diversity recently, but the 'extant-only' plot shows only rapid recent increase.