Question: How many different animals have been thought to exist, but are extinct now?

Edward Bovill answered on 14 Mar 2013:

Of all the known species that scientists have discovered over the years, 98% of them are now extinct! That means that over the entire history of the Earth, there have been 50 times as many species alive as there are in total today. As there are estimated to be 8.7 billion species on the planet at the moment, that means that in total there have been 435 billion species alive at one point in the Earth’s history.