how many animals have been extinct before you’ve found what they are?

Question: how many animals have been extinct before you've found what they are?
- Keywords:
  extinct
Asked by 282evnb42 to Thomas on 18 Mar 2015.
- Thomas Clements answered on 18 Mar 2015:
  
  Many many thousands of types of animals have gone extinct and we are only scratching the surface with all the fossils we have found! Who knows what we will find in the future!

Question: how many animals have been extinct before you've found what they are?