During the course of evolution, only a small percentage of species has survived.

Paleontologists have estimated of all the species that have ever existed, 99.9% are now extinct.

Click on all four of these extinct organisms to learn a bit about who they were.

Ammonite by Sarah Rieboldt, © UCMP; Trilobite and Lambeosaurus skull © UCMP; Golden lion tamarin © Rare Species Conservatory Foundation, www.rarespecies.org; Quagga and passenger pigeon illustrations are in the public domain.