If we just studied the bivalves, foraminiferans, and nannoplankton, we might conclude that the tropics have a higher overall diversification rate. But when we look at the radiolarians too, we can see that this pattern is not universal. For some groups, the tropics seem to be a cradle, but for others — like the radiolarians — they are not. We’d need to study data on lots more organisms to figure out just how common this pattern is. Unfortunately, those data are not yet available.
Now let’s look at another potential explanation for the LDG: extinction.