The word extinction means we have fewer plants and animals than we did before. So we're going to need to count things. But what are we counting? Species? Here's a dirty little secret: Paleontologists don't really have a good species concept. Biologists have a pretty good one.
The biological species concept defines a species as members of populations that actually or potentially interbreed in nature, not according to similarity of appearance.To that we probably need to add that the offspring can, themselves, reproduce but you get the idea. Here's the problem.... "similarity of appearance" is really all we have in Paleontology. You take two fossils, put then on a water bed, give them some privacy and cue the Barry White. Come back a month later and there are still just going to be two fossils there. FOSSILS DON'T REPRODUCE. In fact we can't be really sure those two fossils were the opposite sex. (not that there's anything WRONG with same sex fossils) . So what do we do about the species concept in paleontology?... basically we muddle through as best we can mainly relying on appearance.
The diversity curve we've been looking at is a family level curve not a species level curve. For those of you who are taxonomically challenged remember that species' group into genera and genera group into families. Not all families have the same number of species. A family of insects may have hundreds of species. The family we belong to, Hominidae contains us, gorillas, chimpanzees and orangutans. So taking out a family of insects takes out a whole lot more species than taking out a family of primates.
So you've got a bunch of biologists out there defining extinction by counting species and you've got paleontologists counting what are basically morphoptypes. And really most of our work on broad patterns in extinction and radiation is done two taxomic levels up. Comparing those is gong to be very very tricky.
Next up: Taphonomy. Because if you don't understand that you don't understand anything.